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Contents

16 ·The Masterbuilder - July 2013 www.masterbuilder.co.in

62 CENTRILIT NC: Concrete Additive Based on Pozzolanic Alumosilicate

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Pioneering Foam Concrete Technology

A Testimony to Top Quality and Performance

Greaves Rolls Out High Capacity Concrete Pumps with S Valve Technology

Stay Ahead of the Curve Via Strategic Training and Development

Magnox ILW Interim Storage Facility, Berkeley

Fiber Reinforced Concrete & Its Advantages

Building Trust through Quality Focus

DURAboardHD100: Bitumen Free Joint Filler

Crack Injection System

Interarch's Contribution to Aviation Sector in India and Abroad

Industrial Overhead Doors: Making the Right Choice

Hitech Concrete Solutions Chennai: A State-of-the-art Concrete Testing Center

High Performance Liquid Membrane Series Launched

Advanced Multipurpose Waterproofing Coatings

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Magnox ILW Interim Storage Facility, Berkeley

Evercrete Everwood and Evercrete Top Seal: Waterproof, Preserve and Enhance Life of Wooden Structures

Eco-friendly AAC Block is Here to Stay

Pioneering Innovative and Sustainable Products

Leading the RMC Revolution

Offering a Wide Array of Construction Solutions

Blazing a New Trail in Formwork Systems

Top Notch Engineering Solutions

The Use of Steel and Synthetic Fibres in Concrete under Extreme Conditions

1 2 3 4Don Wimpenny , Wolfgang Angerer , Tony Cooper and Stefan Bernard 1 2Principal Materials Engineer, Halcrow Pacific Senior Tunnel Engineer,

3 4Halcrow Pacific Consultant Elasto Plastic Concrete Consultant, TSE

Concrete Batching Plants: Emphasis on Quality and Variety of Concrete Drives Continual Demand

Concrete Placing Equipment: Application Requirements Dictate Market TrendsM.K. Prabhakar, Associate Editor

Concrete Transportation Equipment: On the MoveM.K. Prabhakar, Associate Editor

Concrete Product Machinery: Shaping Up WellM.K. Prabhakar, Associate Editor

Growing Opportunity in India's Construction Sector Draws Global Attention

The Indian Construction Chemicals Market:Building a Sustainable FutureM.K. Prabhakar, Associate Editor

Concrete: Fibres

Equipment: Concrete Batching Plants

Equipment: Concrete Placement

Equipment: Concrete Transportation

Concrete: Block Machinery

Events

Construction Chemicals: Industry Analysis

M.K. Prabhakar, Associate Editor

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Communication Feature

Contents

Advertisers Index / Classification

AAC and CLC Blocks-Foam making machine

AAC Blocks

Adhesives and Sealants

Automation Doors

Block Making Machinery

Building Materials and Products

Cement Manufacturers

Concrete Cutting Machine

Concrete Cutting Specialist

Concrete Densifier-Lithium Silicate

Concrete Grinding and Polishing Solutions

Concrete Machinery and Equipment

Concrete Products

Concrete testing Equipment

Construction Chemicals

Iyantra 215

Brickwell 281

Citadel Eco-Build Solutions 219

Methra Industries 165

Dow Corning India Pvt. Ltd. 75

Wacker Chemie India Pvt Ltd. 65

Gandhi Automations Pvt Ltd 13

Columbia Pakona Engineering Pvt. Ltd. 255

Hess Concrete Machinery India Pvt. Ltd. Gatefold

Sri Parijatha Machinery Works Pvt. Ltd. 265

Fabtech Sterling Building Technologies Pvt. Ltd. Cover Page

Zuari Cement Italcementi Group 131

Prime Technologies 121

Abcon Tech & Build Aids Pvt Ltd 289

Waltar Enterprises 175

Surie Polex 197

Ajax Fiori Engineering (I) Pvt. Ltd. 163

Everest Equipments Private Limited 283

Greaves Cotton Limited 253

Linhoff India Pvt. Ltd 347

Kyb-Conmat Pvt. Ltd 222

Schwing Stetter (India) Pvt Ltd 35

Toshniwal Systems & Instruments Pvt. Ltd. 273

Universal Construction Machinery & Equipment Ltd. 161 / 235

Buildtech India Corporation 277

Technical & Scientific Sales(TASS) 179

Amit Trading Corporation 105

BASF The Chemical Company 51

Cera-Chem Private Limited 63

Chembond Chemicals Limited Back Inner

Cico Technologies Limited 71

Contech Chemicals 289

Fosroc Chemicals (India) Pvt. Limited 85

Max Civi Chem Private Limited 59

MC-Bauchemie (India) Pvt. Ltd. 53

Multichem Group Flap Cover

S.N. Engitech Developers Pvt. Ltd. 199

Structwel Designers & consultants Pvt. Ltd. 177

Sanrachana Structural Strengthening Pvt. Ltd. 117

Gandhi Automations Pvt Ltd 13

Leister Technologies India Pvt. Ltd. 151

Asons Enterprise 239

Atul fastners 269

Hindalco Everlast Aluminium Roofing & Structurals 22 / 23

Robo Silicon Pvt Ltd 125

Metecno India Front inner

Tekla India Pvt Ltd 141

Bekaert Industries Pvt. Ltd. 29

Kasturi Metal Composites(P) Ltd 227

Stewols India (P) Ltd. 279

Igloo tiles 231

Techny chemie 10

H & K Rolling Mill Engineers Pvt. Ltd. 17 / 19

BASF The Chemical Company 51

Cera-Chem Private Limited 63

Chembond Chemicals Limited Back Inner

Cico Technologies Limited 71

Contech Chemicals 289

Fosroc Chemicals (India) Pvt. Limited 85

Max Civi Chem Private Limited 59

MC-Bauchemie (India) Pvt. Ltd. 53

Multichem Group Flap cover

Nuha Construction Solutions 289

Penetron India Pvt. Ltd. 91

Perma Construction Aids Pvt. Ltd. 279

Pidilite Industries Ltd. Front Inner

Polyflex 31

Razon Engineering Company Private Limited 101

Sika India Pvt Ltd 97

The Supreme Industries 45

Trade Winds 283

JK Cement Ltd. 21

Texsa India Ltd. 95

Repair and Retrofitting

Rolling Shutters

Roofing

Roofing Fastners

Roofing Sheets

Sand Making Machine

Sandwich Panels

Software for Concrete

Steel Fiber Reinforced Concrete

Tiles-Thermal Insulation

TMT Technology Supplier/Waterproofing

Wall Putty

Waterproofing Membrane

Nuha Construction Solutions 289

Penetron India Pvt. Ltd. 91

Perma Construction Aids Pvt. Ltd. 279

Pidilite Industries Ltd Front Inner

Polyflex 31

Razon Engineering Company Private Limited 101

Sika India Pvt Ltd 97

The Supreme Industries 45

Action Construction Equipment Ltd. 259

Wirtgen India 33

Marini India (Fayat Group) 145

Case New Holland Construction Equipment (India) Pvt. Ltd. 39

ConMechAuto Consultants India Pvt Ltd 261

Ammann Apollo 36 / 37

Rotho Robert Thomas 277

Kerneos Aluminate Technologies 15

Excon -2013 291

ICI-CPWD 293

UBM Concrete Show -2014 294

Construction Chemicals Regional Conference (C3R) 295

ICI-IWC (2013) 296 / 297

Big 5 Construct India -2013 298

SEWC -2013 299

World of Concrete 300

Reylon Facility 272

Bekaert Industries Pvt. Ltd. 29

Cipy Polyurethanes Pvt. Ltd. 79

JB Associates 183

Kasturi Metal Composites(P) Ltd 227

Neocrete Technologies Pvt. Ltd. 135

Recron 3S 243

Silicone Concepts Intl. Pvt. Ltd. 83

STA Concrete Flooring Solutions 159 / 193

Stewols India (P) Ltd. 279

Keltech Energies Ltd. 137

Maco Corporation (I) Pvt. Ltd. 281

Interarch Building Systems 9

MNF Metals and Forming Pvt. Ltd. 123

A.N.Prakash Construction Project Managemnet Consultants Pvt.Ltd 41

R & M International 185

Construction Machinery and Equipment

Curing Compounds

Dry Mix Mortar

Events & Conferences

Facility Services

Flooring

Light Weight Concrete

Material Handling Equipment

PEB

Project Management Consultancy

Repair & Restoration/Rehabilation-Service Providers

18 ·The Masterbuilder - July 2013 www.masterbuilder.co.in

56 The Masterbuilder - July 2013 • www.masterbuilder.co.in

New Concrete Technology in Construction Aggregates - Their Role in Concrete and the ‘Green Agenda’Christopher Andrew ClearBritish Ready-Mixed Concrete Association

Sustainability

For a concrete technologist the term ‘Green Agenda’ is a rather more difficult concept to understand than either ‘Aggregates’ or ‘Concrete’. Having said this it is the authors opinion there is a degree of ambiguity associated with all three terms. This paper sets out what ought to be considered important to consider with respect to sustainable construction and a brief consideration of how aggregates and concrete support such an initiative.

The Green Agenda

A problem with the green agenda is that it is often not presented as a list of important environmental, social and economic considerations but as a banner around which those affected by some form of change to their environment gather to resist the change.

However, with respect to building construction the position is changing as a comprehensive technical description of the green agenda is being set out across a set of International and European Standards. EN 15643-1[1] entitled ‘Sustainability of construction works — Sustainability assessment of buildings — Part 1: General framework’. This European Standard forms part of a suite of the European Standards that set out a system for the sustainability assessment of buildings using a life cycle approach. The sustainability assessment is quantified to assess the environmental, social and economic performance of buildings using quantitative and qualitative indicators but benchmarks or levels of performance are not set. It should be noted that currently it is only buildings that are covered by the EN 15643-1 framework, but there are proposals for standards to cover the sustainability assessment of civil engineering and other construction works.

In carrying out assessments, scenarios and a functional equivalent are determined at the building level. This level means that the descriptive model of the building with the major technical and functional requirements has to be been defined Figure 1: EN 15643 concept of sustainability assessment of buildings

Sustainability of construction works is such an important topic that it should not be discussed by rather imprecise and emotive terms such as ‘The Green Agenda’. Therefore this paper deals with the sustainability assessment of building with respect to their environmental, social and economic performance before briefly covering how concrete and its aggregate constituent play a part in sustainable construction.

in the client’s brief or in the regulations as illustrated in Figure 1, abstracted from EN 15643-1.

EN 15643 Part 1 is the general framework, and Parts 2[2], 3[3] and 4[4] the individual frameworks for environmental, social and economic performance respectively. All four parts contain a version of Figure 1, and in their draft stages the ‘Environmental’ box was coloured green, the ‘Social’ box coloured red and the ‘Economic’ box coloured blue. The use of colour in what might otherwise be considered rather dull documents such as standards is to be applauded, however it is unfortunate that in this case it reinforces the misconception that ‘The Green Agenda’ is concerned only with Environmental issues, whereas it should encompass Social and Economic issues as well.

In Figure 1 the boxes within the thick red dashed box are those covered by the European Technical Committee entitled ‘Sustainability of construction works’, CEN/TC 350. As the EN 15643 standards state when describing the integrated building performance at the concept level, the environmental, social and economic performance are one part, and the technical and functional performance are another. Both parts are intrinsically related to each other as shown in Figure 2,

www.masterbuilder.co.in • The Masterbuilder - July 2013 57

where the CEN/TC 350 work programme is represented by the boxes shaded grey as set out in EN 15643-1.

Sustainability assessments can be undertaken for the whole building, for parts of the building which can be used separately, or for elements of the building. This is in a complete contrast to some simplified interpretations of the ‘Green Agenda’ which may largely restrict consideration to just the environmental impacts of the various building products from which the building is made.

15643-4 for Economic performance. The indicators included in EN 15643 Parts 2 to 4 are summarized in Table 1. It should be noted that the items in the middle column come from a pre-Standard dated 2010, and the final lists within EN 15643 parts 2 and 4 changed significantly from the equivalent listings in their pre-Standards.

Although the evaluation of technical and functional performance is beyond the scope of CEN/TC350 standards, the technical and functional characteristics are considered by reference to the functional equivalent. The functional equivalent of a building or an assembled system (part of works) shall include, but is not limited to, information on:

- Building type (e.g. office, factory, school)- Pattern of use (e.g. occupancy)- Relevant technical and functional requirements (e.g.

regulatory framework and client’s specific requirements);- required service life

There is an explicit requirement that the assessments shall be established on the basis of specified realistic scenarios that represent the whole building life cycle.

To ensure the results of the assessment of environmental, social and economic performance can be readily understood they need to be presented in a systematic and transparent method, where Figure 3 shows the groups of information required.

The standard EN 15643-1 makes it explicitly clear that tin the assessment report the results shall be expressed with all the defined indicators set out in EN 15643-2 for Environmental performance, EN 15643-3 for Social performance and EN

Figure 3: The organisation of the result of the assessment in accordance with EN 15643-1 life-cycle stages and the information groups

Table 1 is not a complete list as and EN 15643-2 includes a list of further environmental indicators, and these are listed in Table 2.

The detailed calculation method for the environmental performance of buildings using the environmental indicators set out in EN 15643-2, is set out in EN 159785 entitled ‘Sustainability of construction works — Assessment of the environmental performance of buildings — Calculation method.’ This standard sets out a methodology on how to calculate and most importantly how to display the modular information for the different stages of building assessment. Figure 4 shows the general format for the output where for each stage A1, A2, A3, B1… there will be a number required for each of the environmental impacts as listed in column 1 of Table 1, and those in Table 2 where appropriate. The natural extension is to list the social and economic impacts as well but the Standards for these have yet to be drafted. It is important to remember that the eventual Table of results is for the whole building for its whole life, so a scenario has to be developed to cover the construction, use and end of life stages.

On the specific challenge with respect to Global Warming Potential impact (measured by CO2 equivalent) the concrete options to meet the demand for low-energy housing specific research was commissioned and reported6. Figure 5 shows an example of the results where the cumulative CO2 emissions, embodied and operational, have been modelled for a 60 year period for both lightweight (timber) and medium weight (block work walls) simple semi-detached house. Due to predicted increase in summer temperatures resulting from climate change, the lightweight house needs air conditioning by 2021, whereas for a medium weight home it would not be needed until 2041.

Sustainability

Figure 2: Work programme of European Technical Committee CEN/TC 350 Sustainability of construction works


A typical concrete and masonry house with a medium level of thermal mass has about 4% more embodied CO2 than a lightweight house, but this could be offset in as little as 11 years due to energy savings provided by its thermal mass.

The simple semi-detached house is the sort of scenario

required for the EN 15978 analysis, but for a complete EN 15978 analysis then a further 20 environmental impacts should be considered in addition to the single Global Warming Potential shown in Figure 5. The EN 15978 standard is clear that the communication of results may be limited to a selection of indicators, and so the use of a single indicator is within scope and the standards notes that the graphical representation of results like that shown in Figure 5 may be useful to com-municate results. It is also evident that trying to carry out an assessment to encompass all or even just a majority of the indicators may be an overly cumbersome exercise and so a client may wish to restrict the range to those considered of greatest importance.

What the EN 15643 series of standards clear state is that the ‘The results of possible further aggregation of these indicators shall be clearly separated from the assessment results as additional information’. This is an important facet as it is evident that where environmental indicators are normalised and/or weighted it is possible to present the assessment results in a way that appears to represent the just the bias

Environmental indicators EN 15643-2: 2011 Annex B.1 (informative)

Categories for social aspects prEN 15643: 2010

Economic Indicators EN 15643: 2012Annex C (informative)

environmental impacts (LCIA impact categories)- abiotic depletion potential(elements and fossil fuels)- acidification of land and water resources- destruction of the stratospheric ozone layer- eutrophication — formation of ground-levelozone- global warming potentialresource use (environmental aspects)- use of non-renewable primary energy excluding non-renewable primary energy resources usedas raw materials- use of renewable primary energy excluding renewable primary energy resources used as raw materials- use of non-renewable primary energy resources used as raw materials- use of renewable primary energy resources used as raw materials- use of secondary materials- use of non-renewable secondary fuels- use of renewable secondary fuels- use of freshwater resourcesother environmental information (environmen-tal aspects)- components for reuse- materials for recycling- materials for energy recovery- non-hazardous waste to disposal- hazardous waste to disposal (other than radioac-tive waste)- radioactive waste to disposal- exported energy

Health and comfort- Thermal performance- Humidity- Quality of water for use in buildings- Indoor air quality- Acoustic performance- Visual comfortAccessibility- Accessibility for people with specific needsMaintenance- Maintenance requirementSafety/security- Resistance to climate change- Fire safety- Security against intruders and vandalism- Security against interruptions of utility supplyLoadings on the neighbourhood- Noise- Emissions- Glare- Shock/vibrations

Cost- economic performance expressed in cost termsover the life-cycle Financial value- economic performance expressed in terms offinancial value over the lifecycle

Table 1. Indicators listed in EN 15643-2 for Environmental performance, prEN 15643-3 for Social performance and EN 15643-4 for Economic performance

Further Environmental indicators EN 15643-2: 2011 Annex B.2 (informative)

environmental impacts (LCIA impact categories)— biodiversity— ecotoxicity— human toxicity— land use changeresource use (environmental aspects)— use of non-renewable resources other than primary energy— use of renewable resources other than primary energyother environmental information (environmental aspects)— use of environmentally sustainably managed materials (grouped per ma-terial type e.g. PEFC, FSC, responsibly sourced materials BS 8902:2009)— use of environmentally sustainably managed fuels (grouped per fuel type e.g. Sustainability criteria for bio-fuels ISO 13065)

Table 2. Further indicators listed in EN 15643-2 for Environmental performance

Sustainability


of those responsible for the manipulation system. Figure 6 below shows the results of a particular ‘E-points’ system as applied to a tonne of coarse aggregate and a tonne of CEM I (Portland cement), where the higher the E-points the less environmentally desirable is the material.

Unfortunately, many clients want to be seen to be complying with a green agenda and as all the tools and expertise to carry out a comprehensive sustainability assessment are not readily available the temptation is to go down a simplified route. Such a simplified route, like those based on an E-points type system, can give arguable results. For example we know that the cement represents a global warming potential equivalent to about 850 kg of CO2e/tonne[7], and that such emissions considering world production of over 3,300,000,000 tonnes of cement a year is generally considered to have a negative impact on the environment. According to the E-point system the extraction and use of one tonne of coarse aggregate in concrete is about as half environmentally damaging as the production and use of one tonne of cement, or say two tonnes of coarse aggregate is equivalent to the environmental impact of a tonne of cement. This cannot be sensible as a tonne of cement will initially put about 850 kg of CO2e into the atmosphere whilst even if the two tonnes of coarse aggregate is used to make concrete it is still available. That is the aggregate is still aggregate through the service life of the building but also be an aggregate in a subsequent life where as part of recycled concrete it is likely to be used either as an unbound or bound aggregate application.

The important conclusion is that the term ‘Green agenda’ is the wrong term and its use should be deprecated. Indeed ‘Green Agenda’ is often regarded as synonymous with ‘Environmentally friendly’ and even this is the wrong term. The correct term to use is ‘Sustainability of construction works’ and until the necessary standards are developed for civil engineering works the more appropriate term is ‘Sustainability

Figure 4: Display of modular information for the different stages of building assessment in accordance with EN 15978: 2011

Figure 5: Cumulative Global Warming Potential, CO2 emissions (air-conditioned)

assessment of buildings’ in accordance with the EN 15643 series and supporting standards.

Aggregate

There is a suspicion that as far as a large number of Engineers are concerned aggregates for concrete are natural aggregates, and should be in accordance with BS 882: 1992[9] entitled ‘Specification for aggregates from natural sources for concrete’. To these engineers it may be a bit of a shock that BS 882 was withdrawn on 1 June 2004, when it was replaced by EN 12620 ‘Aggregates for Concrete’ which was first published in 2002 but was subsequently amended in 2008[10]. To the European Standard aggregate is ‘granular material used in construction. Aggregate may be natural, manufactured or re-cycled’ where it is evident that the widest range of usable materials is included. This type of standard supports sustainable construction as it means that any suitable materials can be used to make concrete, but it is up to the specifier to ensure that the correct aggregate and concrete properties are specified to suit a particular application.

In the past it may have been considered reasonable to specify a high performance natural gravel or crushed rock for even the lowest grades or performance concrete classes, and where environmentally, socially and economically a high performance natural aggregate can be justified then that is perfectly acceptable. However, it should be equally acceptable that where there is a technically sound but lower grade of aggregate available to make a particular concrete then this will normally be the more sustainable option. If nothing else it would help preserve the resources of higher quality aggregates for more demanding concrete performances or other applications.

Sometimes it may be possible to use Recycled Concrete Aggregate (RCA) or Recycled Aggregate (RA) providing the source and quality meet all the necessary limits with respect

Sustainability


Sustainability

to particular impurities and other materials that may be deleterious to concrete performance. Recycled aggregates are produced in conformity with the ‘Quality Protocol for aggregates produced from inert waste’[11] where the material produced must also conform to EN 12620. To help ensure that aggregate for concrete is specified in the most appropriate manner guidance is available as BSI Published Document PD 6682-1[12] entitled ‘Aggregates – Part 1: Aggregates for concrete – Guidance on the use of BS EN 12620.’

Concrete

The European Concrete Standard, EN 206-1[13], and its complementary UK counterpart, BS 8500[14], are increasingly being made more flexible, and hence supportive of sustainable construction. The general concept will be that general suitability is established at European level for; natural normal weight aggregates, heavy-weight aggregates and air-cooled blast furnace slag conforming to EN 12620 as well as lightweight aggregates conforming to EN 13055[15] and aggregates reclaimed by the concrete producer. In addition at National level recycled and manufactured aggregates with an identified history of use may be used as aggregate for concrete if the suitability is established. In summary sustainable construction is supported by widening the range of materials deemed suitable for use in concrete, but the specifier may have to give some additional consideration to ensure that the materials specified are suitable for a particular application.

Conclusions

Sustainability is an issue of increasing importance to everyone, and sustainable construction is of particular importance to all those involved in the construction industry. Sustainable construction is not well served by the use of ill-defined jingoistic terms like ‘green-agenda’ and it use of this term should be deprecated by the construction industry and its suppliers. Concrete and aggregates have a role to play in sustainable

construction, but the sustainability assessment needs to be in environmental, social and economic terms at the building level.

Acknowledgements

The author is grateful for all the input received from members of the British Ready-Mixed Concrete Association as well his colleagues and members of the Mineral Products Association.

References

1 British Standards Institution. BS EN 15643-1:2010 Sustainability of construction works. Sustainability assessment of buildings. Part 1 General framework. BSI, London, 25 pp.

2 British Standards Institution. BS EN 15643-2:2011 Sustainability of construction works. Sustainability assessment of buildings. Part 2 Framework for the assessment of environmental performance. BSI, London, 32 pp.

3 British Standards Institution. BS EN 15643-3:2012 Sustainability of construction works. Sustainability assessment of buildings. Part 3 Framework for the assessment of social performance. BSI, London, 25 pp.

4 British Standards Institution. BS EN 15643-4:2012 Sustainability of construction works. Sustainability assessment of buildings. Part 4 Framework for the assessment of economic performance. BSI, London, 36 pp.

5 British Standards Institution. BS EN 15978: 2011. Sustainability of construction works — Assessment of the environmental performance of buildings — Calculation method. BSI, London, 60 pp.

6 HACKER, J. ET AL. Embodied and operational carbon dioxide emissions from housing: A case study on the effect of thermal mass and climate change. Energy and Buildings 40. 2008.

7 Mineral Products Association, Cementitious Slag Makers Association, UK Quality Ash Association. Fact Sheet 18. Embodied CO2e of UK cement, additions and cementitious material. Camberley, Undated but published 2012. 8 pp.

8 World Business Council for Sustainable Development. Cement Sustainability Initiative Progress Report, June 2012. Geneva. 6 pp.

9 British Standards Institution. BS 882: 1992. Specification for aggregates from natural sources for concrete. BSI, London, 14 pp.

10 British Standards Institution. BS EN 12620: 2002+A1:2008. Aggregates for concrete. BSI, London. 56 pp.

11 Waste Resources Action Plan. Quality protocol for the production of aggregates from inert waste, September 2005. 12 pp.

12 British Standards Institution. PD 6682-1: 2009 Aggregates – Part 1: Aggregates for concrete – Guidance on the use of BS EN 12620. BSI London, 28 pp.

13 British Standards Institution. EN 206-1:2000. Concrete — Part 1: Specification, performance, production and conformity. March 2006. BSI, London. 69 pp.

14 British Standards Institution. BS 8500: 2006+A1: 2012. Concrete — Complementary British Standard to BS EN 206-1 – Part 1: Method for specifying and guidance for the specifier. Part 2 Specification for constituent materials and concrete. BSI, London. 59 & 44 pp.

15 British Standards Institution. BS EN 13055-1: 2002. Lightweight aggregates — Part 1: Lightweight aggregates for concrete mortar and grout BSI, London. 40 pp.

Figure 6: E-points for coarse aggregate and CEM I showing contribution for mineral resource extraction and climate change (Global Warming Potential)


Chemical Admixtures for Concrete: An Overview

Use of Chemical Admixtures in Concrete

One of the most important and critical ingredient of modern concrete is the chemical admixture. The introduction of admixtures have changed the way we can work with the cement concrete. As the Power’s Equation states, Vp=100w/c-36.15 . The Porosity of concrete is inversely proportional to W/B ratio. The first and most important role of chemical admixtures is to reduce the water : binder ratio.

Specifications and Test Methods

IS 9103, ASTM C-494, ASTM C 1017, BS 5075 (withdrawn) and BS EN 934 part 1, 2, 6 with supporting test methods are commonly used, for classifying and testing chemical admixtures for concrete. There are different classes of admixtures, based on their effect on concrete properties. ASTM standard C-494 classifies admixtures in 8 types from A to G and S. Apart from reducing the mixing water, chemical admixtures have various functions such as slump retention, set retardation or acceleration, strength acceleration and more. Each type needs to meet different criteria, specified in test specifications. Moreover, there are other types like air entraining admixtures as per ASTM C-260, integral water proofing admixtures meeting the water impermeability requirements as per DIN 1048, anti- wash out admixtures, corrosion inhibitors, shrinkage reducing admixtures, foaming agents, corrosion inhibiting admixtures and shotcrete accelerators.

Polycarboxylate Ether Based Admixtures

Chemical admixtures are based on various chemistries. Water reduction greatly depends on the type of chemistry a formulator uses, to design the admixture. As we know, the best water reduction is achieved using PCE based admixtures. This is a result of very efficient dispersion of the binder which PCEs offer. The effectiveness of PCEs is more evident when the W/B ratio goes below 0.35. The versatile chemistry of PCE polymers, ensures their use in almost every Admixtures mix design. SCC has been mostly associated with PCE based admixtures. Along with the excellent water reduction, they also

Deepak Kanitkar GM – Technology and Business DevelopmentChembond Chemicals Limited (Construction Chemicals Division)

Admixtures

produce flowing concrete without segregation. In a properly designed SCC mix using a PCE based admixture, there is either no need for VMAs or their usage could be limited. Now a days modified PCE based admixtures are available which also offer good rheology control. One of the challenges in earlier PCEs was the increased stickiness of the concrete mix, now there are some molecules which offer excellent reduction in stickiness. This is often necessary while designing very high strength mixes which have a tight water cement ratio and high fines. There are other issues associated with PCEs such as, rapid loss of slump, dosage and temperature sensitivity, sensitivity to moisture content. Due to such factors concrete producers generally avoid the use of PCE based admixtures in lower strength mix designs. Modified PCEs are useful in meeting the requirements of lower strength mixes. Often, due to use of manufactured sand or stone dust, there are issues with regard to slump retention or segregation, these need to be addressed by smart formulations of blended PCE molecules.

Shrinkage Reducing Admixtures

As described earlier apart from the traditional use as water reducing agents and slump retainers, admixtures are nowadays used for more functional roles. Shrinkage reducing admixtures is an example. These are very effective in reducing cracks caused due to drying and autogenous shrinkage, also known as selfdesiccation. They act on very fine capillaries with diameters between 2.5 to 50 nm in diameter, by reducing surface tension within pore solution. This helps in preventing collapse of capillary walls, thereby reducing cracking. Their use in heavy duty industrial floors, enables increased the spans and reduction in the requirement for number of joints. SRAs are mainly based on Ethylene and Propylene Glycol derivatives. While using SRAs, one must consider their effect on final compressive strength and air entrainment. In general, at same W/B ratio, 10-15% reduction in final compressive strengths, have been observed. It is imperative that by adjusting the W/B ratio, one can actually maintain the desired compressive strengths.


The shrinkage could to some extent be determined from an equation derived by Tomita et al

- (sh) = 390 + 2.89W - 21.77X-4.758E (x10-6)

- Where W, X and E corresponds to unit contents 3 (kg/m ) of water, SRA and expansive additives respectively.

- This suggests that SRA are more than 4 times as effective than the expansive additives or the effective dosage of SRAs is only 20-25% that of the expansive additives.

Corrosion Inhibiting Admixtures

Preventing or rather delaying corrosion of steel in concrete is achieved by at least three methods. Use of corrosion inhibiting concrete admixtures, CP systems and protective covering or penetrative treatments. The most important factors are chloride ingress and carbonation, apart from sulphates and other corrosive contaminants. CIAs are mainly based on inorganic chemicals such calcium Nitrate / Nitrite or bipolar acting agents based on amino alcohols and Amino polycarboxylates. Various products are available in the market and are in most of the cases equally effective. The mechanisms are different, so as the dosages.

Internal Curing Agents

Use of Internal curing agents for concrete are also gathering momentum. They act by providing additional moisture in concrete for a more effective hydration of the cement and reduced selfdesiccation. Internal curing means the introduction of a curing agent into concrete to provides this additional moisture. Two major methods currently available.

- Use of saturated porous lightweight aggregate (LWA) in order to supply an internal source of water.

- Super-absorbent polymer (SAP) particles can absorb a very large quantity of water during concrete mixing and form large inclusions containing free water, thus preventing self-desiccation during cement hydration.

For optimum performance, the internal curing agent should possess high water absorption capacity and high water desorption rates.

Admixtures For Sprayed Concrete or Shotcrete

Most tunneling work will never be complete without the use of gunniting or the sprayed concrete. Spraying concrete is a very sensitive application. The nozzlemens’ job will not be easy without the use of an accelerating admixture. Typically, quick setting is as critical as controlling the reheology of sprayed concrete. Accelerators combine both these requirement, thus reducing the rebound to a great extent and achieving quick setting.

There are more and more applications which could be discussed. Admixtures for making pervious concrete, colorants for decorative concrete, admixtures used for stopping the rejected concrete from setting for a period as long as 24hrs are available.

Concrete Mix Proportioning – Science and Art What is mix design?

Concrete proportioning or designing of a concrete mix, is an art more than science. Although it involves a lot of statistics and material science, more depends on the actual feel of the concrete during the mixing and placing stages.

In general concrete mix has 4 main constituents viz. cementitious materials, aggregates, water and chemical admixtures.

More precisely, concrete consists of coarse aggregates which are bound by a mortar made of a paste of hydraulic binders mixed with fine aggregates.

Mix proportioning involves, physical formulation of these ingredients to achieve certain properties. Main criteria remains that of mechanical properties but without achieving the fresh concrete performance, mechanical properties can never really be attained.

In order to achieve the desired level of workability and a cohesiveness concrete mix, the gradation as well as granularity of both coarse and fine aggregates, cement content, water to binder ratio and use of a suitable chemical admixture, play a vital role.

While doing this, apart from getting the desired performance, one has to also remember economy and properties of available resources, including water.

Ingredients Properties

Cement Strength Grade Type Fineness Alkalinity - -

Fine Agregates Zone or Gradation Shape Absorption Reactivity Density Mineralogy

Coarse AggregatesMaximum Size and

GradationShape / Crushing

methodAbsorption Reactivity Density Mineralogy

Admixtures Type Chemistry Dosage - - -

Pozolanic additionsMineralogy / Chemical

CompositionParticle size Dosage - - -

Ingredients and their important properties

Admixtures


Air in concrete is also a critical factor. Controlling the amount of air in the mix can to a certain extent be controlled during mix designing stage. There are two classes of concrete viz normal (Non Air Entrained) concrete and air entrained concrete. The amount of allowed air varies considerably in both types. Air entrainment is achieved through the use of air entraining admixtures. More recently, it is also known that Self Compacting Concrete (SCC) requires a different approach, than these two types.

Essentials of a Mix Design

Following are typically the most important factors, which need to be taken into account while deciding the mix proportions.

- Characteristic Strength- Air content- Workability requirements- Climatic conditions / exposure conditions.- Handling conditions / Automation- Durability- Economy- Aesthetics and appearance

We will take a look on, how some of the above factors help in arriving at a desired mix proportion.

1. Characteristic strength: Compressive strength of a mix is typically the most important mechanical characteristic. Once we know the strength requirements, it becomes simple to select the grade of cement, water content or W/B ratio as well as fine aggregate content. It is well known that reducing the W/B ratio, directly increases the compressive strength.

2. Air Content: Air in concrete is not always desirable. In non airentrained concrete it is desirable to have an air content less than 1 % more than the control mix or a maximum 2.5% by its volume. A little air helps in getting better slump and finishing of the concrete. As the air content increases, it starts decreasing the strength and simultaneously increase the permeability. We need to take this air into consideration, as the density and yield depend on the percentage of air in the mix.

3. Workability: It is the ability of fresh concrete which allows the placement and finishing at a particular point after mixing. The slump as it is called is measured using a slump cone at mixing point and at the point and time of placement. Depending on the actual application and placement technique, the value of slump can vary between almost zero for a pavement grade roller compacted concrete to a flowable consistency for the SCC. Apart from this point measure, the slump or workability may be required to be retained, up to a certain period of time. So, the required workability at a given time needs to be considered, while designing a mix.

4. Climatic conditions and Exposure: Whether the concreting is taking place in winter or summer matters a lot. It is also imperative to understand the variations during the day and nights. Under water placement, needs anti washout admixtures and set accelerators.

We have to also consider the exposure conditions. If there is a chance of moderate to heavy exposure to sulphates, one needs to consider the use of sulphate resistant cement or pozolanic additions such as fumed silica. in case of chloride based environment, use of corrosion inhibitors based on calcium nitrite or bipolar types, may be necessary. Industrial environment, may necessitate use of hydrophobic agents / corrosion inhibitor, along with low permeability mixes.

5. Handling conditions / Automation: Whether the concrete is hand placed, pumped, roller compacted or placed in moulds at a precast facility, will mean a different approach to the mix design. Each method has different requirement of consistency, viscosity and slump retention.

6. Durability: Air entrained concrete in Freeze – Thaw zones, use of pozolans in marine environment, low permeability concrete in areas with heavy rain fall, use of internal curing agents or Shrinkage Reducing Admixtures in concretes prone to selfdesiccation, are some of the examples, how the durability needs to be accounted at mix design stage.

7. Economy: The skill of a concrete technologist is in designing the best mix which meets optimum performance requirements within the allowable economy / cost. Low cost resources such as sand and gravel, need to be taken from the closest possible quarry. Major contribution to the cost of these materials, comes from freight and handling. Adjustment in gradation of these aggregates and minimizing the cement content will automatically result, in good economy of the mix.

8. Aesthetics and Appearance: Whether you require a plain finish or a texture, coloured concrete or a stamped finish, all such factors need to be considered during finalizing the mix design.

Effect of ingredients on Properties of Concrete Cement

Grade of cement decides maximum achievable compressive strength of a mix. Type of cement generally decides exposure conditions as well as water demand and compatibility with admixtures.

Fineness of cement is important to decide water demand, initial workability, setting time as well as retention of slump. Alkalinity is responsible for both short term and long term performance parameters.

IS Codes : 12269, 456, 8112ASTM codes : C-150

Admixtures


Fine Aggregates

Fineness modulus and gradation of sand plays an important part in deciding the workability and cohesiveness of concrete mixes. Shape determines the flow, segregation and bleed characteristics. Absorption value determines the amount of water needed for adjustment after deciding free water content.

Reactivity if any mainly with alkali needs to be mitigated using proper means such as pozolans, Lithium Silicates etc. Density which is generally determined by mineralogy affects yield.

Coarse Aggregates

Size of aggregates decides the amount of mortar required to coat all coarse aggregates. It also decides the maximum thickness up to which the concrete can be cast. Heavy density aggregates are generally used in radiation shielding concrete. The shape and size of aggregates is also critical in achieving desired workability and cohesiveness. Coarse aggregates can have round, angular, or irregular shape. Rounded aggregates because of lower surface area will have lowest water demand and also have lowest mortar/paste requirement. Hence they will result in most economical mixes for concrete grades up to M35. However, for concrete grades of M40 and above the possibility of bond failure will tilt the balance in favour of angular aggregate with more surface area. Flaky and elongated coarse aggregate particles not only increase the water demand but also increase the tendency of segregation. Flakiness and elongation also reduce the flexural strength of concrete.

IS Codes : IS 383ASTM Codes : C 33

Admixtures

Admixtures play a very important role in today’s concrete industry. Faced with a lot of environment, space and resource constraints and add to that the challenges of the modern day structural requirements and design diversities, no concrete today can really be considered without the use of chemical admixtures. We are having a detailed discussion on this topic at the end of this article.

IS Codes : IS 9103ASTM Codes : C-494, C-1017, C-260

Pozolanic Materials

Use of pozolanic materials has now become common. Both separate additions as well as blended cements are available. Microsilica (Fumed Silica), Fly ash, Slag, Meta Kaolin, Risk Husk Ash are predominant. Various specifications and test methods are used to determine the quality of these materials.

IS Codes : IS 3812

ASTM codes : C-1240, C-311, C-441, C-618, C-989

There are different approaches to concrete mix design. We are showing a typical example of M-30 grade concrete, designed as per guidelines from IS 10262 : 2009. Here we have used KEM SUPLAST 128 UT which SNF based Concrete Super plasticizer conforming to ASTM C-494 type G and IS 9103.

Mix Design Calculations as per IS 10262:2009 Stipulation for Proportioning

Grade of concrete 30Type of Cement OPCType of mineral admixture Fly AshMaximum nominal Sizeof aggregate 20mmMinimum cement content 320Kg/cumMaximum water- cement ratio 0.45Type of exposure SevereMethod of concrete placing PumpingDegree of Quality Control Very GoodType of aggregate CrushedMaximum cement(OPC) content 450Kg/cumChemical admixture type SuperplasticizersChemical Admixture brand KEM SUPLAST128 UT

Test Data for Material

A) Cement

a) Cement brand used U/T OPCb) Specific Gravity of Cement 3.15

B) Fly Ash

a) Fly ash brand used Dirkb) Specific Gravity of Fly Ash 2.15

C) Water

a) Source Local

Sieve Size % Passing 50 50 % Passing Specification as per IS 383

10 mm 20 mm 10 mm 20 mm Combined 100% 10 mm 20 mm

40 mm 100.00 100.00 50.00 50.00 100.00 100 100

20 mm 100.00 89.60 50.00 44.80 94.80 100 85-100

10 mm 85.40 0.25 42.70 0.13 42.83 85-100 0-20

4.75 mm 0.00 0.00 0.00 0.00 0.00 0-20 0-5

2.36 mm 0.00 0.00 0.00 0.00 0.00 0-5 -

Admixtures


D) Admixture

a) Type & brand Kemsuplast 128UTb) Specific Gravity of Admixture 1.24c) % of dosage 1.2%

E) Coarse Aggregate

a)Source Local Sourceb) Specific Gravity (SSD) 20mm 2.80c) Specific Gravity (SSD) 10mm 2.78d) Combine Sp. Gr. 2.79e) Water absorption % 20mm 0.0% 10mm 0.0%f) Free surface moisture 20mm 0.0% 10mm 0 . 0 %

Coarse aggregate Sieve Analysis

F) Fine Aggregate

River Sand

a) Source of River sand Local Sourceb) Specific Gravity of River Sand 2.65c) Water absorption 0.0%d) Free moisture 0.0%

Crushed Sand

a) Source of Crushed sand Local Sourceb) Specific Gravity ofCrushed Sand 2.68c) Water absorption 0.0%d) Free moisture 0.0%e) Combine Sp gr of total F.A. 2.7

Combination of Fine Aggregate Fractions

Design Calculation:

Target Mean Strength 30 + 1.65 X 5fck+1.65X SD 38.25

Selection of Water/Cement ratio

From Table 5 of IS 456, maximum water-cement ratio for M30 =0.45

On trial experience base adopted water- cement ratio for this mix design is 0.420.42 < 0.45 Hence OK

Selection of Water content

From table 5 of IS 456, maximum water contentfor 20m – 50 mm slumpfor 20 mm MSA =186Kg/cumEstimated water contentfor 120 mm slump =208Kg/cumSince use of superplastisizerwill reduce 15 – 20% of water & above on trial with presentsuperplastisizer water reduction achieved is 17% So thewater content I =173Kg/cum

Calculation of Cement content in design mix

Water-cement ratio =0.42water content =173Kg/cumTotal Cementitious content will be =410Kg/cumAs per IS 456 minimumcementitious content is =320Kg/cum410Kg/cum > 320Kg/cum Hence OKTotal cement (OPC) content =308Kg/cumFly Ash content % by weightof cement @ 25% =103Kg/cum

IS Sieve % Passing Individual Combination Combine % Lower Limit Upper Limit

R.Sand C.Sand 30 70

4.75 100.00 92.70 30.00 64.89 94.89 90 100

2.36 83.60 72.20 25.08 50.54 75.62 60 95

1.18 52.40 51.60 13.14 36.12 49.26 30 70

0.6 35.60 40.20 4.68 28.14 32.81 15 34

0.3 22.90 20.40 1.07 14.28 15.35 5 20

0.15 12.80 6.40 0.14 4.48 4.61 0 10

Pan 0.00 0.00 0.00 0.00 0 0 0

F.M= 3.17 Zone= ZoneI

I.S. Sieve Size inC.A % F.A% Total %

passingSpecifications

56.00% 44.00% Min Max

40 56.0 44 100.00 100 100

20 53.1 44 97.09 85 100

10 24.0 44 67.98

4.75 0.0 41.8 41.75 30 50

2.36 33.3 33.27

1.18 21.7 21.68

0.6 14.4 14.44 10 35

0.3 6.8 6.75

0.15 2.0 2.03 0 6

Admixtures


Proportion of volume of coarse aggregate& fine aggregates

From table 3 of IS 10262(2009), Vol of CA corresponding to 20 mm size aggregate and FA (Zone I) for W/C ratio of 0.50 = 0.60.

In present case W/C ratio is 0.42 Therefore Vol of CA is required to be increase to decrease the FA content as the W/C ratio is lower by 0.08 the proportion of vol of CA is increased by 0.02 So corrected vol of CA=0.62

For Pumping Mix this value should be reduced by 10%

So Total vol of CA =62 X 0.9 =0.56cumTotal Vol of FA = 1.00 – 0.56 = 0.44 cum

Percentage of Fine aggregate and Coarse aggregate are arrived after conducting sieve analysis and combined grading as follows

Percentage of C.A. Tototal aggregates =56%Combination ratio of 20mm:10mm =50:50 %Percentage of F.A. Tototal aggregates =44%Combination ratio ofR.Sand : C.Sand =30:70 %

Combination Of All Aggregates Fractions

Fine Aggregates % =44%Coarse Aggregates % =56%

Mix Calculations

a) Volume of Concrete = 1cumb) Volume of Cement = Cement Qty./Sp.Gr x 1 /1000 = 0.098cumc)Volume of Fly ash = Fly Ash Qty./Sp.Gr x

1 /1000 =0.047cumd) Volume of Admixture @ = Admixture Qty./Sp.Gr x 1/1000 =0.004cume) Volume of Water =0.173f) Total cementitious material+ water + admixture = 0.322cumg) Total volume of allaggregates =1-f =0.678cumh) Total quantity ofcoarse agg. =g x Vol of CA x Sp. Gr. Of C.A x 1000 =1062Kg/cumQuantity of 20mm =531Kg/cumQuantity of 10mm =531Kg/cumI) Total quantity of Fine agg. =g x Vol of FA x sp. Gr. Of F.A x 1000 =793Kg/cumQuantity of River Sand =238Kg/cumQuantity of Crushed Sand =555Kg/cum

MaterialMaterial Dry wt Moist. W.A. SSD

& Source kg/M3 % % kg/M3

Cemmenti-tious

OPC 53 308 308

Fly Ash 103 103

Aggregates

CA2: 20 MM 531 0 0 531

CA1: 12 MM 531 0 0 531

FA2: R.Sand 243 0 0 243

FA1: C.Sand 567 0 0 567

Admixture Kem Suplast 128 UT 4.9 4.9

Water Local source 173 173

Theoretical plastic density - = 2460 2460

Water / Cement Ration = 0.42 0.42

Constituent Materials

Concrete Temperature 27oC Mix Cohesive

Workability

Initial Slump - Collapse

Slump After 60 Min - Collapse

Slump After 120 Min - 210 mm



Test Results

Age Weight Density Load Strength N/MM2

Days KG KG/Cum KN Strength Average %

1 8.350 2474 250 11 11 37

7 8.300 2459 500 2222

74

7 8.400 2489 510 23 76

28 8.380 2483 890 40

40

132

28 8.390 2486 885 39 131

28 8.370 2480 892 40 132

Compressive Strength Data

Note

The above Mix is for guidelines purpose only, all the aggregates are considered as in SSD condition & their Sp. Gravity are considered according to materials available in Mumbai region. For practical purpose it is always advised to conduct the confirmatory trials at site conditions. Our representatives will be available for further advise if necessary.

Admixtures


Admixtures for Tall Structures

Mumbai saw large-scale rural-urban migration in the 21st century. Mumbai accommodates 12.5 million people, and is the largest metropolis by population in India, followed by Delhi with 11 million inhabitants. Witnessing the fastest rate of urbanization in the world, as per 2011 census, Delhi’s population rose by 4.1%, Mumbai’s by 3.1% and Kolkata’s by 2% as per 2011 census compared to 2001 census. Estimated population, at the current rate of growth, by year 2015 of Mumbai stands at 25 million, Delhi and Kolkata at 16 million each, Bangalore and Hyderabad at 10 million.

Hence with urbanization increasing at such a fast pace, especially so in a developing economy such as India, puts more pressure on available land. This answers why construction companies in India are looking vertical space, kick-starting the trend of high-rise buildings dotting the country scape.

Builders and architects are concentrating on building skyscrapers primarily because they are convenient. It allows them to create a lot of real estate on a relatively small ground area. Until the 1990’s, the world of tall buildings was dominated by the North American continent and the United States in particular. In 1990, 80% of the world’s tallest 100 buildings were located in North America. Two decades later, these numbers have fallen to 35%. This trend is the result of a dramatic and continuing increase in tall building construction in both Asia and the Middle East. The construction of the Petronas Twin Towers in Kuala Lumpur built to a height of 452 m, Taipei 101 standing at 508 m and now the Burj Khalifa in Dubai at 828 m which stands 773 metres higher, or 15 times taller, than the world’s first “tall building”, the Home Insurance Building completed in Chicago in 1885, are a testimony to this fact.

Bruno D’souzaRegional Business Segment Manager, Admixtures, Asia Pacific, BASF

Admixtures

There’s been an increasing trend toward construction of structural concrete super-tall buildings for several good reasons discussed in the following section of the paper. Whilst using such concretes, one needs to pay greater attention not only to aspects such as the mix design but also to its performance with respect to handling, pumping, placing, finishing and curing.

Structural Material

For many years, steel was the material of choice for the tall building, a fact displayed in the first 12 world’s tallest buildings. Currently, composite, concrete and mixed-structure construction is much more prevalent in tall structures. Only 24% of the world’s current 100 tallest structures contain a purely steel structural system, down from 57% in 1990.

Reinforced concrete provides twice the dampening effect compared to steel, reducing forces on super-tall buildings due to wind and the cost of construction. Concrete buildings are quiet and structural concrete is naturally fire resistant. Modern formwork systems for horizontal and vertical castings greatly increase productivity and improvements in concrete pumping equipment and techniques, make easy and fast delivery of concrete possible.

Advancements in concrete technology because of newly developed materials such as chemical admixtures have assisted in improving the properties of concrete, including strength and modulus of elasticity (E) making high-rise construction more attractive. Self-Compacting Concrete is increasing in use too, mainly due to the utilization of admixtures classified as Viscosity Modifying Agents (VMA) and Viscosity Enhancing Agents (VEA) and the availability of more economical fines or fillers.

Tall buildings or skyscrapers are constructed for several reasons. One reason being the creation of a status symbol, for example, the Burj Khalifa, the world’s tallest building which was constructed to symbolize Dubai as a world city. Other more obvious reasons for the presence of such structures would be part and parcel in the construction of central business districts in cities and also urbanization of populations. Urbanization is taking place at a faster rate in India than most places in the world. Urbanization in India was mainly caused after independence, due to adoption of a mixed system of economy by the country which gave rise to the development of private sector. Population residing in urban areas in India, according to 1901 census was 11.4%. This count increased to 28.53% according to 2001 census, and crossing 30% as per 2011 census, standing at 31.16%. According to a survey by UN State of the World Population report in 2007, by 2030, 40.76% of country’s population is expected to reside in urban areas. As per World Bank, India, along with China, Indonesia, Nigeria and the United States, will lead the world’s urban population surge by 2050.


Chemical Admixtures

Super-tall construction requires that the concrete to be economical and yet deliver high performance characteristics. High performance characteristics being properties such as high strength, low water/binder ratio, flowable for extended periods at ambient temperatures fluctuations from say 10 deg. C to 50 deg. C and most of all, pumpable to heights in excess of 600 m.

To meet these challenges posed by various stakeholders including engineers, contractors and ready mixed concrete producers for the construction of such super-tall structures, BASF embarked on an intensive R&D project. The aim of the project was obviously to develop & deliver products capable of meeting the demands and needs necessary for the manufacture of such high performance concrete even in harsh environments. The R& D work resulted in an innovative concept being developed. This new concept is termed Total Performance Control (TPC). TPC concept ensures that the stakeholders achieve a concrete that is of the same high quality as originally specified; starting from production at the batching plant, to the delivery and application into place and followed by its hardening process. TPC is the state-of-the art technology that provides improved short and long term performances of concrete by controlling the two distinct features essential for high-quality concrete: extended workability and low water/binder ratio. These features are the key to the success of such an admixture system.

The key element of the Total Performance Control is the Glenium SKY superplasticiser. Glenium SKY is an innovative superplasticiser based on second-generation polycarboxylate ether (PCE) polymers. It is derived directly from the TCP concept and is specially engineered to provide high water reduction and slump retention for ready mix concrete simultaneously. As compared with other PCE superplasticisers, it is possible to obtain a high quality concrete mix with accelerated strength development and extended workability without delayed setting characteristics. Glenium SKY is made using nanotechnology. A nanometer is a millionth of a millimeter – the dimension of molecules and polymeric chains. In-house expertise in nanotechnology allows BASF to control the chemical and physical behavior of polymers and their interactions with cement by augmenting chain length, side chain length and density, and electrical charges as well as free functional groups. For the first time, nanotechnology allows local requirements and conditions to be better met.

Self-Compacting Concrete produced using the TPC concept provides a concrete mix with exceptional placing characteristics, accelerated cement hydration for high early strength development and quality concrete. The addition of a Viscosity Modifying Agent (VMA) to Self-Compacting Concrete with Glenium SKY superplasticiser enhances the

robustness of the mix by providing excellent cohesion and anti-segregation properties. Robustness of a mix is desirable, especially when such mixes are expected to perform under high pump pressures and also flow for long horizontal distances and remain stable when dropped from heights into structural members such as columns and beams.

Mechanism of Action

The dispersion effect of superplasticisers is based on the adsorption of molecules on cement particles, imparting a negative charge that causes electrostatic repulsion and steric hindrance between them and, therefore dispersion. The hydration, and particularly the ettringite formation, works against the superplasticiser. Already adsorbed molecules are covered by the ettringite lawn, thus are ineffective. The particular configuration of the Glenium SKY molecules allows its delayed adsorption onto the cement particles and disperses them efficiently over a long period of time.

The molecular structure is essential for the early development of strength. With superplasticisers based on conventional polycarboxylate ether, the molecules cover the entire surface of the cement grain and build a barrier against contact with water. Therefore, the hydration process takes place slowly. The Glenium SKY molecules, on the other hand, leave sufficient room on the cement surface to allow a rapid hydration reaction, resulting in high early strength development.

A schematic representation of the mechanism of action of a normal PCE superplasticiser versus Glenium SKY is given below.

Application

Burj Dubai, called Burj Khalifa since its opening, is the tallest building in the world by a large margin. Concrete admixtures from BASF have made a substantial contribution to its construction. The world’s tallest construction, reaching up 828 metres into the Dubai sky with the number of floors reported at 189, was opened on 4 January, 2010. Both, the second largest construction, the 610-metre Canton TV Tower in Guangzhou, China, and the second largest “house”, the 509-metre Taipei Financial Center in Taiwan, pale in comparison with these

Admixtures


80MPa – which is roughly equal to the pressure the total weight of a small car would exert on a big toe. The high strength also provides the concrete structure with a long service life and ensures the sustainable usage of the building.

Conclusion

Concrete with specialized & tailored admixture systems utilizing nanotechnology & unique polymer science can deliver the following benefits to stakeholders of tall structures:

- Ensuring a constant high-quality concrete even at a low water/binder ratio

- Providing concrete with extended workability at high temperatures, without delayed strength development

- Guaranteeing a concrete that meets the original specification from the fresh to the hardened stage

- Offering a single, versatile admixture system for many types of applications and conditions

Refrences

- Abalos, I., & Herreros, J. (2003). Tower and Office: From Modernist Theory to Contemporary Practice. Cambridge, MA:MIT Press.

- In L.S. Beedle & D. Rice (Eds.), Proceedings of the 4th World Congress of the Council on Tall Buildings and Urban Habitat: Tall Buildings- 2000 and Beyond. Chicago, IL: Council on Tall Buildings and Urban Habitat, 3-12.

- Ali, M.M. (2001). Art of the Skyscraper: The Genius of Fazlur Khan. New York: Rizzoli.

- Ali, M.M. (2005). The skyscraper: epitome of human aspirations. In Proceedings of the 7th World Congress of the Council on Tall Buildings and Urban Habitat: Renewing the Urban Landscape [CD-ROM]. Chicago, IL: Council on Tall Buildings and Urban Habitat.

- Ali, M.M., & Armstrong, P.J. (Eds). (1995). Architecture of Tall Buildings. Council on Tall Buildings and Urban Habitat Monograph. New York: McGraw-Hill.

- Taranath, B. (1998). Steel, Concrete, & Composite Design of Tall Buildings. New York: McGraw-Hill.

figures. Concrete admixtures from BASF’s GLENIUM® SKY product line brand have made a substantial contribution to this new world height record.

Construction went on for five years in extreme climatic conditions. Burj Khalifa put great demands not only on the 2,400 construction workers deployed around the clock. The staggering heights, a demanding architectural design, and the climate required exceptional performance from concrete admixtures as well: The hyperplasticiser of the GLENIUM SKY brand enabled the concrete to be pumped up to a height exceeding 600 metres without interruption. In all, about 180,000 cubic metres of concrete with GLENIUM SKY were used for the structure’s foundation plate as well as the superstructure construction.

The climatic conditions posed a further challenge during the construction of the world’s tallest building: Temperatures in Dubai not only fluctuate between 10 °C in winter and 50 °C in summer, but there are also temperature differences between day and night of up to 20 °C. GLENIUM SKY and the BASF team’s expertise counteracted these conditions and ensured a consistently high concrete quality.

In addition, the staggering height of the Burj Khalifa and the resulting weight of the construction cause enormous pressure within the concrete structure. Through the use of GLENIUM SKY, the concrete acquires a compressive strength of up to

Admixtures


Performances and Benefits of Concrete Admixtures in Concrete Durability

As larger and larger amounts of concrete will be required worldwide in the future, the need for developing sustainable construction solutions has been pro-

gressively growing in our building industry, depending on each country sensitivity and awareness. Concrete is in fact is one of the most durable –and cost effective – construction material in use. Its durability is characterized by the capacity to keep over time its values in use for which it was designed (structural purpose, safety, comfort of users), while maintaining maintenance costs as low as possible.

In order to guarantee this durability, it is still common in India, as well as in other countries, to see project designers prescribing a fixed concrete mix design, based on ultimate strengths associated with a workability target. However, based on the experience collected over the last decade, we can state that durability should be rather understood by considering a larger set of criteria: physico-chemical characteristics of the concrete, production and placement process, as well as a perfect knowledge of the exposure conditions.

In a new approach considering durability more in terms of performance aspects, to take better into account all the criteria mentioned above, we will see that the role of concrete admixtures becomes of great importance, and we will detail how they can directly or indirectly improve some durability factors.

Approaching Concrete Durability Performance Wise

Traditionally, many project designers are used to specifying the durability of a concrete in terms of ultimate strengths, together with a pre-established mix design fixing typically a maximum Water / Cement ratio, a minimum cement content, even sometimes a definite dose of admixture. This “prescriptive” approach is able to provide a certain durability for the concrete, as it will ensure a reasonable compacity. Nevertheless, the mechanical strength of a concrete does not determine alone the durability. As an example, two concrete with same strengths can show very different behaviours in terms of resistance against sulphate attacks or alkali-silica reaction.

Philippe OrtegaVice Technical Director, CHRYSO SAS, France

Admixtures

Even though the compacity would be comparable, the way the pores are distributed in the concrete, their size, will influence for example the diffusion properties of aggressive chemicals, and lead finally to different durability performances.

The experience collected in the last years allows now to better optimize concrete mix designs in terms of durability while taking into account the real performance of the concrete. This “performance” approach requires knowing precisely the conditions playing a role on the durability:

The concreting process: it is essential to respect the best practices in terms of concreting process. From mixing to curing, the way the concrete is handled will affect its quality, and, as a consequence, its durability. For example, a concrete can be properly designed for the exposure conditions, but may finally lack durability if not properly cured, because of a non optimal cement hydration. Another key point is the rheological behaviour of the concrete: workability, viscosity, cohesiveness, will provide more or less ability for placement and consolidation, which is also essential for keeping concrete structures durable.

The physico-chemical concrete properties: beside the classical particles packing density that determines the compacity of the concrete, the chemical nature of the raw materials (aggregates, cement, SCM materials…) plays an important role: it is well documented for example that slag and pozzolanic materials have a positive impact on durability, since they are able to reduce the corrosion rate by increasing the resistivity of the concrete. Furthermore, the potential reactivity between con-stituents has also to be considered (aggregates sensitive to ASR reaction, e.g.).

The exposure conditions: taking precisely into account the environment in which the concrete structure will be located and the risk of attacks it will be exposed to during its service life is necessary to optimize the durability performance of the concrete. In Europe, a first step towards this performance approach has been achieved, by introducing in the EN 206-1 (European Concrete standard) some specific exposure classes


related to corrosion induced by chlorides, by carbonation, freeze-thaw attacks and chemical. Using these classes, the project designer will be able to prescribe a minimum ultimate strength, cement content and maximum water to cement ratio, as well as the type and nature of some constituents.

In some countries like France, this performance approach has been further investigated. Some specific durability indicators have been defined, in association with operating modes (like oxygen permeability, chloride diffusion, speed and depth of carbonation). Work is still in progress to link such parameters with the real life span of concrete structures. As an example, we can mention the construction of the Euro-tunnel, designed for 120 years: water permeability tests had been performed, which confirmed the low porosity (7 % to 8 %) of the concrete, allowing to select the final mix design parameters, especially the water to cement ratio of 0,32 (Rc 28D between 66 to 69 MPa).

Approaching this way the durability performance wise, many benefits can be be found using concrete admixtures, since they will help to optimize some physico-chemical characteristics of the concrete and even the concreting conditions.

How Admixtures can Improve Durability Related to the Concreting Conditions:

One condition for guaranteeing durable concrete is a proper cohesiveness. It starts with the mixing process: according to the Indian standard IS 456, concrete should be mixed until

there is a uniform distribution of the materials and the mass is uniform in color and consistency. Mixing aid admixtures can help in that regard, as they will reduce efficiently the mixing time, especially in case of concrete containing high volumes of cementitious materials, particularly hard to defloculate.

Also the placing of the concrete around the reinforcement, fully compacted, has got a very important influence on the final durability. It is therefore essential for the workmanship to respect the best practices of pouring and vibrating concrete. In addition, with the most advanced admixture technologies, like PCP based superplasticizers, it is possible to optimize this process, by producing highly flowable concrete that will require less effort to place. Some of these products allow also keeping a better control over other rheological parameters of the mix, like thixotropy: this reversible thickening force can lead to an early concrete stiffness, especially at low water cement ratios, disturbing consequently the placement process. Admixture suppliers have now developed specific molecules to decrease this effect.

Not only the easiness of placing , but also its regularity is another key point for a consistent and guaranteed durability: the new generation superplaticizers allow to make for example Self Compacting Concrete, that do not require any more vibration. As the quality of compaction does not depend any more on the human factor, it is possible to expect a more regular placement quality, favourable for the durability. The regularity is also obtained by improving the robustness of the

Class Des-ignation Description of the environment Informative examples where exposure classes may occur

4 Corrosion induced by chlorides from sea water

Where concrete containing reinforcement or other embedded metal is subject to contact with chlorides from sea water or air carying salt originating from sea water, the exposure shall be classified as follows:

XS1 Exposed to airbome salt but not in direct contact with sea water

structure near to or on the coast

XS2 Permanently submerged Parts of marine structures

XS3 Tidal, splash and spray zones Parts of marine structures

5 Freeze/thaw attack with or without de-icing agents

Where concrete is exposed to significant attack by freeze/thaw cycles whilst wet, the exposure shall be classified as follows:

XF1 Moderate water saturation, without deicing agent Vertical concrete surface exposed to rain and freezing

XF2 Moderate water saturation, with deicing agent Vertical concrete surface of road structures exposed to freezing and airbome de-icing agents

XF3 High water saturation, without de-icing agent Horizontal concrete surface exposed to rain and freezing

XF4 High water saturation, with de-icing agent or sea water

Road and bridge decks exposed to de-icing agents:Concrete surface exposed to direct spray containing de-icing agents and freezing splash zones of marine structures exposed to freezing

6 Chemical attack

XA1 slightly aggressive chemical environment Concrete exposed to natural soil and ground water according to table 2

XA2 Moderately aggressive chemical environment concrete exposed to natural soil and ground water according to table 2

XA3 Highly aggressive chemical environment concrete exposed to natural soil and ground water according to table 2

Extract from the EN 206-1: Description of some exposure classes

Admixtures


fresh concrete to different variables: first cement chemistry variations (the soluble alkali content especially) can lead to non regular water and superplasticizer demand, thus impacting workability. Today, products like alkyl phosphonate based superplasticizers have been designed to cover a large range of cement chemistries, and are proved to be much less “cement specific” than the regular PCPs. Then, other variations come also often in India from the aggregates particle size distribution: variable fine contents, in crushed sands particularly, increase sometimes a lot the risk of bleeding and segregation, and therefore the durability. For that, admixture suppliers have now in their offer some efficient molecules, known as Viscosity Modifying Agents, able to smooth up these variations and ensure a more regular concrete cohesiveness.

Finally, another variation that may affect the placing process is the workability loss over time. Whereas PNS based old generation superplasticizers were hardly able to retain workability more than one hour, some PCP based superplasticizers, along with other newly developed technologies called “slump extenders” allow concrete open times up to 3 hours or more. Also, being

able to maintain constantly over time a cohesive concrete -from the time of mixing until the time of placing - avoids producing a concrete with a too high initial workability at the edge of segregation, as observed sometimes in India, which minimizes the risk of getting a non homogeneous concrete at the pouring point.

After placing, curing is the next step which is essential for avoiding a too quick drying of the concrete surface, leading to drying shrinkage responsible for cracks. Most admixture manufacturers offer already different types of curing agents, mineral or water based, that protect efficiently the concrete surface, and therefore improve durability. Considering the relatively high outside temperatures in India, it makes fully sense to develop the use of such products compared to traditional solutions - typically water spraying - less efficient as they require many more repeated applications. To some extend, it is also possible to minor the shrinkage phenomena by using two other chemical types of solutions, known as Shrinkage Compensating Agents (SCA) and Shrinkage Reducing Agents (SRA). These liquid admixtures are able to help reduce the capillary forces while concrete is drying and their efficiency is excellent. As much as the SCA use remains confidential and some time difficult to control, the SRA technology is widely available and robust.

How Admixtures can Improve Durability Related to the Physico-Chemical Characteristics

Getting a microstructure as dense as possible is a well known key driver for improving concrete durability. Beside waterproofing agents dosed in the concrete mass, able to close to a certain extent the capillaries, and decrease consequently the risk of penetration of aggressive substances, the super-plasticizers play a major role, as the most recent ones – the new generation PCPs - are able to offer almost an unlimited reduction of the water to cement ratio. The consequence is the possibility, together with mix design optimization, to increase regular concrete compressive strengths up to 120 MPa and Ultra High Strength concrete (UHPC) up to 200 MPa in commercial applications. The consecutive reduction of the porosity allows today the designers to build concrete infrastructures up to sometimes 300 years.

However, reducing too drastically the water to cement ratio will increase significantly the solid volume fraction in the paste, and consequently the viscosity, as it is predicted by the universal Krieger and Dougherty law. In India, due to high proportions of cementitious materials necessary to reach safely the strengths requirements, this practical limit is often exceeded, and people start to complain about too much “stickiness”. Some further admixture developments – modified PCPs and alkyl phosponate based superplasticizers – can now help about this issue, by reaching an acceptable compromise between rheological and strengths properties.

From a bleeding mix…to a cohesive SCC mix using a Viscosity Modifying Admixture

Admixtures


, by increasing the freeze-thaw resistance of concrete. Also, the so-called “corrosion inhibitors”, based mostly on calcium nitrite or more recently on amine technologies, will directly fight against the aggressive agents - chlorides in that case. These admixtures will chemically neutralize the chlorides, slowing down or stopping their penetration in the concrete. Some simplified tools (e.g. the American software Life-365) can be actually used to simulate their benefit on the life span of the structure.

Unlike these admixtures specifically designed for durability purposes, it is interesting to bring up the case of products that found a secondary function as durability enhancers. Surface retarders is one typical example: normally designed for aesthetic purpose (it allows to produce exposed aggregate concrete, by removing the outer ‘skin’ of cement paste to uncover decorative coarse aggregate), they can be advantageously used to ensure the continuity of reinforcement at construction joints between two concrete pours, which is a guarantee for a good durability of the whole structure. Instead of hammering the upper surface of the first concrete to make it rough, as we can still see on some Indian job sites, this surface can be treated instead with a surface retarder, providing a more regular surface attack. The aggregates are then perfectly exposed for receiving the new concrete, ensuring a good bound at the joint.

Conclusion

Following a mix design prescription that fixes ultimate strengths has been often considered as a sufficient guaranty for qualifying concrete durability. However, many other factors related to concrete properties (more than just the porosity), environmental conditions and production process, have a direct influence on the durability. As we saw, concrete admixtures producers can play a significant role in all these fields, as they have now well understood the new potential of their product on the durability aspect. Their contribution however - especially with the newest superplasticizer technologies - will be only complete if the project designers accept to approach durability more performance wise, a trend seen more and more in European concrete regulations.

Impact of the W / C ratio decrease thanks to a PCP based superplasticizer on water permeability

The admixture chemistry can be also very helpful to support the use of some particular concrete raw materials, like the Supplementary Cementitious Materials (SCM). Within blended cements or added separately as a partial replacement for the Portland clinker, SCM materials contribute to a significant reduction of the carbon footprint, improving directly the durability of the concrete, sustainability wise. Furthermore, many of them, like fly ash or slag are able to improve some physico-chemical characteristics of the concrete related to durability: one the physical side, they improve further the compacity of concrete mixes thanks to their fineness, on the chemical side they can fight against aggressive exposure conditions, like corrosion induced by chlorides, sulphate disorders or alkali silica reactions (ASR). Also, the risk of thermal cracks, due to young concrete heat generation wave and its volume variation (expansion / contraction), especially in massive concrete works, can be minimized using SCM materials. Well documented, the synergy between such materials and conventional retarding admixtures has been proven years ago, to help to maintain temperature gradients into concrete at acceptable levels. However, putting high amounts of SCM materials can affect some concrete properties: often a decrease of the workability window, due to less soluble alkalis in the total binder, or a delay of setting times and early strengths. Admixture suppliers are now well aware of these issues, and they adapt their chemistry, especially the PCP based superplasticizers, to be fully compatible with SCM materials, by speeding up the strengthening process or by adding some specific slump extender admixtures to retain properly the workability.

It is worth mentioning that some admixtures were specifically developed in order to improve specific durability performances related to physical or chemical concrete characteristics: air entraining agents for example, that avoid cracks due to the volume expansion of the frozen water left in the concrete pores. Such admixtures, providing a closely and uniformly distributed network of air bubbles, work therefore as real durability enhancers

Author Bio

Philippe Ortega is a graduate in Chemistry from Lille university (France). He joined CHRYSO in 1998 in order to start the first developments on Self Compacting Concrete in the German Precast and Readymix industry. From 2002 up to now, he works in the CHRYSO concrete business unit, in charge of the technical development and support in many countries (India, USA, European countries…), where he has been given the opportunity to participate to the achievement of big job sites (ex. Nancy creek Tunnel, USA, Millennium Bridge, Poland…).

Admixtures


Positive Waterproofing from Negative Side

With the increase in infrastructural projects, use of underground constructions is tremendously increased. Scarcity of land at ground level necessitates underground

metros in crowded cities. Distances of rail and roads are lowered by tunelling through mountains and under the bed of water. Deeper basements are part of construction activities civil and geotechnical engineers are facing challenges firstly to gather geotechnical data and later to select construction techniques to complete projects on schedule. In spite of technological advancements in geological strata mappings, the real problems are faced while actual construction is going on. Since the exact nature of strata cannot be predetermined, civil engineers have to resolve problems at site. Rehabilitation of underground structures pose major problems as usually only the internal face is accessible and the problem is always on the other side. Movement of water is one of the major impediment to underground construction. Leakages can be uncontrollable and have to be repaired immediately. If leakages are not arrested early, the consequential damages to structures can be time consuming and costly to repair throwing the deadlines out of gear and endangers the very stability of structures. Fig 1 shows different types of Cracks in Tunnel linings. Fig 2 shows critical water ingress.

Any repair based on predetermined material is bound to be ineffective. Sound principles of civil engineering is the

Samir SurlakerManaging Director, MC-Bauchemie

Waterproofing

base to all repairs, rehabilitation or retrofitting. So positive waterproofing is possible from negative side.

In many waterproofing projects, damp-proofing systems are specified and waterproofing is expected of them. There is a major difference between waterproofing and damp-proofing. As per ACI Committee 515 report, waterproofing and damp-proofing is defined differently. Waterproofing is a treatment of a surface or structures to prevent the passage of water under hydrostatic pressure. Dampproofing is a treatment of surface or structure to resist the passage of water in the absence of hydrostatic pressure. While designing the waterproofing system, actual service conditions are to be borne in mind. It should be decided at this stage whether damp-proofing is required or waterproofing is desired.

Positive side waterproofing systems are applied on the same side as the applied hydrostatic pressure and negative side waterproofing systems are placed on the side opposite the applied hydrostatic pressure. Previously negative side waterproofing was only adopted when access to positive side was unavailable as in case of common boundary lines. Negative side waterproofing systems were also used as repair strategy because the positive side was inaccessible and the back fill or protective layers had to be removed. There is an increasing trend in the country to specify negative side waterproofing.

If the negative side waterproofing systems are unavoidable, extreme care should be exercised during the construction process. First and foremost, the concrete cast should be of very low permeability without honeycombing, cracks, crevices or any other surface defects. Efficient drainage systems are mandatory. Provision of water stops is also mandatory, as this is first line of defense in case of negative waterproofing. Fig 3 shows schematically the positioning of positive side and negative side waterproofing.

It should be clearly borne in mind that negative side waterproofing accords no protection to concrete if soils contain corrosive chemicals. Extreme precautions should be taken at joints


between walls and floors and careful detailing is required to maintain integrity and water tightness. In every case it seems that negative waterproofings are chosen over positive waterproofing on account of ease of application and this trend should be discontinued in favour of positive waterproofing. Negative side waterproofings are also inefficient against high vapour permeability and therefore are ruled out when the interiors are to be used for humidity sensitive applications.

Waterproofing as a System

Waterproofing treatments cannot be carried out by application of single material on new concrete or existing treatments. There cannot be a single material that is right for every structure. Most of the failures in the waterproofing treatments are on account of this misconception. The only way to ensure reliable treatment is by considering the waterproofing treatment as a system. System for waterproofing can be defined as combination of materials, preparation of specifications, application techniques designed by taking into considerations the requirement of the client or home owner, which would provide efficient, reliable and long term protection to concrete structures with minimum maintenance costs. Waterproofing should never begin with a specific material in mind. The properties of material are to be stated and then the material is to be selected as per the merit.

In case of underground structures the design has to consider the soil mechanics aspect like variable strata, unsuspected

Crack Injection

• Structural injection for dry cracks• Structural injection for damp cracks• Sealing of cracks and cavities for waterproofing• Sealing against pressurized water• Injections for imparting stability in Masonry structures• Frictional Sealing of loose masonry• External sealing using curtain injection technology• Grid injection for dampness

Table 1 : Different types of Crack Injection

Fig 2: Water Ingress which can be stopped

Fig 3 : Comparison of Positive Side & Negative Side Waterproofing

streams etc. which may contribute to the hydrostatic pressure. Efficient drainage systems should be designed to avoid build up of hydrostatic pressure due to stagnation of water against waterproofed surfaces. If necessary, the path of water should be re-routed with proper grading to control surface runoffs. Well connected network of perimeter drains connected to vertical drainage system is the key to achieving perfect waterproofing of subgrade structures. Fig. 4 shows basement waterproofing system designed with efficient drainage for a very high water pressure. Geotextile filter fabrics are normally designed for a narrow range of permeability. The filters are efficient guard against silt collection in aggregate drainage system as they avoid clogging of drainage conduits. Difficult ground water conditions can be overcome by providing well designed geodrainage systems which would eventually lower the hydrostatic pressure on waterproofing barrier system. If a sump is provided the pumping system should be proportionate to collection of water.

Reasons for Crack Injection

The other major avenue for water entry into structures are cracks. Cracks threaten the durability of buildings. Moisture ingress restricts the usage of buildings and may ultimately lead to adverse economic effects inclusive of collapse of

Waterproofing


effectively at a later date. The material used for such cases must display excellent flow properties. Fig 7 shows usage of different materials for different Injection types.

Guideline for Material Selection

The selection of material for injection in the crack largely depends upon the investigation and primarily the following factors:

- Pattern of cracks- Width of the crack- Movements in the crack faces- Due to temperature variations- Due to dynamic loadings- Moisture in the crack- Dirt in the crack

Available Options

Duromer Resins: Duromer Resins have been used for many

Fig 4: Subgrade waterproofing system with drainage

Fig 5 : Grid Injection Principle

structures. The combined effect of moisture ingress and frost may even aggravate existing damage. Due to its increased volume, ice causes more serious crack damage and large-area frost heave. Cracks can endanger the stability of entire buildings. Apart from the structural aspects, the entire appearance of a building – an important criterion for the valuation and assessment of buildings – is considerably impaired by cracks and deterioration that is brought about by water entry and chemicals ingress. Table 1 shows various reasons for Injection. Usually three different systems are specialised applications to solve critical problems:

Grid Injection: Water penetration is often caused by bad concrete compaction, honey combing or defective seals. This is rectified by using grid injection systems a special technology developed from the standard injection process. Fig 5 shows a Grid Injection Principle and Fig. 6 shows the process.

Waterbar Injection: A special injection application is waterbar injection where waterbars are used to seal joints in moving structures against pressurised water. However, the concrete often proves to be a weak point in the area of the waterbar because of inadequate compaction. Waterstop Injections effectively clear off this defect and render structure waterproof.

Reinjectable hose Injection: Other critical areas with regard to the watertightness of a building or structure are expansion joints that are not sealed with waterbars. Inserting injection pipes provides the possibility of sealing expansion joints

Various Injection Material Bases

PU/Hydrostructural Elastomers• Water foam and bubble• Adhere extremely well to wet substrates• Good Performance with waterCementitious• Water is a parpt of the curing process• Must have water to build healing crystals• Good Adhesion Good Performance with waterEP / Duromers• Foam and bubble with water• Bubbles contain air, reduced strength• Poor adhesion• Bad Performance with waterAcrylic Gel/ Hydrostructure• Consists of 80% water• Adhesion is reasonable• movement is excellent• Good Performance with water

Table 2 : Different Injection Materials

Fig 6 : Process of Grid Injection

Waterproofing


years for the rigid injection of cracks in dry structural sections. Epoxy resins of very low viscosity can be injected into the finest of cracks. This guarantees a strong permanent bridging of both sides of the crack, thus restoring bearing capacity for the designed loads planned. This process can also be carried out where the structure is also subject to vibration. Duromer resins of varying viscosity can be used for injection and impregnation depending on the width of the crack.

Elastomer Resins: If rigid injection material is used the cracks become visible after injection or new cracks develop adjacent to old cracking. High quality elastomer resins are a solution for avoidance of such occurences. Elastomer resins can be designed with distinct pore structures. An homogenous closed cell structure is formed which enables safe sealing. The integrated compression and decompression reserves in the cell structure absorb the expansion and the contraction in the crack. The effectiveness and permanency of the seal is guaranteed by the ability to expand and contract in sympathy with the crack movement while maintaining the tenacious bond to crack surfaces.

Hydrostructural Resins: Hydrostructural resins cure to form elastic membrane and impervious seals when encountered with water. This property is very useful when the external of building is not accessible like in buried structures and when the external waterproofing envelope is damaged. By drilling

How to choose the right product - Job Conditions

These are problems which injection can solveSealing Coming from• movement cracks• stationary cracks• construction joints• expansion joints• bad construction• All needs flexible fillingStrengthening Cracks coming from• mechanical damages damages caused by earth movements• general wear and tear• Bad construction• All needs rigid filling

Table 3: Selection of Materials

Fig 7: Crack V/s. Materials to be used

Fig 8: Showing Hydrostructure Resin PU Foam

through the structure to the interface between existing water-proofing membrane or protection board, hydrostructure gels can be injected to recreate the seal. Hysdrostructure resins can be designed for high chemical and mechanical resistances. Fig 8 shows water reactive Pu Foam. They guarantee permanent elasticity, tenacious and isotopic bonding and unique skin effect. Fig. 7 different material for different injection types.

The Process of Injection

After completion of diagnosis and selection of materials for injection the work of injection passes through following stages.

- Preparation of the crack- Location of points (nipples) for injection- Fixing of injection points- Surface sealing of cracks- Injection of resin proper- Removal of nipples and plugging- Removal of sealing material- Final surface treatment after injection resin/grout hardens

Components and Machinery

Proprietary materials and machineries are available for treating the cracks by injection system. The materials are mostly synthetic resin based or cement based. The synthetic resins are usually two component materials based on epoxies and polyurethane. The cement based materials are invariably modified with polymers to impart flowability, non shrinking characteristics, better bonding etc.

The first step is selection of packers. It is important that packers are in the form of metal or plastic tubes. They should be able to be connected to the injection nozzle, so that the pressure if any should not be lost. Thereafter, it should be possible to tie or seal the packers, so that the resin is not lost and they should be removable to enable the surface smoothening. Normally there are two types of packers.

Waterproofing


Waterproofing

The packers, which can be stuck to the surface of the structure along the line of crack, if the surface is even and packers which are to be introduced in the structure after boring and inclined at 450C to the crack plane. The spacing of injection points depend upon the width of crack as well as the porosity of concrete. However, as a thumb rule, in case of adhesion packer, the spacing should be about 50% of concrete cross section. Adhesion packers should not be used for high pressure injections exceeding 60 bars. Fig. 9 shows both types of packers.

Packer or Packer Systems are the link between the structure and face of the crack and the injection nozzle. Packers must be of adequate size to gurarantee the flow of injection resin to the desired place with or without being displaced or debonded due to injection pressures or rebounds. The critical selection depends upon the access to crack, quality of surface, surface condition as well as pressures used in injection process. Table 5 shows Technical data about packers.

There are Normally Three Types of Packers used Under General Conditions

Adhesion Packers: for the injection of dry cracks, cavities and

substrates with epoxy and polyurethane resins where surface conditions are suitable.

Drill or Bore Injection Packers: for the injection of dry, moist and water bearing (pressurized and Non-presurrized) cracks, cavities and substrates with epoxy, Acrylic and Polyurethane resins

Hammer Packers: for the injection of cement injections and acrylic gels. Fig. 10 shows different packers for Injection.

The more sophisticated the machinery, the better the control and therefore the performance. Secifications written in office can be perfectly adhered to at site and controlled via good supervision. Present day gel injections with a very low setting and reaction times require 2 component machineries which can mix materials at the nozzle. Fully computerized attachments can measure the pressure, control per point, idle time, time taken for injections and this data can be mentioned for documentation purpose. Further such data can be used by owner to control the quantity of repair process and materials respectively. The supervision at site is very essential to ensure that the specifications are strictly adhered to. The temperature plays very important role in the performance of some resin based systems: and therefore manufacturers instructions as to the environmental temperature as well as the temperature of the component in which the material in injected are to be followed.

The simplest of the injection method is the brush injection. The resin is brushed on the non moving surface cracks and is absorbed in capillary action. In case of pressureless injection the material is poured into the packers especially in case of pipes acting as packers, the use of such injection depends largely on the dimensions of the crack. In case of structural cracks of the width 0.2 - 1.0 mm, it is advisable to resort to low pressure injection. This low pressure can either be created with hand guns (sealant guns, grease guns etc) or a normal compressor used at site. The pressure developed is around 6 - 10 bars. Depending upon the crack widths and depths, high pressure injections can be resorted to for structural crack repairs. It is possible to develop pressures to the tune of 500 bars using mechanical or pneumatic transmissions. The injection method should be clearly specified prior to the commencement of the work and should be supervised to conform with the specifications. Fig 11 shows Injection Machineries.

Modern Injection Techniques

When problems cannot be resolved by conventional methods of grouting and injections the resort can be made to modern injection techniques in which external envelope can be created by working from inside the structures. This means creating positive waterproofing from negative side. This

Fig 9: Different Packers for Injection

Fig 10: Showing Different Packers


technology involves new machineries, new materials and trained applicators as the technology is state of art. In order to ensure durability and, in particular, bearing capacity, the permissible crack width in the relevant norms is confined to a harmless size. If this width is exceeded, there is a danger of further secondary damage to the building or structure. Cracks smaller than the permissible maximum width can, when penetrated with water, also detrimentally affect the structure concerned. To maintain and restore operational use, any cracks appearing in a building or structure must be sealed. Various injection procedures are used depending on the cause of the crack and individual requirements. The material is injected into the crack through injection packers and a specially designed machine. The objective is the closing and sealing of the crack as well as the expandable or rigid bridging of the sides of the cracks.

Epoxy resins have been used for many years for the rigid injection of cracks in dry structural sections. Epoxy resins of very low viscosity, can be injected into the finest of cracks. This guarantees a strong permanent bridging of both sides of the crack, thus restoring bearing capacity for the loads planned. This process can also be carried out where the structure is subject to vibration. Epoxy resins of varying viscosity can be used for injection and impregnation depending on the width of the crack. Mineral injection systems also fulfil the requirements of rigid injection.

The newly developed cement based suspensions are suitable, in particular, for injecting moist cracks bearing pressureless water as well as dry cracks and can even be used for the rigid injection of cracks down to 0.2 mm vide.

Even newly erected buildings and structures often show signs of defects in the form of cracks or cavities caused by damage. Mineral injection systems really come into their own in these cases because they are not sensitive to the moisture

contained in new concrete. Rectifying defects with mineral polymer cement injection makes the building or structure appear as a unified entity for inspection purposes as well as restoring the structural integrity.

Structures located in groundwater and pressure water environments, e.g. tunnels, reservoirs, etc., often display pervious areas, which can considerably impair the use of the structure in question. These pervious areas can be caused by cracks, voids or faulty seals. Water-bearing cracks make particular demands on injection technology. The objective is to seal an existing structure to prevent water penetration without impeding the normal expansion and contraction of the structure. Structures with water-bearing cracks are normally accessible from one side only. If the actual seal can only otherwise be repaired at a great expense, the only truly economical way of achieving permanent success is to use injection technology developed polyurethane-based injection systems for such cases and these have proved successful over many years. Permanent seals are achieved by using PU Injection with its outstanding material properties which was by forming an even closed pore structure. The result is a plastic workable product which makes flexible injection sealing possible. Even cracks bearing pressurized water can permanently sealed. In this case, a fast-foaming water-stopping polyurethane, can be used.

Water penetration is often caused by bad concrete compaction, honey combing or defective seals. This is rectified by using grid injection systems, a special technology developed from the standard injection process. Fig. 5 shows the principle of grid injection of creating positive waterproofing from negative side. Fig. 6 shows the process of injection and grid matrix.

A special injection application is waterbar injection where waterbars are used to seal joints in moving structures against

How to choose the right product - Site Conditions

Few are only 2 site conditions which has to be taken into con-dition when injection shall be attempted

Wet Cracks • movement cracks• stationary cracks• construction joints• expansion joints• bad construction• Needs Hydrostructure Resins and gels for flexible fillingDry Cracks• Non moving cracks• mechanical damages• damages caused by earth movements• general wear and tear• bad construction• Needs Duromers for Rigid filling

Table 4 : Selection of Materials Fig 11: Injection Machineries

Waterproofing


pressurized water. However, the concrete often proves to be a weak point in the area of the waterbar because of inadequate compaction. This technology is further proof of the effectiveness of solving structural problems with special injection systems.

Other critical areas with regard to the watertightness of a building or structure are expansion joints that are not sealed with waterbars. Inserting injection pipes provides the possibility of sealing expansion joints effectively at a later date.

The material used for such cases must display excellent flow properties. Viscocities play a major role in injection techniques. The reaction times also are very important. Secondary injections are a must.

All cracks are different. They vary depending on the construction material, cause, location and environment. Quite often, cavities in the fabric of construction materials, construction joints or foundations with insufficient load-bearing capacity require injection measures. One single system is not able to achieve durable and reliable results. Various solutions based on different materials which are tailored to specific application needs are now available to users. Table 2 shows different available materials. A range of solutions is essential depending upon job and site conditions. Table 3 and 4 shows selection of materials with respect to these conditions. The use of unsuitable materials or techniques may necessitate costly subsequent reconstruction, which are more expensive than correct action right at the outset. These additional costs are avoidable through technical approach. Experience has shown that a superficial solution will necessitate further repair measures.

Very low-viscosity, highly cross-linked series of duromers

easily penetrate into the crack also filling the so called crack root. This ensures a seamless, rigid bonding of the crack edges. These highly cross-linked duromer resins are also the right choice for critical joints, where all important static forces are being transmitted. The properties of the injection material must be compatible with the parent material. The structural mechanics of the injected element should remain unaffected. This is an important aspect when carrying out construction works on concrete or masonry buildings that are classified as historical monuments. Cement suspensions are insensitive to varying moisture levels in the building. Even large volumes of injection are possible with cementatious suspensions suitably modified to low viscosities and having non shrink properties. They ensure a high degree of efficiency enabling a reduction in restoring costs.

Conclusion

Occurrence of cracks is practically unavoidable in structures. The modern building chemical technology coupled with proper equipment can solve almost all types of rehabilitation problems thereby providing economical solution in comparison to demolition and reconstruction of structures. The specifications should be very clear and unambiguous. The specifications should at least cover points like material, viscocity, techniques to be adopted, the equipment to be employed, type of nozzles and spacings, pressure to be applied etc.

The supervision at site is very essential to ensure that the specifications are strictly adhered to. The temperature plays very important role in the performance of some resin based systems: and therefore manufacturers instructions as to the environmental temperature as well as the temperature of the component in which the material in injected are to be followed.

New Injection technology accommodates not only new materials but also advanced machineries and trained applicators. The latest injection methods and processes can arrest heavy water leakages in couple of minutes and therefore require precision mixing proportions and split second timing to achieve immediate gelation. It must therefore be first determined what is the purpose of injection and from this decision one should proceed to selection of material and adequate packers and machineries. Right decision at this stage is prerequisite for avoidance of failure. Planning therefore and technical guidance becomes key factor in the process of decision making sound knowledge of soil structure interaction, technical know how of material chemistry coupled with trained applicators are required to successfully carry out the job under actual site and job condition. Any repair based on predetermined material is bound to be ineffective. Sound principles of civil engineering is the base to all repairs, rehabilitation or retrofitting. So positive waterproofing is possible from negative side.

Packers

Adhesion Packer

Drill PackerHammerPacker

Material SteelAluminium-base alloy

Plastic

MeasurementsPlate 38 x 43 mm shank 23

mm115 x 13 mm 115 x 23 mm

Valve Orifice 1.5 mm 1.5 mm 4.5 mm

Permitted max pressure in concrete

60 bar 200 bar 30 bar

Permitted max pressure in masonry

20 bar 20 bar 30 bar

Loss of pressure 10 - 15 bar 10 - 15 bar < 1 bar

Table 5 : Technical data about packers

Waterproofing


High Performance Concrete using Microsilica

Surendra SharmaDeputy General Manager (Concrete) Elkem South Asia Pvt. Ltd.

High Performance Concrete

Need for High Performance Concrete

Concrete structure gets deteriorated on account of chemical reactions such as corrosion of rebar or physical effects like porosity which permits easy access of harmful ions into the main body of concrete which makes concrete resistance weak there by shortening the life cycle of concrete. Some of the factors which affect durability of concrete are reinforcement corrosion, chloride diffusion, carbonation, sulphate attack, alkali silica reaction, frost action, leaching of concrete etc.

In practice, most of this deterioration of concrete cannot be seen as they developed as concrete grow older.

Fig.1 below shows some of the damages and therefore as a practicing engineer one need to design and place concrete which is durable and sustainable in the long run.

Therefore as a good practicing engineer, the best way of protecting these damages is to make concrete less permeable as high degree of water saturation is seen as one of the causes for durability problem. It becomes very essential to design concrete with less porosity to protect the concrete as water

Fig. 1

is only medium through which chlorides, sulphate and other harmful trace element can enter into concrete structure and cause damages to concrete from inside .i.e. corrosion.

All major infrastructure projects are designed for 100+ years and capital intensive. It becomes very essential to increase the life cycle and produce the concrete which can last for longer period and boost the nation’s economy. Replacing concrete or structure is a costly affair.

ACI defines High Performance Concrete as “concrete meeting special combinations of performance and uniformity require-ments that cannot always be achieved routinely using con-ventional constituents and normal mixing, placing and curing practices.

Microsilica: Elkem Microsilica® is one of the principle products supplied by Elkem Silicon Materials. It finds a wide range of applications in high-strength concrete, High Performance Concrete other building materials such as roof tiles and facade cladding, and fire proof products for heavy industry.

Microsilica is also used for sealing tunnels and drilling oil wells. Microsilica is proven mineral additive which has been in use since many years to provide high performance concrete solution to construction Industry. Worldwide 10 Million m3 or even more concrete is produced with Microsilica. The ACI defines Silica fumes as “very fine non crystalline silica produced in electric arc furnaces as a by- product of the production of elemental silicon or alloy containing silicon. Microsilica® is having a very fine particle. The average particle size of Microsilica is 0.15 µm.

The specific surface tested on BET method is minimum 15000 m2/Kg (See SEM Fig 2). Which provide better filler effects and make concrete less porous and robust and it helps in improving concrete resistance to chloride and sulphate attack.

Usually Microsilica available in condensed from. There are two type of Microsilica available in the market.

1) Undensified Microsilica


2) Densified Microsilica

During Microsilica production, Undensified Microsilica is densified by use of hot air which helps in increasing its bulk density. It is essentially a non-hazardous material and fall into general category of nuisance dust, which is similar to cement and many other fine particles.

The relevant standard for Microsilica in India is IS 15388-2003. Please see below the other physical and chemical properties specification of Microsilica as outlined by different standard.

How does it work?

Microsilica has minimum 85%-99% of SiO2 which, make it very reactive pozzolanic material in concrete. As we all know, water is an essential element in the cement hydration process

and when one adds water to concrete; generally following chemical reaction takes place.

Cement hydration

Portland cement (C3S, C2S) + water = CSH + Ca (OH)2

Secondary Pozzolanic reaction with Microsilica

Pozzolana (SiO2) + Ca (OH)2 + water = CSH

Meaning you get more,

CSH-gel (binder)

Denser concrete/good interface

Higher compressive strength.

Also, above chemical equation shows that free lime, Ca(OH)2 which detrimental to concrete is turned to something good meaning more C-S-H (Calcium-Silicate-Hydrate) gel meaning additional binder for given concrete which provide improved strength and durability characteristics both at early age and later age besides improving plastic properties.

Other value attributes of Microsilica in Plastic stage of concrete

Generally the recommended dosage of Microsilica is 5 to 10%

Fig. 2

High Performance Concrete 0.45um

Bulk Density - Undensified Microsilica Densified Microsilica

130 to 480 Kg/m3

480 to 720 Kg/m3

Specific Gravity 2.2

Physical properties of Microsilica.

USA ASTM C

1240-2004

EUROPE CEN prEN

13263: 1999

INDIA IS 15388: 2003

CANADA CSAA23.5-98

NORWAY NS 3045

AUSTRALIAAS 3582

Sio2 %> 85.0 85.0 85.0 85.0 85.0 85.0

SO3%< 2 1 3

Cl %< 0.3 Report if > 0.10 Report

CaO %< 2

MgO

Si (free) %< 0.4

Total Alkalies 1.5

Free C

Moisture Content %< 3 3 2

LOI %< 6 4 4 6 5 6

Specific Surface m2/gm>

15 15-35 15 12



by wt of cementitious content depending upon the specific characteristics that need to be improved in the concrete mix. Microsilica as stated earlier is a very fine material and it need proper dispersion well in concrete matrix and it occupies space between cement grains thereby improving permeability of concrete mass. Typically it is said that in one grain size of cement, 100 Microsilica grains can exits. It also helps in improving mobility of concrete when energy is applied to concrete.

Through the permeable structure of concrete the ingress of chloride and sulphate ions can damage the concrete as stated earlier and addition of Microsilica largely improved this and protects the concrete.

Fine particles also hold other constituent of concrete better and therefore other value addition of Microsilica is better cohesion. This property largely helps in short creating operation as well as helps in pumping the concrete. In addition it reduces bleeding and segregation and aids in efficient finishing and helps in slip form because of better finish.

These plastic properties have its own value and are of equal

important while placing and finishing concrete. The first and foremost principle a concrete mix designer should bear in mind is to make the concrete cohesive enough with limited stickness, good mobility and finishability at a given workability which enable concrete to hold its constituent together under pressure which will be excreted during mixing, transportation and placing of concrete. What is the use of a great mix design made in the laboratory which cannot be placed in field? And therefore these plastic properties becomes equally important, Once cohesion of concrete is satisfactory then mechanical properties such as compressive strength, E-Modulus etc and durability characteristics tested on durability standard in terms of permeability ,I-Sat, Rapid Chloride Penetration, Sorptivity, Water Penetration, chloride diffusivity etc are to be taken care of. These plastic, harden and durability characteristics have been seen as remarkable improved by addition of Microsilica. If someone is using higher content of cement, by adding Microsilica he/she can reduce cement content and make concrete less sticky too this would be a desirable largely in pumping operation. It also help us to reduce heat of hydration while strength is maintained which ultimately help to prevent early age cracking.

Other value attributes of Microsilica Hardened stage of concrete.

In fully hydrated Portland cement paste, approximately 1/4th of the hydration product by mass consists of oriented, heterogeneously distributed and weekly bonded layers of calcium hydroxide crystals; which de-bond easily under tensile stress thereby serving as potential sites for formation of micro cracks. Therefore, transformation of most of this calcium hydroxide into the calcium silicate hydrate paste (which is the predominant phase produced by hydration of Portland cement) would result in a much more homogeneous hydration product.

At hardened stages, it helps in,

- Improving Strengths,

Bulk Density Report Report

Pozzolanic Activity Index %>

105%-7d accel cur-ing, w/cm=variable

100%-28d Normal Curing w/p ration=0.5

85%-7d IS 1727 Factor N=1

85%-7d accel’dcuring

95%-28d Normal curing w/cm=0.5

Report

Retained on 45 micron sieve %<

10 10 10

Density, kg/M3

Autoclave Expansion %< 0.20%

Canadian Foaming Test No visible foam

NotesCharacteristic values. Not an official standerd in the approval process

Requirement of limiting alkali only in case of reactive aggregares

charateristic values

Fig. 3 Fig. 4



Building Name Location App. Height (M) Date

Burj Khalifa Dubai 828 Tallest structure in the world, completed 2010

International Commerce Centre Hong Kong 484 Completed 2010

Petronas Twin Towers Kuala Lumpur 452 Tallest twin buildings in the world, completed 1998

Taipei 101 Taipei 508 Completed 2004, tallest LEED rating building

Guangzhou West Tower Guangzhou 432 Completed 2010

Trump Tower Chicago 423 Completed 2009

Two International Finance Centre Hong Kong 413 Completed 2003

Burj Al Arab Dubai 321 One of the world’s tallest hotels, opened 1999

311 South Wacker Drive Chicago 293 Completed 1999

Trianon Frankfurt 186 Completed 1993

World One Mumbai 442 On going

Polais Royal Worli Mumbai 320 On going

Oasis Towers Worli Mumbai 372 On going

Minerva Tower Mumbai 302 On going

Ireo Victory valley New Delhi 178 On going

- Reducing Permeability,

- Improving Acid Resistance,

- Improving Abrasion-erosion Resistance,

- Improving Durability

As you all, are aware addition of extra cement beyond certain mass depending upon cement type, per cu.m of cement does not increase your strength and highly uneconomical. And if one wants to design for higher strength and high performance concrete, addition of Micro silica helps a lot to achieve the desired plastic and hardened properties of concrete with robust and workable design. There are other mineral admixtures available based on Fly ash and GGBS, however they have their own limitation on SiO2 content and other and largely influence setting characteristics of concrete as they influence hydraulic properties of concrete and in today’s age everyone need high early strength to speed up the construction and high ultimate strength to go vertical in building high rise structure.

If you see, fig 3 and fig 4, fig 3 is a hardened concrete surface of concrete made with only cement and fig.4 is hardened concrete surface of concrete made with Microsilica and if you closely look at the interface of matrix or a cement glue (see letter A in the Fig 3 & 4), you will find that Microsilica concrete is having much less thickness of cement paste or glue or matrix and holding the aggregate better meaning economical

use of cementitious materials.

Besides this, there are enumerable advantages of Microsilica in the concrete and Microsilica is playing very important role to redefine High Performance concrete and allowing structural engineer to think of reduced size of structural elements and dream higher in the sky. And incorporation of Microsilica at designed stage of structure results in direct saving for the project. As a concrete mix designer we all should strike a balance between performance and economy and it is seen that reduced material cost per cu.m of concrete is taken only as means of economic mix design which is not true.

Therefore concrete cost must not be compared with material cost per cu.m but a comprehensive cost analysis is needed which includes all cost associated right from placing to maintenance of the structure till its design period is to be taken in to account which will the serve the purpose in longer run. Also there is need to go for performance based specification rather prescriptive based specification and Microsilica will ensure to meet the specifications.

Please see the summary of high rise executed in the recent past where Microsilica has been used in the project which one proves value additions of Microsilica.

References

- Silicafume user’s manual 2005- Internal references from Elkem ASA



Development of Modern Prestressed Concrete Bridges in Japan

Prestressed Concrete

For more than half a century prestressed concrete (PC) is one of the most important construction materials in not only Japan but also all over the world particularly in the field of bridge construction. The increased interest in the construction of PC bridges can be attributed to the fact that the initial and life-cycle cost of PC bridges, including repair and maintenance, are much lower than those of steel bridges. Moreover, comparing to the reinforced concrete (RC) bridges, PC bridges are more economically competitive and aesthetically superior due to the employment of high-strength materials. In Japan, the first PC bridge, Tyousei Bridge, was built in 1951 and since then, the construction of PC bridges has grown dramatically (Figure 1). However, deterioration of bridges is becoming a big social issue since many bridges are getting older over 50 years.

In recent years, many PC bridges have been deteriorating even before their designed service-life due to corrosive circumstances, alkali-silica reactions, and other environmental effects. Hence, the durability has become a particular concern and should be seriously considered in the design and con-struction of PC bridges apart from structural safety, aesthetical appearance and economical viewpoint. In Japan, a number of innovative techniques have been developed to enhance not only the structural performance but also the durability of PC bridges. These include the development of novel structural system and the advance in new construction materials. This paper presents an overview of such innovated technologies of PC bridges including a brief detail of their development and background as well as their applications in the actual construction projects. Figure 1 Trend of construction of different types of bridges in Japan

Hiroshi Mutsuyoshi1, Nguyen Duc HAI1 and S.V.T. Janaka Perera1

1Structural Material Lab, Department of Civil and Environmental Engineering, Saitama University

Prestressed concrete (PC) is being used all over the world in the construction of bridge structures. In Japan, the application of PC was first introduced in the 1950s, and since then, the construction of PC bridges has grown dramatically. This is largely due to several advantages such as lower initial and life-cycle cost compared to steel bridges, and superior characteristics concerning economical and aesthetical aspects compared to reinforced concrete bridges. However, many PC bridges have been deteriorating even before their designed service-life due to corrosion and other environmental effects. Therefore, the durability has become a particular concern and should be seriously considered in the design and construction of PC bridges. In Japan, a number of innovative techniques have been developed to enhance both the structural performance and the durability of PC bridges. These include the development of new materials such as pre-grouted internal tendon, high-strength concrete and structural systems such as external prestressing, highly eccentric external tendons, extradosed prestressing and corrugated steel web. This paper presents an overview of such innovated technologies of PC bridges including a brief detail of their development and background as well as their applications in the actual construction projects.

Development of Innovative Materials for PC Bridges

In Japan, most of PC bridges were constructed using internally post-tensioning tendons with grouting in sheaths. Recently, however, problems regarding grouting condition have been of much concern because the insufficient grout of internal tendons was found in some existing PC bridges (Mutsuyoshi 2001). Many researches have been carried out recently for the development of new materials to enhance the performance and longterm durability of the PC bridges. In this section, application of pre-grouted internal tendons and high-strength concrete are explained with brief overview on their applications in actual PC bridge projects.

Pre-Grouted Prestressing Tendon

Pre-grouted prestressing tendon was first developed in Japan in 1987. It is made by coating a prestressing strand with unhardened epoxy resin and a polyethylene protective tube (Figure 2) and is embedded directly into concrete with the


polyethylene protective tube as a tendon for posttensioning. Time of hardening is set for the epoxy resin filled in the polyethylene protective sheath so that post-tensioning process can be completed before hardening or the epoxy resin. The resin has viscosity like grease before hardening, and it naturally hardens with certain time after the completion of tensioning of prestressing steel and bonds with concrete to form an integrated member.

As the grout is injected into the polyethylene sheath, complete grouting is ensured in this technique. Furthermore, con-struction work can be saved as neither in-situ insertion of prestressing tendonds nor grouting process is required. Sheath and epoxy resin also provide double layer corrosion proection to the prestressing tendons. This technique also ensures stronger bond with concrete than conventional cement grouting technique. Moreover, smaller diameter of sheath makes concrete placement relatively easier and provide higher efficiency in prestressing can be achieved as polyethylene sheath and unhardened resin reduce the friction during prestressing.

Pre-grouted prestressing tendon, primarily in the form of single strands, was generally applied to transverse prestressing of deck slabs or other work. Application to main tendons has just started in view of the above benefits (see Figure 3). When pre-grouted prestressing strands are used for main tendon, more prestressing strands are required than when conventional multiple strands are used. Numerous prestressing strands therefore should be anchored in a limited anchorage region. Hence, attention should be paid in design for the arrangement of pre-grouted prestressing tendons and details of anchorage.

High-Strength Concrete

High-strength concrete (HSC) has become common in building construction in recent years as it enables the use of smaller cross-sections, longer spans, reduction in girder height

and enhanced durability. In addition to this application, there are a few instances of HSC being applied to PC structures (Figure 4) (Mutsuyoshi et al. 2010). The chief advantage of using HSC is the possibility of achieving higher prestressing force compared to the normal concrete which will lead to smaller crosssection and reduction in the overall weight of the structure. Hence, the use of HSC has a good potential in the construction of large structures.

The lower water/binder ratio in HSC may, however, cause the increase in autogenous shrinkage which will lead to decrease in effective prestressing force and cracking due to restraining caused by the reinforcing steel. Conventional method of reducing autogenous-shrinkage-strain is to use expansion-producing admixture and shrinkage-reducing agents. However, these materials are expensive. This problem has been overcome by the development of new type of artificial light weight aggregate called as “J-Lite” (Figure 5). J-lite is made from environmentfriendly coal ash and is twice as strong as conventional light weight aggregate. Low shrinkage ultra HSC termed as “Power Crete” with compressive strength as high as 120 MPa, has been developed with the use of J-lite

Figure 2 Pre-grouted prestressing tendon

Figure 3 Use of pre-grouted prestressing strand for main tendons

Figure 4 AKIBA pedestrian bridge constructed with HSC (f’c = 120 MPa) in Akihabara



together with expansion-producing admixtures and shrinkage reducing agents. As strength development in lowshrinkage HSC is independent of the curing condition, it can be used for cast-in-place applications as well.

flexural failure (so-called second-order effect). One possible method of enhancing the flexural strength of externally PC structures is to make the tendons highly eccentricity (Figure 7). This kind of construction is possible only when external prestressing is used, since this allows the tendons to be placed outside the concrete section of girder. In this concept,

Figure 5 J-lite

Development of Modern Structural Systems in PC Bridges

External prestressing technique is widely being used in the construction industry. Externally prestressed PC bridges are designed with prestressing tendons placed outside the concrete section, but still remaining within the bounds of the cross section of girder (Figure 6). The concept of external prestressing has become increasingly popular in the constructions of medium- and longspan bridges due to its several advantages such as reduced web thickness, possibility to control and adjust tendon forces, and ease of inspection of tendons during construction. The Japan Highway Public Corporation (abbreviated for JH; it is changed to three highway companies at present), has actively adopted the concept of fully external tendons for box girder bridges (Figure 6) since 1999 due to the improved durability performance compared to that of internally grouted tendons. It is of importance to note that, recently, a new construction of PC bridges with internally grouted tendons has been forbidden by the JH due to the bad quality of grouting of internal tendons in some existing PC bridges (Mutsuyoshi 2001). For the better performance of the externally prestressed concrete bridges, various new technologies have recently been developed in Japan.

PC Bridges with Highly Eccentric External Tendons

Although externally prestressed PC bridges are well recognized to have several advantages, however, they have lower flexural strength compared to that of bridges with internally bonded tendons (Virlogeux 1988). This is due to the smaller tendon eccentricity, which is limited by the bounds of concrete section of girder (i.e., at the bottom slab in case of box-girder bridges) as well as the reduction in tendon eccentricity at the ultimate

Figure 6 Typical layout of an external PC box girder bridge

Figure 7 Ordinary vs. highly eccentric external tendon

the compressive forces are taken by concrete and the tensile forces by external tendons, thus taking advantages of both materials effectively (Mutsuyoshi 2000).

There has been extensive research conducted at Saitama University both analytically and experimentally to study the fundamental behavior of girders with highly eccentric external tendons (Aravinthan et al. 1999, Witchukreangkrai et al. 2000). From the test results of single-span beams, it was found that by increasing tendon eccentricity, the flexural strength can be significantly improved or, conversely, the amount of prestressing reduced; the result is more economical structures. By extending the concept of highly eccentric external tendons to continuous girders, the structural performance can be further improved. In addition, the girders consisting of linearly transformed tendon profile were found to have the same overall flexural behavior (Figure 8). This gives the designer to take advantage of arranging the external tendon layout freely, depending on the site conditions.

To verify the application of this concept to the segmental



construction method, the behavior of segmental girders with highly eccentric external tendons was also investigated and found to be nearly the same as that of monolithically cast girders. Hence, this gives considerable flexibility in selecting the method of construction when designing the bridges with highly eccentric external tendons. One of the concerns raised for this type of structure was the shear capacity as the girder height is considerably reduced. It was verified, however, from the experiment on shear characteristic of model specimens that the shear capacity of the girder with highly eccentric external tendons is much higher than that of the conventional girders due to the large increase of tensile force in external tendons.

The world’s first application of the prestressing with highly eccentric external tendon to a continuous-span girder was in the construction of the Boukei Bridge in Hokkaido, Japan. Considering the site conditions, the bridge was designed with a two-span continuous and unsymmetrical girder having a total length of 57 m as shown in Figure 9. The effective width of the bridge varies from 3.0 m at the abutments to 6.0 m at the central pier. A completed view of the bridge is shown in Figure 10.

The characteristic of this innovative bridge is that the external tendon layout takes the similar shape of the bending moment diagram as shown in Figure 11. The structure was designed to form a pseudo truss, with the main girder made of concrete as compression chords, the external tendons as tension chords, and the steel deviators as diagonal members. This allowed the girder height to be reduced significantly, thus making the bridge lightweight. The external tendons are placed below the girder in the midspan region by means of

steel struts, the function of which is similar to a truss. At the intermediate support region, it is placed above the bridge deck and covered with a fin-shaped concrete web member. The combination of the subtended tendons and the fin-shaped concrete web makes this bridge a unique one with aesthetically pleasing appearance.

Figure 8 Linear transformation of tendon layout

Figure 9 Layout diagram and dimension of the Boukei Bridge

Although having several advantages, PC bridges with highly eccentric external tendons should be carefully designed and constructed concerning the following important points. Since the main girder, struts and highly eccentric external tendons form a truss in this type of structure, construction precision of individual members has a significant influence on the structure. For this, it is necessary to give special consideration to the techniques and procedures for constructing the falsework, formwork and external tendons. Moreover, the vibration characteristics under service load may be of concern due to the smaller stiffness of the main girder caused by the reduction in girder height. Nevertheless, the authors believe that this new concept of prestressing would pave way to a wider use of external prestressing technology in the construction industry, leading to improved structural performance as well as cost effective PC bridges. The research is in progress regarding the possibility of applying these kind of structures to highway bridges using precast segmental construction.

Extradosed PC Bridges

An extradosed prestressing concept, which was first proposed by Mathivat in France (Mathivat 1988), is a new type of structural system in which the tendons are installed outside and above the main girder and deviated by short towers located at supports. Considering its definition, this type of bridge is placed between cable-stayed bridges and ordinary girder bridges with internal or external tendons.

Extradosed PC bridges have several positive characteristics.



The girder height may be lower than that of ordinary girder bridges, thus reducing self-weight of structures. As shown in Figure 12, the ratio of the girder height to the span length (H/L) in extradosed bridges ranges between 1/15 and 1/35, while it is approximately 1/15~1/17 for box-girder bridges. Comparing to cable-stayed bridges, the height of the main tower in extradosed bridges is lower; hence, a reduction in labor costs of construction can be achieved.

The major difference among box-girder, extradosed and cable-stayed bridges can be further revealed by comparing the relationship between materials used with span lengths. In box-girder bridges, the average concrete thickness increases with the span length, since the girder height is a function of the span length. On the other hand, in cable-stayed bridges, there is almost no increase in the average depth of concrete because the girder height is generally designed to be 2.0~2.5 m, regardless of the span length. It is interesting to note that extradosed bridges are placed between these two types, and the rate of increase is also thought to be midway between the rates of the other two types of bridges.

Similarly, with increasing span length, the quantity of prestressing tendons in box-girder bridges shows a more increase than that in cablestayed bridges, whereas extradosed bridges yield the intermediate value between the other two types. From the above discussion, it can be concluded that an extradosed bridge is similar in construction and appearance to a cable-stayed bridge. In the light of structural properties, however, an extradosed bridge is closed to ordinary PC girder bridges, and the design specifications may be considered to be the same for both types of bridges.

Figure 10 Completed view of the Boukei Bridge

Because of a lower main tower in extradosed bridges, vertical loads are partly resisted by main girders and stress variations in stay cables produced by live loads are smaller than those in cable-stayed bridges. This is quite similar to the behavior of box-girder bridges, where the main girder itself has a decisive influence on the structure rigidity and live loads produce only limited stress variations in tendons. Based on these facts, the Japan Road Association has recommended that the safety factor for the stayed cables in extradosed bridges under design loads shall be taken as 1.67 (0.6 fpu; fpu = tensile strength of tendons), which is same as that for tendons in ordinary girder bridges. For cablestayed bridges, this value is specified to be 2.5 (0.4 fpu) (Japan Road Association 2002).

Figure 11 Schematic view of layout of external tendonFigure 12 Comparison among externally box-girder, extradosed and cable-stayed PC bridges



In case of extradosed bridges it is necessary to provide an structural rationale rather than simply assuming an allowable stress of 0.6fpu in design of the bridges. In this point, attention focuses on the distribution ratio of vertical load carried by the girders and the stay cables. Figure 13 shows the relationship between the distribution ratio of vertical load ( ) and maximum stress change of stay cable due to design live load ( ) of various cable-stayed bridges and extradosed bridges con-structed in Japan. As shown in the figure, it is difficult to clearly distinguish between extradosed bridges and cable-stayed bridges in terms of structural mechanics since many of the cablestayed bridges are very similar to extradosed bridges. In designing stay cables, stress change due to design live loads provides an effective index that can be easily determined through the design process.

Approximated Design Method for Stay Cables

The fatigue limit state is usually critical in the design of stay cables. When bridge structures reinforced by stay cables, the design of stay cables would be structural rationale by focusing on the stress change in the stay cables rather than defining whether bridges are cable-stayed or extradosed by assuming allowable stress for the stay cables. This would make it possible to design each stay cable separately and enable the allowable stress to be set individually for each stay cable. Unlike suspension bridges, the stress change in a cable-stayed bridge will differ depending on the stay cables and it is not rational to define the allowable stress using a single value of 0.4fpu. This is reflected in the “Specifications for Design and Construction of Cable-Stayed PC Bridges and Extradosed Bridges” (Japan Prestressed Concrete Engineering Association 2009). The specification allows two kinds of design method. Method A is normal fatigue design using fatigue load and design lifetime of a bridge. However, it is usually difficult to estimate the amount of future traffic and heavy trucks, especially in local roads. In that case, method B using stress change in stay cables due to design vehicular live loads is introduced. Figure 14 shows the relationship between the allowable tensile

stress (fa) of stay cable for highway bridges and the stress change due to live load regulated in the specifications. The difference in fatigue strength between prefabricated wire type and strand type is considered. By using prior experience in Japan with cable-stayed, extradosed and similar bridges having spans of up to about 250 m, method B is defined so as to ensure adequate safety in comparison with bridges designed using method A. Fatigue design was performed for the estimation line of stress range for two million cycles ( 2E6) including secondary flexural bending due to girder deflection (determined according to design conditions on a design service life of 50 years and average daily traffic of 70,000 mixing 50% trucks) by using the structural models of the Odawara Blueway bridge, the Tsukuhara bridge, and

the Ibi River bridge as shown in Figure 13. Based on the calculations the stress change due to fatigue load is about 1/3 of that due to design live loads and the stress level due to secondary flexural bending is the same as that due to axial forces of stay cables. It is noted that the estimation line of

2E6 is assumed to be 2(1/3)(Max L). The safety margin of method B can be confirmed by comparing 2E6with fatigue strength (fscrd) divided by a safety factor ( b).

In the stay cables designed by method B, L is determined to require a safety factor of about 2.0 for 2E6 with respect to fscrd / b, in order to take into consideration the fact that the method includes more uncertainties than method A, and in order that the safety of stay cables does not vary greatly from that of cable-stayed and extradosed bridges constructed to date. In the most of extradosed bridges and some cable-stayed bridges, the tensile stress of 0.6fpu can be used because stress changes are low (20 to 50N/mm2). Moreover the most rational point of this specification is that we can choose the tensile stress in each stay cable from 0.4fpu to 0.6fpu continuously. This is based on the concept that one value of tensile stress in one bridge is not structurally rational.

Figure 14 Allowable tensile stress and stress change of stay cable

Figure 13 Distribution ratio of vertical load and stress change of stay cable



Figure 15 shows the Odawara Blue-Way Bridge, which is the first extradosed PC box girder bridge in the world and was completed in 1994. This bridge was designed with a three-span continuous box-girder with extradosed prestressing, having a middle span length of 122 m, a tower height (h) of 10.5 m, and a girder height at supports (H) of 3.5 m. The ratios of h/L and H/L are approximately 1/12 and 1/35, respectively.

Figure 16 shows the prospective view of the Shin-Meisei bridge on Nagoya Expressway No. 3 crossing the class-1 Shonai River in western Nagoya. From both aesthetic and economic viewpoints, the bridge was designed with a threespan continuous rigid-frame structure with extradosed prestressing, which is to become a landmark of Nagoya’s western threshold. The length of the middle span (L) is 122 m, a tower height (h) of 16.5 m, and a girder height at supports (H) of 3.5 m, giving the ratios of h/L and H/L of 1/7.4 and 1/35, respectively.

Corrugated Steel Web Bridges

In PC bridges with corrugated steel webs, light-weight corrugated steel plates are used instead of concrete webs.

The corrugated steel plate webs are capable of withstanding shear forces without absorbing unwanted axial stresses due to prestressing, thus enabling efficient prestressing of top and bottom concrete deck slabs, thus resulting in an “accordion effect” (Figure 17). Moreover, the corrugated webs also provide high shear buckling resistance. Use of light-weight corrugated steel plates for webs causes a reduction of self weight of about 25% of main girders. Therefore, this enables the use of longer spans and reduction of construction cost. The weight of a segment to be cantilevered during erection can also be reduced, thus longer erection segments can be adopted and construction period can be shortened. This also eliminates assembly of reinforcement, cable arrangement and concrete placement for concrete webs. Thus, saving of construction manpower, quality enhancement and improvement of durability are expected. In addition, replacing the damaged deck slabs is easier than that in ordinary PC bridges.

Recently, the use of corrugated steel web has been applied to a variety of new constructions of PC bridges in Japan (Figure 18). In addition to the rigid or box girder bridges, the concept of corrugated steel web was also successfully adopted in the constructions of extradosed and cable-stayed PC bridges.

Figure 15 Odawara Blue Way bridge

Figure 16 Shin Meisei bridge (prospective view)

Figure 17 Typical section of PC bridge with corrugated steel web

Figure 18 Ginzanmiyuki bridge with corrugated steel web



Rittoh Bridge located in the southern edge of Lake Biwa, is the first extradosed PC bridge with corrugated steel web whose main girder has a three-celled cross section, making it suitable for a bilaterally suspended structure with a wide roadway. The bridge consists of four-span and five span continuous rigid-frame structure with total span length of 495m and 555m, respectively (Figure 19).

term durability, which is becoming one of the serious problems in concrete structures nowadays.

Considering the development of new construction materials, the application of pregrouted internal tendons and use of low-shrinkage HSC were discussed. In light of new structural systems, external prestressing with highly eccentric tendons and extradosed prestressing are excellent examples of a wider use of external prestressing technology to achieve a PC bridge with improved structural performance as well as cost-effective outlook. The corrugated steel webs, which take advantages of steel and concrete, have proved to be one of promising solutions that can reduce the selfweight of main girders, thereby enabling the use of longer spans and reduction of construction cost.

Acknowledgement

The authors wish to acknowledge the members of the international committee of Japan Prestressed Concrete Engineering Association (JPCEA) for providing valuable information. The authors would also like to extend their gratitude to Nippon Expressway Company (NEXCO).

References

- Aravinthan T, Mutsuyoshi H, Hamada Y, and Watanabe M (1999) “Experimental Investigation on the Flexural Behavior of Two Span Continuous Beams with Large Eccentricities”, Transactions of JCI, 21, pp. 321-326.

- Japan Prestressed Concrete Engineering Association (2009) “Specifications for Design and Construction of Prestressed Concrete Cable-Stayed Bridges and Extradosed Bridges”, (in Japanese).

- Japan Road Association (2002) “Japan Specification of Highway Bridges-Part I Common Part, Part II Steel Bridges, Part III Concrete Bridges”, (in Japanese).

- Mathivat J (1988) “Recent Development in Prestressed Concrete Bridges”, FIP Note, Feb., pp. 15-21.

- Mutsuyoshi H (2000) “State-of-the-Art Report on External Prestressed Concrete Bridge with Large Eccentricity”, Concrete Journal, 38(12), pp. 10-16 (in Japanese).

- Mutsuyoshi H (2001) “Present Situation of Durability of Post-Tensioned PC Bridges in Japan”, Proceedings of the 1st fib/IABSE Workshop on Durability of post-tensioning tendons, Belgium, pp. 75-88.

- Mutsuyoshi H, Ichinomiya T, Sakurada M, and Perera S V T J (2010) “High-strength concrete for prestressed concrete structures”, CPI Trade Journal for the Concrete Industry, August, pp. 42- 46.

- Virlogeux M P (1988) “Non-linear Analysis of Externally Prestressed Structures”, Proceedings of FIP Symposium, Israel, pp. 319-340.

- Witchukreangkrai E, Mutsuyoshi H, Aravinthan T, and Watanabe M (2000) “Analysis of The Flexural Behavior of Externally Prestressed Concrete Beams with Large Eccentricities”, Transactions of JCI, 22, pp. 319-324.

Figure 19 Rittoh Bridge (Extradosed PC bridge with corrugated steel web)

Conclusions

Recent techniques in design and construction of PC bridges in Japan were presented in this paper, with emphasis on their background and development as well as their applications in actual structures. Not only to improve the structural properties in terms of safety, aesthetic and economical aspects, such innovated technologies were developed to enhance the long-



A Review of Fresh Cement and Concrete Rheology

Rheology is the science of the deformation and flow of matter, and the emphasis on flow means that it is concerned with the relationships between stress, strain,

rate of strain, and time. Available literatures have helped to overcome perceptions of the difficulty of rheology with its often mathematically complex relationships. Flow is concerned with the relative movement of adjacent elements of liquid and in shear flows liquid elements flow over or past each other, while in extensional flows elements flow towards or away from each other. In a shear flow, imaginary parallel layers of liquid move in response to a shear stress to produce a velocity gradient, which is referred to as the shear rate, equivalent to the rate of increase of shear strain. Elongation or stretching flows are rarely found in cement systems but there may be some elongation at the entry or exit of a pipe. They will not be considered further here. The rich variety of material behaviour can be characterised in various ways, of which the flow curve showing how shear stress and shear rate are related is very common, but equally data may be presented as the variation

of viscosity (the ratio of shear stress to shear rate) with shear rate or time.

Basic Information on Rheology

Rheology is the scientific description of flow. The rheology of concrete is measured with a concrete rheometer, which determines the resistance of concrete to shear flow at various shear rates. Concrete rheology measurements are typically expressed in terms of the Bingham model, which is a function of:

- Yield stress: the minimum stress to initiate or maintain flow (related to slump)

- Plastic viscosity: the resistance to flow once yield stress is exceeded (related to stickiness)

Concrete rheology provides many insights into concrete workability. Slump and slump flow are a function of concrete rheology. Refer Figure 1 for a typical rheometer and typical flow curve.

Sonjoy Deb, B.Tech, Civil Associtate Editor

Concrete: Rheology


Figure 1: Rheometer and Typical Flow Curve

Workability versus Rheology

- Workability tests are typically empirical- Tests simulate placement condition and measure value

(such as distance or time) that is specific to the test method- Difficult to compare results from one test to another- Multiple tests needed to describe different aspects of

workability- Rheology provides a fundamental measurement- Results from different rheometers have been shown to be

correlated- Results can be used to describe multiple aspects or

workability

Some Concrete Rheometers are shown in Figure 3 below:

Rheology Testing Methods for Cement Based Materials

There are well−established rules for the sizes of apparatus and sample to ensure that rheological measurements are reliable, chiefly that any gap must be 10 times the size of the largest particles and that the ratio of outer cylinder radius to inner must be less than 1.2. For coarse granular materials like concrete this means that a coaxial cylinders viscometer is impracticably large, requiring a sample volume of 2.5m3 [1], whereas a specially designed one for mortar is feasible [16,17] and cement pastes are well within the capability of any of the wide range of laboratory instruments available commercially. These principles are equally applicable to other geometries and mean, for example, that the cone and plate viscometer cannot be used for suspensions because the gap is zero under the apex of the cone. This led to the development of the truncated and annular plate and cone geometries [8].

Fresh concrete

Because of the impracticability of using a coaxial cylinders viscometer of anything like ideal dimensions for fresh concrete, Tattersall and co−workers developed a highly successful and practical apparatus in which an interrupted helical impeller rotates in a cylindrical bowl of fresh concrete and the

Concrete Flow Curves

- Flow curves represent shear stress vs. shear rate - Bingham model is applicable to majority of concrete - Other models are available and can be useful for specific

applications (e.g. pumping) - Very stiff concrete behaves more as a solid than a liquid.

Such mixtures are not described by these models.Refer a Flow curve in Figure 2.

Rheology Measurement: Typical Geometry

- Rheometers must be uniquely designed for concrete (primarily due to large aggregate size)

- Results can be expressed in relative units (torque vs. speed) or absolute units (shear stress vs. shear rate)

Concrete: Rheology

Figure 2: Flow Curve


behaviour is analysed using the theory of mixing [1]. This has been developed further by Domone and Banfill [18] and the current computer assisted model of the Two−point apparatus is available commercially [19]. Following calibration it can deliver the yield stress and plastic viscosity of fresh concrete in fundamental units. Other, broadly equivalent, approaches to the measurement of fresh concrete rheology have produced the IBB rheometer [20], the BML rheometer [21] and the BTRHEOM [22]. These instruments were developed in different countries and the question naturally arose as to whether the results can be compared. The first attempt to answer this was a programme of comparisons achieved by bringing all four instruments together at a single location with a fifth, the Cemagref−IMG [23], a large (0.5m3) coaxial cylinders instrument used as a standard, all under the sponsorship of the American Concrete Institute [24]. While each instrument characterised fresh concrete as a Bingham material and the yield stresses and plastic viscosities measured on the 12 concretes tested remained in the same rank order, the results fell into two groups. The Cemagref−IMG and BTRHEOM agreed well, and the Two−point and BML agreed well, with the second group giving a generally lower yield stress. Pairwise correlations were highly significant and enable the result of one test to be predicted from another. Since the BTRHEOM uses parallel plates, the Cemagref− IMG uses coaxial cylinders, the Two−point uses an interrupted helix rotating in a cylinder and the BML uses coaxial cylinders this agreement is encouraging.

Mortar

Mortar can be considered to be fresh concrete without the coarse aggregate and its testing has attractions for the study of the effects of ingredients at small scale. A coaxial cylinders viscometer, while feasible, proved to be inconvenient and Banfill described the use of the Viskomat as a small calibrated mixer for mortar testing [25]. More recently Jin [26] used a scaled down interrupted helix (like the Two−point impeller)

in an extensive study of the mortar fraction for design of self compacting concrete and demonstrated that its rheology could be predicted with a high degree of certainty from tests on the rheology of the mortar.

Progress with cement paste

Experimental challenges for testing cement pastes and slurries are the risks of slippage at the walls of the viscometer, sedimentation of the particles and plug flow. Depletion of particles at the viscometer surface can result in a thin (<1mm) layer of water which facilitates bulk flow of the sample, superimposed upon the shearing flow within the rest of the material. The result is an underestimate of the stiffness of the sample [27]. The slip can be avoided using a roughened surface and Mannheimer [28] showed convincingly that slippage reduced measured yield stress by 85%. This is supported by comparisons between smooth coaxial cylinders and a vane−in−cup apparatus: slippage in the former reduced the measured yield stress by 50% but oscillatory measurements at lower stresses were indistinguishable [29]. However, proof that slippage does not occur with roughened surfaces above the yield stress has been elusive. At the high water contents representative of concrete, the particles in cement pastes may separate gravitationally and centrifugally and this can cause errors. When measurement geometries include devices to keep the paste homogeneous the results are much more satisfactory. These include angled blades to lift the particles [30], recirculating pumps [31], blades with interlocking fingers [32] and more conventional mixers [33]. The problem of plug flow, when the shear stress does not exceed the yield stress everywhere in the sample and some part of the sample does not shear, was first raised by Tattersall and Dimond [34] but has never been satisfactorily resolved. They found that hitherto irreconcilable anomalies in breakdown measurements were explained when filming the flow in the gap of a coaxial cylinders viscometer revealed that a solid plug of paste formed and was either stationary (rough cylinders) or slid round slowly (smooth cylinders). No satisfactory explanation has ever been offered for this anomalous plug flow but its existence casts doubt on all experimental data where full shearing flow has not been confirmed visually.

Rheological Results of Cement Based Materials

It might be expected that the rheology of the more complex material, concrete, containing a wider range of particle sizes, would be more complicated than that of one of its constituent materials, cement paste, but in fact fresh concrete has proved to be simpler and considerable practical progress has been made with it and, more recently, with mortars.

Concretes

Much work has been done on the effects on the rheology of

Concrete: Rheology

Figure 3: Rheometers


concrete of mix constituents and their relative proportions, cement properties and admixtures and cement blending agents [1,17,20,21,35]. Concrete conforms to the Bingham model and does not show structural breakdown over the range of shear rates used in the test. Yield stress and plastic viscosity vary in a complex fashion with composition and this makes rheology measurement a versatile way of controlling the quality of fresh concrete production: tests carried out on the fresh concrete can show up changes in the mix composition which may have implications for the concrete’s hardened properties and performance in use [35]. With the recent advent of self−compacting concrete, characterised by a very low yield stress, it has been found that the thickeners used to prevent segregation in use by raising the viscosity of the water also change the flow curve from the normal Bingham behaviour to Herschel− Bulkeley type behaviour (see below) [26].

Mortars

Mortars undergo structural breakdown and measured data are sensitive to the previous shear history of the sample, but the equilibrium flow curve conforms to the Bingham model [25] The effects of composition are similar to those observed in fresh concrete and mortar tests can be used as small scale predictors of concrete rheology [26, also Banfill, unpublished].

Cement Pastes

There are qualitative and quantitative disagreements between the results for cement paste reported by different research workers. The flow curve has been reported to fit several different

mathematical forms, all of which indicate the existence of a yield stress:

(1)

(2)

(3)

(4)

(5)

(6)

(7)

Where A, B and C are constants.

Additionally the numerical values reported for the rheological parameters cover a very wide range, which cannot be wholly explained by variations in the materials used. It can only be accounted for by accepting that differences in experimental technique and apparatus of different workers have a much greater effect than has been generally realised. Differences in the shear history at the time of test, undetected plug flow and slippage at the smooth surfaces of a viscometer could all combine to give experimental variations as large as those reported. However, there is general agreement on two fun-damental qualitative aspects of the behaviour of cement pastes.

Material Cement paste, grout Mortar Flowing concreteSelf-compacting

concrete Concrete

Yield stress N/m2 10-100 80-400 400 50-200 500-2000

Plastic viscosity Ns/m2 0.01-1 1-3 20 20-100 50-100

Structural breakdown Significant Slight None None None

Figure 4: Rheology of cement paste, mortar and concrete

Concrete: Rheology


Concrete: Rheology

Comparison of Cement Pastes, Mortar and Concrete

Figure 4 shows that there is a trend in the rheological properties of cement−based materials, as quoted in the literature, which can be explained semi quantitatively by the presence of aggregate in the coarser grained materials. The flow properties of suspensions are governed by the interfaces between solid and water and, in terms of the surface area of contact, the dominant contribution is due to the cement−water interface. This is progressively diluted by the presence of aggregate. Thus, for example in one comparison, two cements which gave pastes whose rheological parameters differed by a factor of two produced concretes of indistinguishable flow behaviour [1].

The yield stress and plastic viscosity increase as the maximum particle size increases. This is because in a typical concrete at least 50% by volume is in the form of aggregate which is capable of withstanding the applied stresses without deformation: consequently the yield stress is higher, a point confirmed by the increase with increasing aggregate content in concrete [1]. The increased plastic viscosity is partly due to the increased interparticle contact and surface interlocking, as demonstrated by the fact that for two concretes with the same yield stress containing rounded and angular coarse aggregates, the plastic viscosity of the latter is higher. It is also partly due to the inability of the aggregate to be sheared: when an overall shear rate is applied to an imaginary concrete consisting of aggregate and paste 50:50 per cent by volume, the shear rate within the solid aggregate particles is zero and that in the paste is. This higher shear rate results in a higher stress and resistance to flow in the paste which in turn accounts for the increase in measured plastic viscosity of the bulk material.

In contrast, the yield stress and plastic viscosity of cement paste increase as the cement gets finer [44], which reflects the dominance of the water−cement interface in this system. Evidently the influence of particle size is a surface area effect in fine grained pastes and a simple volume effect in the coarser grained concretes. Perhaps further work on particles suspended in dispersions will suggest the particle size range where the

change from one influence to the other occurs.[45]

Models for Particular Situations

Pumping

Transport of fresh concrete by pumping through pipes to the point of placement has been used since the 1930s and is an obvious candidate for rheological study, to help select pumping equipment and conditions. Pipe flow of a Bingham material is well characterised and the variation of shear stress from a maximum value at the wall of the pipe to zero at the centre line means that a plug of solid unsheared material moves surrounded by a zone of shearing flow from which pressure−flow rate equations have been derived [1,7]. However, this assumes that the material is homogeneous, whereas pumped concrete actually forms a layer of paste which lubricates the wall and facilitates flow. Therefore the pumpability of a concrete is mainly governed by its ability to form and maintain this layer under the pumping conditions and an acceptance test has been developed [48]. In fact, practical problems with blockage of pipe work have meant that most pumping trials have had to be done at full scale, which is both costly and inconvenient.

Interactions at the Surface of Formwork

A related problem requiring a knowledge of friction at a concrete−wall interface is the pressure on formwork, which is lower than the equivalent hydrostatic pressure because of the yield stress within the material and the friction at the wall [50], but empirical predictions underestimate the actual pressures measured for modern highly fluid concretes. Friction between steel and fresh concrete was measured in a tribometer based upon moving a steel plate between opposed pressurized cylinders filled with concrete which exert a known stress normal to the surface [51,52,53]. Applying the coefficient of friction between steel and concrete determined in this apparatus enabled preliminary estimates of the formwork pressures exerted by fluid and self compacting concrete to be compared with those measured in full size formwork up to


12 meters high. This confirmed the complexity of the factors affecting formwork pressure and showed that it is about 25% less than hydrostatic for a self−compacting concrete, but the contribution of friction depends on surface roughness, concrete rheology and particle size distribution.

Vibrational Compaction

Vibration is the most popular means of compacting fresh concrete into formwork and around reinforcement and there is an extensive literature on the effects of such features as frequency, amplitude and acceleration, but several recent papers have significantly advanced understanding. Practically vibration removes the yield stress of fresh concrete which then flows under its own weight [54,55] and the important characteristic of the vibration is the peak velocity. The fluidity of vibrated concrete, defined as the reciprocal of its low shear rate viscosity, is proportional to peak Vibrational velocity up to a critical value, above which it is constant, and the viscosity of the vibrated concrete is proportional to the plastic viscosity of the unvibrated concrete [56]. This work enabled the effect of vibration to be defined phenomenologically but a more rigorous investigation was recently completed by Teixeira et al [57].

The Action of Superplasticisers

It has long been established that superplasticisers can have spectacular but sometimes unpredictable effects on the rheology of cement systems. The yield stress of cement and concrete are reduced to very low values by the dispersion of flocculated cement particles [1]. However, the progress of the hydration reactions causes stiffening (slump loss) and this can be a serious practical problem. Flatt and Houst proposed that there are three components of the behaviour of the superplasticiser added to the system [59].The first part is consumed by intercalation, coprecipitation or micellization within the hydrating cement minerals, forming an organo−mineral phase. The second part is available for adsorption at the surface of cement particles and is effective in dispersing the flocs, but the adsorbed amount is not easily measured since current analytical methods are based on the amount removed from solution and this cannot distinguish between admixture consumed in the first part and adsorbed on surfaces [60,61].

Conclusion

Rheology is important because of the scope it offers for characterising fresh cement paste, grout, mortar and concrete, and for understanding how they perform in practical applications. Without satisfactory fresh properties it is unlikely that the desirable properties of the hardened materials can be achieved. Their rheology is dominated by the structure that exists in the cement paste, but in mortar and concrete the structure has been partially or fully broken down during mixing. As a result they conform closely to the Bingham model and their behaviour

during pumping, vibration and in formwork can be explained by reference to that model. Reliable instruments for testing the coarser grained materials are available and experience in comparing the data is growing. In contrast there remain apparently conflicting results for cement pastes, which are probably due to the different experimental techniques used by different workers. The important effects of shear history, mixing energy and wall slippage on the results obtained in viscometers are only now being generally understood. Rheology can be optimized to ensure concrete performance

Reference

[1] Tattersall, G.H., Banfill, P.F.G. The rheology of fresh concrete, Pitman, (1983), 356pp

[2] Banfill, P.F.G. (editor) The rheology of fresh cement and concrete, Spon, (1991), 373pp.

[3] Bartos, P.J.M, Marrs, D.L., Cleland, D.J. (editors) Production methods and workability of concrete, Spon, (1996), 541pp.

[4] Skarendahl, A., Petersson, O. (editors) First international RILEM symposium on self−compacting concrete, Spon, (1999), pp.786.

[5] Nonat, A. (editor) Why does cement set? An interdisciplinary approach, 2nd International RILEM workshop on hydration and setting, RILEM Publications Sarl, (1997), 419pp.

[6] Barnes, H.A., Hutton, J.F., Walters, K. An introduction to rheology, Elsevier, (1989), 199pp.

[7] Barnes, H.A. A handbook of elementary rheology, Institute of Non−Newtonian Fluid Mechanics, University of Wales, (2000), 200pp,

[8] Banfill P.F.G., Kitching, D.R.Use of a controlled stress rheometer to study the yield value of oil well cement slurries, The rheology of fresh cement and concrete, Spon, (1991), pp125−136.

[9] Bombled, J.P. A rheograph for studying the rheology of stiff pastes: application to cement setting, Revue des Materiaux de Construction, no. 673, (1970), pp.256−277.

[10] Barnes, H.A., Carnali, J.O. The vane−in−cup as a novelrheometer geometry for shear−thinning and thixotropic materials, Journal of Rheology, vol.34. (1991), pp.841−866.

[11] Gregory, T. The measurement of early strength development in polymer modified cement pastes, 5th International congress on polymers in concrete, (1987), pp.205−208.

[12] O’Keefe, S.J. Rheological properties of polymer modified cement pastes, PhD thesis, Bristol Polytechnic, (1991), pp.232

[13] Banfill, P.F.G., Carter, R.E., Weaver, P. Simultaneous rheological and kinetic measurements on cement pastes, Cement and Concrete Research, vol.21, (1991), pp.1148−1151.

[14] Schultz, M.A., Struble, L.J. The use of oscillatory shear to study flow behaviour of fresh cement paste, Cement and Concrete Research, vol.23, (1993), pp.273−282.

[15] Keunings, R. A survey of computational rheology, 13th international congress on rheology, (2000), vol.1, pp.1.7−1.14.

[16] Banfill, P.F.G. Feasibility study of a coaxial cylinders viscometer for mortar, Cement and Concrete Research, vol.17, (1987), pp.329−33.

[17] Banfill, P.F.G. A coaxial cylinders viscometer for mortar: design and experimental validation, Rheology of Fresh Cement and Concrete, Spon, (1991), pp.217−226.

[18] Domone, P.L.J., XuYongmo, Banfill, P.F.G. Developments of the two−point workability test for high−performance concrete, Magazine of Concrete Research, vol.51, (1999), pp.171−180.

For complete list of the reference kindly view the digital edition.

Concrete: Rheology


Low Carbon Cements and Concretes in Modern ConstructionA John W HarrisonManaging Director, TecEco

There are some significant problems facing humanity including climate change, ocean acidity and food shortages, land degradation and water pollution by wastes. At the 2007 “Driving CO2 Reduction” National Conference held in Melbourne on 13 & 14 September 2007 M K Singh stood up and said “I want the cement industry to be the saviour of the world”. Concrete can be and those in the industry who take on this challenge will succeed. We must however think outside the square and develop new technical paradigms1.

Cement production was 3.4 billion tonnes in 2011 and the concrete produced with it roughly 28 billion tonnes. The annual carbon emissions from the cement in this huge material flow amount to roughly 2.9 billion tonnes of carbon dioxide, or 8.8%2 of total anthropogenic carbon emissions making cement a significant source of emissions. If other associated supply chain releases are included, significantly more.

China and India between them are now consuming 40 times more cement and concrete than the USA. India currently produces around 210 million tonnes of cement, second to China at around 2 billion tonnes (25).

There is a solution to global warming, salinity and many other global problems and it is potentially very profitable. TecEco Gaia Engineering utilises the huge flow of concrete to create an enormous CO2 sink and I will explain this as I go through the options and issues for cement and concrete in modern construction.

According to the British Research Establishment (BRE) we cannot address de-carbonation without changing the composition of cement and fuel derived emissions will diminish slowly for purely economic reasons (19). Papers describing numerous different binder formulations abound and I have written some of them.

The BRE are applying the wrong emphasis. The composition of Figure 1 - Predicted Global Cement Demand and Emissions (19)

Cements

A multidisciplinary analysis and review of low carbon cements and concrete is provided. Options are set out in a table and explained in the context of a wide range of disciplines and concepts. The paper takes a lateral thinking approach and calls for a change in mindset, teaching methods and the way standards are written so that the business model of cost cutting prevalent in the industry can change. It finds that significant de-carbonation will result not so much by changing the chemistry of existing cements or by developing new ones but by focussing on properties affecting lifetime energies and making CO2 and other wastes resources to manufacture synthetic carbonate aggregates and introducing carbon capture during manufacture or Portland and other hydraulic cements.

cement does not have change so much as the composition of concrete. Cement is only around 10% of concrete. The use of a high proportion of SCM’s coupled with synthetic carbonate aggregates made from flue CO2 and waste magnesium ions and other materials effecting properties3 would make concrete made with any hydraulic cement currently associated with emissions a very green material with net sequestration.

Mehta summarised some of the techniques used by architects for dematerialisation (15) but did not consider the effect of dematerialisation on lifetime or operational energies and TecEco have realised the potential of carbon capture during manufacture.

These and other alternatives are summarised with reference to Table 1 that follows. I then address the options, dealing with those that have been dealt with in the literature adequately more briefly than others that have not.

Alternative Binders

There are a large number of alternative binders and many options to improve the energy and emissions associated with their manufacture. Given necessary brevity and the fact that they have been dealt with extensively by others most of them are presented as Table 2 - Future Binder Contenders with


Differentiated Supply Chain Options on the next page. Data on production emissions with and without capture in most cases is included.

Noticeably the hydraulic cement group dominated by Portland cement is the largest and there are many variants providing high early strength, sulphate resistance etc. Many of the new cements are variants of a Portland cement theme.

Hydraulic Cements

There is significant potential for carbon capture with cements made with a calcination step such as most if not all hydraulic compositions. This potential is covered in more detail under the heading Alternative Manufacturing Processes.

Slag-lime or slag – Portland Cement (PC) cements. Slag is made from ground granulated blast furnace slag (GBFS) which is a waste and latently hydraulic. It can be used with activators such as Portland cement (PC), lime and/or reactive magnesia

(according to our patents) to make a cement4.

Calcium sulfoaluminate cements & belite calcium sulfoaluminate cements are low energy cements that can be made from industrial by products such as low calcium ( class F) flyash and sulphur rich wastes. The main hydration product producing strength is ettringite. Their use has been pioneered in China and more recently in the UK.

Calcium aluminate cements are hydraulic cements made from limestone and bauxite. The main components are monocalcium aluminate CaAl2O4 (CA) and mayenite Ca12Al14O33 (C12A7) which hydrate to give strength. Calcium aluminate cements are chemically resistant and stable to quite high temperatures.

Belite cements can be made at a lower temperature and contains less lime than Portland cement and therefore has much lower embodied energy and emissions. Cements containing predominantly belite are slower to set but otherwise have satisfactory properties. Many early Portland type cements such as Rosendale cement were rich in belite phases. (See http://www.tececo.com/links.cement_rosendale.php.)

Reactive magnesia blended with other hydraulic cements and Supplementary Cementitious Materials (SCM’s)5 Reactive magnesia (rMgO)6 is a powerful new tool in hydraulic cement blends. 15-30% improvement in compressive strength and greater improvements in tensile strength, much faster setting, better rheology and less shrinkage and cracking, less bleeding and long term durability have been demonstrated with 50 % replacement and more of PC by flyash and GBFS. We believe autogenous shrinkage has been solved.7 rMgO is an ideal

Figure 2 - Mehta’s Triangle (15)

Option Description Players Drivers BarriersStandards &

Guides

Alternative Binders Numerous and described in Table 2 - Future Binder Contenders with Dif-ferentiated Supply Chain Options

Scientists(CementChemists)

Sustainability andprofit.

Conservatism, out dated software. Prescrip-tion standards and approvalssystems (see http://www.tececo.com /sustain ability.permissions_rewards.php.)

A few guides anddraft standards (e.g. with Geopolym ers)

CO2 Capture duringManufacture

Reduce process emissions. Cements that involve calcination can be made without releases.

Scientists.TecEco andCalix.

Sustainability,carbon taxes.

Inability to think laterally. Fear of change Common sense!

Replacement of Portland cement by Limestone

Blending with Limestone with ce-ment to reduce net emissions has met with some success and is now incorporated into many standards. There are however issues.

Cementtechnol ogists

Economiccost/benefit,sustainability,Leed, GBC, r & d& procurementPolicies.

Buyer hesitancy. New standardsemerging becauseindustry driven.

Replacement of cement by SCM’s

Blended cements that contain a high volume of replacement materials (SCM’s) such as fly ash, slag ce-ment (gbfs), pozzolans, silica fume, rice husk ash etc. High replacement cement concretes often have im-proved properties such as rheology, less shrinkage, greater durability etc. The use of reactive MgO makes the use of higher proportions ofSCM’s possible.

Cementtechnol ogists

Economic cost / benefit,sustainability,Leed, GBC, r & d& procurementPolicies.

Conservatism, out dated software. Prescrip-tion standards and approvalssystems (see http://www.tececo.com/sustain ability.permissions_ rewards.php.)

Mix design methods.LCA & LCCA. New better software.

Cements


additive as it aids the dissolution of SCM’s and contributes positively to many other properties (See REPLACEMENT OF CEMENT BY SCM’S 7

Chemical Cements

Magnesium Phosphate Cements are chemical cements that rely on the precipitation of insoluble magnesium phosphate from a mix of magnesium oxide and a soluble phosphate. They include some of the oldest binders known8 and are potentially very green if the magnesium oxide used is made with no releases or via the nesquehonite (N-Mg route) which is part of the TecEco Gaia Engineering solution (See ALTERNATIVE MANUFACTURING PROCESSES. It would also be good if a way is found to utilise waste phosphate from feedlots9.

Carbonating Cements

Reactive Magnesia Based Carbonating Binders can, like lime, be used for full thermodynamic cycle binders such as carbonating mortars. Reactive Magnesia (rMgO) has the advantage of also taking on water of crystallisation so the solid produced for input mass ratio is higher than for lime based carbonating binders. rMgO can also be made without releases10.

Lime Based Carbonating Binders can, like magnesia, be used for full thermodynamic cycle binders such as carbonating

mortars. As water of crystallisation is not also taken up, the solid produced to input mass ratio is lower than for magnesium based carbonating binders which is a disadvantage.

Other Cements

Geopolymers are potentially very green but suffer from a number of fundamental flaws that will restrict their use and increase risk outside factory environments where they are currently being mainly used. They suffer from the nanoporosity durability flaw and the fact that water is not consumed in their setting with the result that making them fluid enough for easy placement is difficult.

Because geopolymers are nanoporous soluble aggressive agents can get into them and attack aggregates. What makes them risky to use is the variability of results obtained. The problem is that the amount of water added is critical - too much and they are insufficiently alkaline or too little and they cannot be placed. Getting over these problems has been the main area for research and some success has been achieved as geopolymer premix concretes are commercially available as at the time of writing in Australia for non - structural applications.

Sialites are a neologism for rocks made in a manner mimicking natural rock forming processes. 11 The technology is not new

Dematerialisation Innovative architecture and engineer-ing. More durable concretes.

Architects & Engineers

Economic cost / benefit,sustainability,Leed, GBC, r & d& procurementPolicies.

Prescription standards and approvals systems (see http://www.tececo.com/sustainability.permissions_rewards.php.)

Design codes, LCA & LCCA

Mix Optimisation Appropriate particle packing, better admixtures and use of brucite hydrates to release water for more complete hydration

Cementtechnolo-gists

Economiccost/benefit,sustainability.

Conservatism, inappropriate software. Prescription standards and approvals systems (see http://www.tececo.com/sustainability.permissions_rewards.php.)

Mix design methods. LCA & LCCA.New better software.

Product Differentia-tion and Specialisa-tion

Mineral composites other than concrete with just stone aggregate can improve sustainability. E.g. composites with a high “R” value

Materialsscientists

Economiccost/benefit

Inability to think outside the box. Fear of change

Standards, LCA & LCCA

Changing the emphasis

An emphasis other than on the bind-er to improve sustainability. E.g. Use of synthetic carbonate aggregate. A greater focus on properties having a high impact on lifetime energies.

Scientists Sustainability,economic cost/ben-efit. Technical merit

Inability to think outside the box. Fear of change. Technical issues (?).

Common sense!

The Right BusinessModel

Although not a technical matter the right business model is essential for progress to be made.

Consultants Profitability in achanging businessenvironment

Conservatism, standards and legislative environment.

The Right Framework to Operate in

Legislative restrictions and standards throughout the world are prescrip-tive in nature and this and a lack of training is holding back innovation. There is a strong need to throw away dogma for what it is and get back to science.

Scientists andConsultants

The need tochange

Conservatism, inability or unwillingness to change.

Table 1 - Ways to make Cement and Concrete More Sustainable.

Cements


PC Conventional .266 0.498 .764 .004 .760 Ordinary portlandcement

Most denseconcretes

Normal premix. No sup-plementary cementious or pozzolanic materials

PC Permeableblockformulation

.266 0.498 .764 .144 .620 Carbonated ordinary Portland cement blocks

Gas permeablesubstrate

No supplementarycementious orpozzolanic materials

42% PC 8% MgO 25% Flyash 25%GBFS

.199 .209 .408 .001 .407 Terniary mix withMgO additive.

Most dense concretes Faster setting and higher early strength

PC With capture .266? .266? .004 .266? Recapture duringcalcination.

Most denseconcretes

No supplementarycementious orpozzolanic materials

MgO 750-1000oC

Conventional .240 1.092 1.322 -1.092 .240 Eco- Cements sorel & magnesium phosphatecements.

Historic andconventional

Sorel and Mg phos-phate cements. TecEco Eco-Cement Force carbonated pure MgO (Cambridge University)

MgO <750oC Tec-Kiln(with capture)

.240 .240 -1.092 -.851 Eco-cement Brucite (MgO) boards

TecEco, CambridgeUniversity

Sorel and Mg phos-phate cements. TecEco Eco- Cement Force carbonated pure MgO (Cambridge University)

MgO <450oC MgCO3 .3H2OConventionalcalcination

.378 .007 .385 -1.092 -.706 Eco-cement concrete, pure MgO concretes

TecEco, Universityof Rome La Sapienza.

TecEco, University ofRome La Sapienza.

MgO <450oC MgCO3 .3H2O Tec-Kiln(with capture)

.378 -1.085 -.706 -1.092 -1.798 Eco-cement concrete, pure MgO concretes Novacem concretes?

TecEco N-Mg route TecEco

MgO <450oC20% PC 80% MgO

MgCO3 .3H2OTec-Kiln(with capture)

.369 -.743 .374 -.874 -1.248 Eco-cement concrete, pure MgO concretes Novacem concretes?

TecEco N-Mg route TecEco

Silicate route Novacem, Calix? After Klaus Lackner?

CaO Conventional .266 .785 1.051 .785 .266 Carbonating lime mortar Calera, British Lime Assn & many others

CaO CaCO3 Tec-Kiln(with capture)

.266 .266 .785 -.518 Carbonating lime mortar Calera, British Lime Assn & many others

Small net sequestration with TecEco kiln

C3S Conventional ? 0.578 >0.578 ? >0.578

C2S Conventional ? 0.511 >0.511 ? >0.511 Belite cement Chinese & others

C3A Conventional ? 0.594 >0.594 ? >0.594 Tri calcium aluminatecement

Increased proportion

C4A3S Conventional ? 0.216 >0.216 ? ? Calcium sulfoaluminate cement

Chinese & others

Geo polymers Flyash + NaOH 0.16 0.16 Geopolymer Alliance, Geopolyer Institute, University Melbourne

Cements

Cem

ents

Bas

ed o

n

Proc

ess

Proc

ess

CO2 (

tonn

es C

O 2 /

tonn

e ou

tput

)

Deca

rbo

natio

n CO

2(t

onne

s CO

2 / o

nne

outp

ut)

Tota

l Em

issi

ons

(ton

nes

CO

2 / to

nne

outp

ut)

Re –

abs

orpt

i on

(ton

nes

CO

2 /to

nne

outp

ut in

1 y

ear)

Net E

mis

sion

s (S

eque

stra

-tio

n) (t

onne

s CO

2 / to

nne

outp

ut in

1 y

ear)

Exam

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of C

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tTy

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Type

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t


Table 2 - Future Binder Contenders with Differentiated Supply Chain Options

Notes to Table 2 - Future Binder Contenders with Differentiated Supply Chain Options

1. http://www.tececo.com/files/spreadsheets/TecEco-CementLCA15Jan2013.xls, 2. Quillin, K. and P. Nixon (2006). Environmentally Friendly MgO-based cements to support sustainable con-struction - Final report, British Research Establishment., 3. http://www.geopolymers.com.au/science/sustainability Much of the thermodynamic data used in this table has been calculated using TecEco’s LCA Tool downloadable from the TecEco web site at http:// www.tececo.com/files/spreadsheets/TecEco-CementLCA15Jan2013.xls. The tool is in the form of an editable Excel spread sheet with no password that calculates energy and emissions to the factory gate only. Please send any corrections or suggestions to [email protected]

Cements

as it has been known by some for years how to solidify some fly ashes12 for example.

Alternative Manufacturing Processes

So far the industry response has mostly been to:

- Modernise and upgrade plant- Convert wet to dry plant processes- Convert shaft to rotary kilns- Install preheating, more efficient burners etc.- Improve grinding and other efficiencies.- Burn cheaper waste fuels. Burning waste materials with

high calorific value including timber, tyres, solvents, waste oil, animal fats, carbon waste from the aluminium industry (14) etc. has met with opposition in some countries because of the associated pollution.

- Reduce kiln temperatures and adjust the composition of cement accordingly with more aluminates and less alite13.

Capture During Manufacture

Significant sequestration can be achieved with capture of CO2 from kilns but the problem, which has not been solved yet for any form of sequestration, is what to do with it. One approach taken by Ramesh Suri of ACC in 2006/7 was to use Algae to consume the CO2 and produce bio fuel14. The problem lies in the sheer volume of CO2 produced and in making the sequestration process profitable. To meet this challenge TecEco is developing the N-Mg sub-process of Gaia Engineering that will sequester huge amounts of CO2 as synthetic carbonate that can be used as aggregate or as feedstock to make rMgO for its cements15.

Manufacturing Taking Advantage of Full Thermodynamic Cycles

The manufacture and use of Portland cement does not involve a full thermodynamic cycle and there is therefore little point in splitting the process into the endothermic16 and exothermic17 sub processes18 with one or two exceptions such as the manufacture of Syngas19.

It is a different situation with carbonating cements containing rMgO such as TecEco Eco-Cement. Recapture occurs with net sequestration possible if CO2 is also captured during calcination.

The real game changer and pinnacle of industrial ecology is our N-Mg sub process of Gaia Engineering that will produce large quantities of nesquehonite (MgCO3.3H2O) from waste

magnesium cations such as found in oil process water and bitterns and if this source runs out then from any brine containing Mg++(step 1).

Nesquehonite can then be calcined in our Tec-Kiln without releases (step 2) to make rMgO and the CO2 fed back into the process (step 1) to precipitate more nesquehonite.

The rMgO is then used as a binder to agglomerate massive amounts of nesquehonite to make synthetic carbonate aggregate or in TecEco Eco-Cements where it re carbonates (step 3).

The sequestration into synthetic carbonate aggregate without saturating the market for aggregate is sufficient to solve the global carbon problem as can be seen from the graph in Figure 6 below.

Eco-Cement binders according to TecEco Pty. Ltd.’s patents are ideal for agglomerating synthetic carbonate aggregate and from the graph the total sequestration possible given 2011 – 12 concrete production is over 22 billion tonnes which is around 2/3 of that needed to consume all anthropogenic emissions. As a matter of proportion, the particular cement used in the future becomes less important in relation to the total sequestration possible in concrete if the focus is also on aggregate.

TecEco call this breakthrough technology Gaia Engineering and it is a whole new way of thinking about industrial ecology and associated molecular flows. Gaia Engineering can profitably solve the problem of global warming and related problems such as ocean salinity and in doing so mitigate other problems such as pending food shortages15.

Replacement of Portland Cement by Limestone

Limestone is now routinely being added to Portland cement in varying proportions around the world and some success is being claimed mainly as a result of the improved particle packing.

Compton and Chandler make it clear that Portland cement is not the best possible additive however when they say “limestone is generally considered to be the poorest potential performer of the available suite of mineral additions and as such considerable effort is focused on developing Portland-limestone cements that achieve the current general purpose Portland cement performance” (2)

By adding limestone to Portland cement the industry may be


reducing the effectiveness of better and more suitable mineral additions such as rMgO and supplementary cementing materials (SCM’s) classified and ground as necessary to optimise particle packing. The SCM’s most commonly used include fly ash, ground granulated blast furnace slag and silica fume. Greater consumption of such wastes should be encouraged not compromised by a lowering of pH caused by the addition of limestone.

Hooton, Nokken and Thomas make it clear “One area where there is very little data is on the influence of limestone cements when used in conjunction with SCM’s. A question that needs to be answered is whether the use of limestone cements will reduce the replacement levels of SCM’s that can effectively be used.” (8) A theoretical analysis of the issue is on our TecEco and Gaia Engineering web sites20

At the present time “considerable effort is focussed on developing Portland-limestone cements that achieve the current general purpose Portland cement performance” (2). The goal is to find out just how much ground limestone can be added without compromising the properties of normal cements too much. The argument is that “limestone is the obvious choice for most cement manufacturers. It is a fundamental raw material at cement plants, and high grade limestone is readily available through its use as a supplement to amend raw meal chemistry. Limestone is abundant, pure and soft and makes an ideal mineral addition to be inter-ground with clinker and gypsum during cement milling” (1)

Given the theoretical evidence I present on the TecEco web site20 it is essential that cement that consists of clinker and gypsum in the right ratio with 5 % or less of limestone21 and nothing else continues to be made available for downstream blending as the economics of other mineral additions will change and many work much better such as rMgO which would otherwise compete with limestone for interstitial sites between cements grains.

Reactive magnesia blended with Portland cement and SCM’s results in significant improvements in properties as detailed in the next section. The problem is that best results are achieved

if it is fine enough to also fill interstitial sites between grains of Portland cement.

Replacement of Cement By SCM’S

Portlandite should not be left in a concrete because it is far too reactive. On the other hand consuming it all in the pozzolanic reaction also has technical issues. Mistakes are routinely made. Portland cement can be blended with pozzolans such as flyash which will consume Portlandite (Ca(OH)2) in the pozzolanic reaction. It is important however that not all of the Portlandite is consumed as calcium will start leaching from CSH22 if it is. As an alternative pH buffer we recommend the addition of rMgO which hydrates to brucite. The equilibrium pH of Brucite is approximately 10.5 and the pH of CSH

Figure 3 - Options for Portland Cement Manufacture

Figure 4 – Manufacture of MgO from Magnesite with and without Capture

around 11.2 (22). The pH of a CSH and Brucite assemblage in equilibrium will not fall much below 10.5.

According to the 12th plan in India around 10 million tonnes of GBFS are produced annually (10). In a recent article by Dr Yashpal Singh a figure of around 175 million tonnes of flyash was estimated for 2012 (20).

As most SCM’s are wastes their use is obviously more sustainable than digging up limestone, grinding it and adding it to cement and the paradox is that the use of limestone may compromise the use of SCM’s as explained in the previous section.

In many parts of the world builders are negative about the use of SCM’s because when added, concretes take longer to gain strength. Grinding cement finer to compensate for the negative chemical effects of limestone should have been considered as one way of making it more reactive with SCM’s such as flyash however this costs money. Another is to air classify pozzolans increasing their reactivity.

A way of accelerating the setting of mixes containing a high proportion of SCM’s is to include about 8-10 % rMgO as a proportion to PC in a mix. The reason is because when dissolved in water Mg2+ has a profound effect on the polarity of all species in solution that can be polarised. Of particular

Cements


dimensional distress in the proportions we recommend and does not act in the same way.

Dematerialisation

Dematerialisation is a technique for reducing the materials used by design and has been practiced for many years by some architects. As the subject was covered by Mehta extremely well (15) I will not elaborate. I refer readers to Figure 2 - Mehta’s Triangle on page 3 and comment that the impact of dematerialisation on lifetime energies must be considered.

Mix Optimisation

Mix optimisation is mostly an art and should be a science. It is not practiced widely enough and there are a lot of shortcuts in the software used. More often than not the “rule book” of prescriptive standards is religiously followed without questioning why. For example the use of gypsum to prevent flash set even when blended with SCM’s.

An important aspect is particle size and charge. It is not just what is in the mix but the particle size range of each component and how they fit together in 3D space. The smaller

Figure 5 - The N-Mg Sub-Process of Gaia Engineering

interest in relation to cement are water and its disassociation species.

Mg++ is a strong kosmotrope and strongly attracts electrons and brucite (including microcrystallites) has a strongly charged surface. Water dipole strength is increased and propagated in mix water. Water and its disassociation species such as hydroxides have reduced negative electron clouds around protons. Dissolution of SCM’s by proton wrenching occurs more readily, speeding up reactions and making their use more acceptable to builders24.

With about 8-10% of reactive magnesia (rMgO) added, 50% or more of (SCM’s) can be used and the resulting concretes still outperform ordinary Portland cement concretes25. This may not be achievable with higher additions of inter-ground limestone. Surely the goal should be to use SCM’s most of which are wastes.

The focus of my efforts in the past few years has been to find ways of making reactive magnesia much more cheaply so it will be blended with Portland cement and SCM’s as a matter of routine as it should be. Readers should however be aware that some manufacturers are actively selling magnesia for use in concrete without stating the reactivity in possible breach or circumvention of our patents and possibly dangerously as well because magnesia that is not highly reactive can cause

Figure 6 – Sequestration capturing CO2 from the air in the N-Mg process using TecEco’s Tec-Kiln Figure 7 - The Sequestration Potential for Synthetic Carbonates in Concretes

Assumptions

Tec-Cement concret with synthetic magnesium carbonate aggregate

Percentage by weight of cement in concrete 12.00%

Percentage by weight of rMgO in Tec-Cement 9%

Percentage by weight Ca (OH)2 in cement 29%

% of Ca(OH)2 in concrete that carbonates 10.00%

Proportion cement that is flyash and/or GBFS 20%

1 tonne Portland Cement 0.867 Tonnes CO2

Proportion concrete that is aggregate 80.0%

CO2 captured in 1 tonne aggregate 1.084 Tonnes CO2

Net CO2 sequestration 1 tonne rMgO (N-Mg route, 1 complete recycle)

1.794 Tonnes CO2

CO2 captured hydration and carbonation of tonne Cao (in PC) 0.785 Tonnes CO2

Cements


Cements

the average size of each addition the more particle charge becomes important26.

What is or is not included should be carefully considered with end use in mind and many changes are suggested in this paper. RMgO for example accelerates first set because it goes negative at the surface as the pH rises as demonstrated in Figure 10 above/

It is important to realise that packing considerations apply to all components in a mix, not just the cement and that small component substitutions can make a big difference to pro-perties and the amount of cement required to achieve a given strength. Less cement for a given strength is more sustainable.

A pioneer in this field is de Larrard from France (13)(12) and the TecSoft27 project at TecEco has been initiated to implement some of his math.

Mix optimisation is also too focussed and should be better connected with need as discussed in the section with the title Product Differentiation and Specialisation.

Product Differentiation and Specialisation.

It has been said in the industry that “all that is grey is great, all we make goes out the gate”. Significant amounts of energy would be saved with specialisation for differentiated market niches. Market penetration would also increase with the development of new concrete product.

With rising energy costs and an urgent need to improve sustainability additions to cementitious composites that improve lifetime or operating performance are an import part of a profitable future. Many waste streams can offer a wider range of properties for purposes such as thermal insulation or weight reduction. With the use of rMgO any toxics are encapsulated as well as immobilised and bonding to alternative included materials such as agriculture or domestic wastes is dramatically improved as are fire resistant properties. Concretes with high thermal mass for heat retention and

concretes with greater elasticity and plasticity for road pavement are other examples.

New mineral composites incorporating waste streams with low thermal flow characteristics will be in high demand (e.g sawdust blocks) in the future and will drive this differentiation. Cementitious composites such as concrete can take a lead role in reducing lifetime energies and become part of the solution instead of the problem.

Changing the Emphasis

It is essential to think of concrete not just cement as each component has a role in the performance at every stage. It we think whole of concrete as recommended by Ken Hover (9) and many others then it becomes much easier to understand the material and issues concerning it such as sustainability.

There is too much emphasis on strength and not enough on durability and properties. As discussed under the heading Product Differentiation and Specialisation. Concretes with a wider range of properties such as low conductance or light weight could play a major role in reducing the lifetime or operational energies of structures.

About seven years ago now I realised that aggregate is 80% or more of most concretes and therefore represented an opportunity to sequester huge amounts of carbon dioxide as synthetic carbonate in our Gaia Engineering process explained in this document under the heading Capture during Manufacture and in a lot more detail at www.gaiaengineering.com.

The Right Business Models

For a long time a cost cutting model has dominated the efforts of players in our industry however if we are to move forward and fulfil the potential of solving many of the world’s problems this will have to change. New innovation based business models will have to be adopted.

Most governments have realised that innovation is important and in relation to this some interesting statistics are coming

Figure 8 - Gaia EngineeringFigure 9 –The Mg++ ion drags electrons to it exposing more electro positive protons23


out of the Department of Industry, Innovation, Science, Research and Tertiary Education in Australia that conclusively demonstrate that innovative companies perform better (See Figure 11)

trading the industry should also consider using standards, modified as suggested, as a way of providing benchmarks for minimum embodied energy and emissions. Existing standards could be relegated to a role as guides.

Legislative Frameworks fall into the same trap as standards with too much restriction although some governments such as my own here in Australia pay lip service to the need for change as evidenced by renaming departments with the word “innovation” included. The way forward surely lies in better education and training and the conversion of standards to guides. The concrete industry produce and place the most used and important materials in construction yet in many countries no qualifications of even basic training are required by those practically involved. This deplorable situation must improve and hopefully the training is not rote but such that it awakens minds to the possibilities.

The Right Policies to Support Research and Development to Improve Sustainability

Few governments have ever managed to get the mix of stick, incentive and procurement right or even remotely efficient. The level of control continues to increase without the leadership to drive it. As Lord Stern made clear in his review of the economics of climate change (23) there are huge opportunities for emissions reduction in building and construction, not just in reducing the embodied energy and emissions of the materials we make but by changing the way we design structures and the way we utilise the materials we use to build them.

The biggest problem is that governments do not follow their policies through the supply chain. It is no good supporting the Research and development of for example carbon capture methods without making sure there are financial sticks and/or incentives to encourage the changes required to the process of making cement.

Back to Science

There are too many non - scientific dogmas in the cement and concrete industry.

At TecEco we have relied on science to explain what we

Figure 10 The Change in the Surface Charge of Metal Oxides with pH. (21)

Companies in the construction sector including the concrete industry do not spend much on research as shown in Figure 12. The main reason why is because our industry is bound by a framework of standards, legislation and conservatism that has resulted in low margins. It is no wonder cost based business models prevail.

The Right Framework to Operate in

The concrete industry in most countries operates with a restrictive framework of standards and guides and supporting legislation that breeds conservative managers who do not innovate and a cost cutting business model. Our engineers are taught to rely on out of date dogma not rely on science or their common sense. Change must occur if we are to move forward on sustainability and take advantage of emerging opportunities for carbon trading. Given the lack of training and education at the base level this will be a difficult challenge.

An Inappropriate Permissions and Rewards Systems

At Concrete Solutions 09 (6) I spoke about the tremendous potential for players in the concrete industry to make money as a result of inevitable change yet many barriers still exist as I have discussed. One of the greatest remains our ill-conceived permissions and rewards systems28 designed with the false notion that they protect the status quo. There is a rising current of change that I helped initiate and prescriptive standards and inappropriate legislation is getting in the way.

Standards for cement and concrete are still prescription based in most countries and even the Green Building councils in the US, Australia and elsewhere have fallen into the same trap of locking in the status quo and stifling innovation. Prescriptions should be confined to guidelines on how to do things. Standards should set out minimum performance requirements in a chosen range of categories. In order to take advantage of carbon Figure 11 - Increases in Business Performance by Innovation status 2008 – 9(4)

Cements


observe. By using science rather than applying dogma it is easier to understand what is really happening and see a way forward for improvement.

Conclusions

It is time cement companies adopted a business model that connects innovation with profitability. In many other industries profitability is understood to be a function of research and development and that the regular release of new technically improved product generates revenue growth. Progress towards de-carbonation will be slow unless this occurs.

Engineers should be taught science not dogma and standards need to be rewritten as benchmarks not prescriptions. The legislative framework in many countries needs a broom through it and governments need to realise that the concrete industry could in fact be the saviour of the world as M K Singh bravely arose and said29.

All this can only be achieved if there is a change in mindset, a strong desire to move the agenda towards greater sustain-ability forward, a willingness to throw away the rule book, a whole new modern scientific lateral thinking.

Last century there were many different mineral cement contenders and Portland cement was only one of them. By 1900 it emerged as the dominant formulation. In the future a new differentiation of product based on properties will probably occur and this will be a good thing as it will result in greater margins and product more suited to particular use such as, for example, the development of binders suitable for utilising wood waste to make insulating composite products for the outside of buildings including rMgO for fire retardation. The major barrier to implementation will be the mindset of our managers and out dated prescription based standards.

John Phair said at the conclusion to his paper that “Further developments and new techniques must continue to be introduced into the cement and concrete industry. Green chemistry will play a significant role in facilitating a holistic industrial ecological approach to cement from a fundamental level. This will provide distinct alternatives to an OPC dominated

cement market.” (18). There will be greater diversification and alternatives but in my view the market will still be PC and PC derivative based but only if new formulations and production processes that include capture are implemented such as our N-Mg process which produces synthetic carbonate aggregate.

Chemistries that fix known “sleeper” issues such as Portlandite content will need to be embraced including our own Tec and Eco-Cement technologies. With the use of a high proportion of SCM’s, particularly if pre - blended the addition of gypsum and limestone are questionable. As Paul Hawken makes clear in “The Ecology of Commerce,”concrete that is more durable is more sustainable(7).

By adopting a whole of concrete approach there is much more scope for sustainability. The obvious target in construction is to lower lifetime or operational energies so we should be thinking properties as well as strength and durability and this will require product diversification.

Paradigm changes such as our Gaia Engineering project will modify the supply chain to focus on carbon capture and then use the CO2 produced to manufacture synthetic carbonate aggregate.

A new approach to cement and concrete formulation cannot evolve without the realisation that concrete can be part of the solution not the problem.

Portland cement concretes are the most ubiquitous and will probably remain so because immense economies of scale make them relatively cheap and sustainability problems can be overcome by carbon capture with CO2 used as synthetic carbonate aggregate and a reformulation excluding limestone and gypsum and including reactive MgO and a mix of classified fine and normal SCM’s in high proportion.

Footnotes

1 The technology paradigm defines what is or is not a resource.2 The Chinese government (16) estimate that 861 kg (net) of CO2 are

emitted for every tonne of Portland cement clinker produced. The production of 3.4 billion tonnes cement would results in emissions of 2.9 billion tonnes CO2. Global emissions are around 33 billion tonnes (17) so the current contribution of cement production globally as 2.9/33 x 100%, or 8.8%.

3 The inclusion of additions that introduce properties such as thermal capacity and lower heat transfer rates have significant scope to reduce lifetime or operational energies. Many of these additions can be sourced from waste streams.

4 We include for convenience GBFS in our definition of hydraulic cements in our patents in most countries and claim the right to reMgO GBFS mixes. Beware of infringements. We do not think reactive magnesia (rMgO) a good activator but it is an excellent additive and facilitates more rapid dissolution

5 The NRMCA’s CIP 30 – Supplementary Cementitious Materials includes pozzolans which by themselves do not have any cementitious properties and other materials such as ground blast furnace slag that do.

6 The magnesia must be reactive and be wary of imitations that are notFigure 12 – Australian R & D Expenditure by Industry Size (Bubble diameter) Employment and Gross Value Added. (3)

Cements


7 Much more detailed information is available in the TecEco web site in the downloads area.

8 Dung + MgO in ancient Indian stupas.9 Thereby solving an environmental pollution problem.10 See under the heading Alternative Manufacturing Processes.11 The name Sialite is attributable to Dr Henghu Sun and others of the

Pacific Resources Research Center in California, in collaboration with Tsinghua University in Beijing; see (24) for a description.

12 Depending on the composition13 Pers comm. WA (Tony) Thomas – Chief Engineer Concrete, Boral

Construction Materials14 http://www.tececo.com/files/newsletters/Newsletter64.htm15 See www.gaiaengineering.com home page and some of the latest

movie downloads at http://www.gaiaengineering.com/movies.php16 Calcination of limestone.17 Reaction of quicklime (CaO) with clays, shales etc to produce clinker.)18 More processes result in more process energy.19 Dr Sheila Devasahayam at the SMaRT Centre, School of Materials

Science and Engineering, The University Of New South Wales, is researching pyroprocessing CaCO3 with an additive to produce Syngas

20 In the technical areas of www.tececo.com and www.gaiaengineering.com under the heading “Ground Limestone in Portland Cement - A Good Idea or Lost Opportunity”.

21 Preferably none at all22 Properties will change and CSH is thought to become more brittle (5).23 Water is in equilibrium with hydroxide and hydronium ion ions. When

acidic or basic compounds are dissolved the equilibrium is pushed towards more hydronium or hydroxide ions respectively. e.g.: H2O <=> H3O+ + OH The hydronium ion is highly solvated and H5O2 +, H7O3 + and H9O4 + are increasingly accurate descriptions of the environment of a proton in water. Chemists represent a hydronium ion as just a hydrogen ion (H+, as in the figure) in place of H3O+

24 See the web page Reactive Magnesia - A Theoretical Explanation of Properties in the technical area at www.tececo.com

25 See Presentation 50 at http://www.tececo.com/document.conference_presentations.php

26 It is not well known that electrostatics plays a significant role in the setting of concrete (11)

27 See www.tecsoft.com.au28 See also http://www.tececo.com/sustainability.permissions_rewards.php29 See Introduction

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2 Compton M, Chandler J. Elevated limestone mineral addition impacts on laboratory and field concrete performance. Concrete in Australia. 2012, 38(1):27 - 33.

3 Department of Industry, Innovation, Science, Research and Tertiary Education (Australia). Australian Innovation System Report 2012 [Internet]. Department of Industry, Innovation, Science, Research and Tertiary Education (Australia); 2012. Available from: http://www.innovation.gov.au/Innovation/Policy/AustralianInnovationSystemReport/AISReport 2012.pdf

4 Department of Innovation, Industry, Science and Research (Australia). Australian Innovation System Report 2011 [Internet]. Department of Innovation, Industry, Science and Research (Australia); 2011. Availablefrom:http://www.innovation.gov.au/Innovation/Policy/Australian Innovation System Report/ AISR2011 /wp-content/uploads/2011/07/Australian-Innovation-System-Report-2011.pdf

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7 Hawken P. The Ecology of Commerce. New York: Harper Collins; 1993.

8 Hooten, R. D, Nokken, M, Thomas, M.D.A. Portland-Limestone Cement: Stte-of-the- Art report and Gap analysis for CSA A 3000. University of Toronto for the Cement Association of Canada; 2007.

9 Hover, Kenneth C. Concrete Design and Construction from the Inside Out. In: Concrete in the Third Millenium. Brisbane, Australia: Concrete Institute of Australia; 2003.

10 Indian Bureau of Mines. Indian Minerals Yearbook 2011 [Internet]. 2011, [cité 2013 janv 15] Available from: http://ibm.gov.in/IMYB%202011_Slagl.pdf

11 Labbez C, Nonat A. The Cement Cohesion: an Affair of Electrostatics. In: Iutam Sympiosium on Swelling and Shrinkage of Porous Materials. Petropolis, Brazil: 2007.

12 De Larrard F, Sedran T. Mixture-proportioning of high-performance concrete. Cement and Concrete Research. 2002, 32(11):1699.

13 DE LARRARD F. Concrete Mixture Proportioning: A Scientific Approach. E & FN Spon; 1999.

14 MCGRATH TM. Sustainable cement and concrete. Concrete in Australia. 2012, 38(1):15-15.

15 MEHTA PK. Global Concrete Industry Sustainability. Concrete International. 2009, Vol 31(2):4.

16 MEP. Technical Requirements for Environmental Labeling Products: Low-carbon Cement (Discussion Paper) [Internet]. Ministry of Environment Protection, China; 2012. Available from:http://www.mep.gov.cn/gkml/hbb/bgth/201112/W020111208396803781008.pdf (in Chinese)

17 PETERS GP, MARLAND G, LE QUERE C, BODEN T, CANADELL JG, RAUPACH MR. Rapid growth in CO2 emissions after the 2008-2009 global financial crisis. Nature Clim. Change. 2012, 2(1):2-4.

18 PHAIR JW. Green chemistry for sustainable cement production and use. Tutorial Review. 2006,

19 QUILLIN K. Low-CO2 Cements based on Calcium Sulfoaluminate [Internet]. Available from: http://www.soci.org/News/~/media/Files/Conference%20Downloads/Low%20Carbon%20Cements%20Nov%2010/Sulphoaluminate_Cements_Keith_Quillin_R.ashx

20 SINGH, YASHPAL. Fly Ash Utilisation in India [Internet]. [cité 2012 janv 15] Available from: http://www.wealthywaste.com/fly-ash-utilization-in-india

21 SMALL RJ, PETERSON ML, ROBLES A, KEMPA DK. Using a buffered rinse solution to minimize metal contamination after wafer cleaning [Internet]. MicroMagazine.com. 2005, Available from:http://www.micromagazine.com/archive/98/01/small.html

22 SPENSE, ROGER FP. Chemistry of Cement Solidified Waste Forms. In: Chemistry and Microstructure of Solidified Waste Forms Symposium. Oak Ridge: D. Lewis Publishers; 1992. p. 1 - 39.

23 STERN N. The Stern Review on the economics of climate change [Internet]. 2007, Available from: http://www.hmtreasury.gov.uk/independent_reviews/stern_review_economics_climate_change/stern_review_report.cfm

24 SUN, HENGHU, JAIN, R, NGUYEN, K, ZUCKERMAN, J. Sialite technology: Sustainable alternative to Portland cement. Clean Technologies and Environmental Policy. 2010, 12503 - 516.

25 USGS. Mineral Commodity Summary - Cement [Internet]. 2012, (2012): Available from: http://minerals.usgs.gov/minerals/pubs/commodity/cement/mcs-2012-cemen.pdf

Cements


An Insight into Liquid Floor Densifiers

In many cases, the best floor-covering is nothing. Leaving the concrete slab exposed and finishing it as the floor can be economical, aesthetically successful, and sustainable.

It is so durable that it may well be the ‘ultimate’ floor – the last floor that will ever need to be installed in the space. The key to optimizing the performance of concrete as a floor is densifiying. Densifiers make concrete floor more durable, and abuse-resistant. Densification eliminates wear - induced powdering of the surface known as dusting. It makes the surface less permeable to liquids, improving stain-resistance and making cleaning easier and more eco-friendly.

A dilemma sometimes faced by concrete floor contractors and owners is what to do about a recently installed or existing industrial floor that has a weakened surface. After concrete has been Installed, how can contractors most effectively correct the dusting, poor abrasion resistance, and high porosity that characterize such a surface? (Weakened floor surfaces are typically caused by adding excess water to the concrete, poor

finishing techniques, and a poor-quality cure.) Obviously, removing and replacing the concrete is not a favorable option, because it is disruptive, labor-intensive, and costly. A simpler, less expensive option is to use a liquid chemical treatment to harden and densify the concrete surface. Refer Figure 1 for a hardened concrete floor.

Need for Densifier

New or existing concrete can be improved by densification. It transforms concrete into a more practical flooring solution, and is applicable to a wide variety of projects. Owners of retail, commercial, and institutional environments are opting to expose the concrete floor instead of covering it. Densification expands appearance finishing options, enabling the success of finishing techniques such as burnishing and polishing. A densified, diamond polished concrete slab can achieve a high gloss that can rival the look of natural polished stone at a fraction of the cost. A slab burnished to a sheen after den-sifiying can achieve almost as attractive a result, but faster

Ankita Adhikary

Concrete Flooring: Densifiers


and even more affordably. Combined with techniques like dyeing, staining, or grinding to expose aggregate, a broad palette of visual options are available for taking concrete beyond a featureless gray surface. The cost of densification and polishing is competitive with the least expensive floor coverings available. Even with the added expense of coloring, it is still more affordable than the lowest end carpet or stone floors. Densifiers are also used where appearance is not a concern because of the improved performance of the slab. As it is easy to keep clean and safe, the densified floor is suitable for warehouses, factories, and maintenance facilities such as automobile service bays. Chemical densifiers (also called hardeners or densifier / hardeners) are a small investment that leads to long-term savings and improved performance for decades.

About Liquid Densifier

Liquid floor hardener formulations vary from manufacturer to manufacturer, but they usually contain inorganic compounds that undergo a series of complex chemical reactions with the available lime in mature concrete. Some liquid hardeners also contain special proprietary ingredients to make treated floors more resistant to chemical attack and wear and to improve the aesthetics of the finished surface. Unlike solvent-based, membrane forming sealers, which usually contain resins and hazardous solvents, most liquid floor hardeners contain inorganic compounds that are water soluble and comply with today’s environmental, health, and safety regulations. Wetting agents (surfactants) are usually added to a liquid hardener to help the product penetrate the pores of the concrete substrate. The efficiency of the floor hardening treatment increases with the depth of penetration, which usually ranges from 1/8 to 1/4 inch.

Though manufacturers of liquid floor hardeners sometimes use the word “sealer” to describe their products, a liquid hardener is actually a treatment, not a surface coating like most membrane - forming organic sealers. A drawback to applying a membrane to concrete is that the membrane tends to wear away in high-traffic areas. Not only is the worn coating unattractive, the unprotected concrete is susceptible to chemical attack by acids and caustics. Liquid floor hardeners not only protect concrete surfaces, they protect the concrete down to the depth of penetration. Also, the hardeners don’t form a coating or membrane on the concrete surface, so they don’t scratch; peel, show tire marks, or require recoating.

Working Principle of Liquid Densifier

As soon as a liquid floor hardener is applied to a concrete substrate, a chemical reaction takes place between the inorganic compounds and lime (whether hydrated or unhydrated) in the pores of the concrete matrix. The primary product of this reaction is a mixture of dicalcium and tricalcium silicate compounds, which hydrate (react with water) even further to produce a chemical compound called calcium silicate hydrate, or tobermorite gel. The ultimate strength and binding properties of hydrated Portland cement are primarily due to the presence of tobermorite gel in the concrete matrix. Therefore, liquid hardeners increase concrete strength by increasing the concentration of tobermorite gel. Liquid floor hardeners can also increase concrete’s density.

When the tobermorite gel forms in the concrete pores, its crystalline growth effectively blocks voids in the concrete, decreasing the pathways for moisture movement. Since chemicals attack concrete by penetrating the matrix, the presence of an insoluble gel in the substrate’s pores and on its surface greatly increases the concrete’s chemical resistance. In addition to the benefits of strength gain and chemical resistance, a liquid hardener can enhance the beauty of a troweled floor by giving it a polished look. This high sheen results when the treated floor is polished by mechanical means. Floors treated with liquid hardeners will not dust when abraded or polished.

The drawing below shows a cutaway view of a rough, porous floor prior to floor- hardener treatment. When light strikes the

Figure 1: A typical hardened concrete floorFigure 2: Working Principle of concrete hardened floor versus untreated concrete floor



irregular surface, the light is reflected in all directions. This scattering of light makes the concrete surface appear dull. After a floor has been treated with a liquid hardener, the pores are filled with tobermorite gel and the concrete surface can be polished smooth. When light strikes this smooth surface, it is reflected uniformly, so the floor shines. Refer Figure 2 for how concrete hardened floor works as compared to normal concrete floor.

Application Tips of Liquid Densifier

Ideally, concrete floors should be treated with liquid floor hardeners at least seven to 14 days after placement, or after the cement has had sufficient time to hydrate. Cement hydration increases the amount of available lime in the concrete, thus increasing tobermorite gel formation. In addition, this waiting time allows the pores of the concrete to dry, so the liquid hardener can penetrate the concrete surface rather than merely lying on it. It is much easier for a liquid hardener to displace air than water in the concrete voids. True liquid floor hardeners should not be applied to fresh concrete at the time of initial cure, because the concrete is still saturated with moisture. This saturated condition prevents the hardener from penetrating the surface. Also, liquid hardeners should not be applied as curing compounds, since they do not meet the requirements of ASTM C 309, “Standard Specification for Liquid Membrane- Forming Compounds for Curing Concrete.” Always check the manufacturer’s recommendations for when to apply its product.

The type of cure used on new floors prior to application of chemical floor hardeners is very important. Liquid hardeners must penetrate the concrete surface to undergo the chemical reaction that imparts density and hardness, but they can’t penetrate a membrane forming curing compound. If a curing compound has been used, be sure to remove it before applying a liquid hardener. ACI 302.1R- 89, “Guide for Concrete Floor and Slab Construction,” recommends moist curing floors that will later be treated with a liquid floor hardener or other surface treatments. Surface preparation of the floor before hardener application is also important. The surface must be

thoroughly cleaned to open the concrete pores and allow for hardener penetration. Typical cleaning methods include chemical cleaners and high-pressure water. Some chemical floor hardeners contain magnesium fluosilicates, which are low- grade toxic chemicals. When applying these products, be sure to wear protective clothing such as rubber gloves, boots, and goggles. Typically, fluosilicate hardeners are supplied in concentrated form and must be diluted with water before application.

Non fluosilicate floor hardeners, that are more commonly used today are colorless, odorless, biodegradable, and VOC- compliant. Many manufacturers offer 10- year warranties with these products. When using a non fluosilicate floor hardener, apply a slight flood coat to the concrete surface, covering about 200 square feet per gallon. Next, scrub the material into the surface with a stiff-bristle broom or janitorial floor- scrubbing machine for 15 to 30 minutes, until the product begins to gel or become slippery. Wet the material lightly with a water spray, and then rework it into the surface for another five to 10 minutes. After this process, rinse the floor and remove any excess material with a mop or squeegee. This final step is important, because residue is more difficult to remove if it is allowed to dry.

Application Areas of Liquid Densifier

Typical applications for liquid hardeners include floors in warehouses, industrial plants, shopping malls, stores, schools, food-processing plants, and hospitals. Installation costs vary, depending on the required surface preparation and size of the project. The degree of surface hardness and density that can be achieved with a liquid hardener depends on the quality of the concrete surface. Liquid hardeners can improve the abrasion resistance and reduce the dusting of a lower-quality concrete floor. On higher-quality concrete surfaces (those with



a lower water-cement ratio and denser finish), the need for a chemical floor hardener diminishes.

Many manufacturers claim that liquid floor hardeners improve the chemical resistance of a concrete floor. This is true to a degree, but one should carefully examine the chemical- resistance requirements of the floor before proceeding with hardener application. Though liquid floor hardeners improve the chemical resistance of a concrete surface, they do not make the surface 100% chemical resistant. Unfortunately, liquid hardeners are sometimes sold as chemical-resistant products meant to replace truly chemical-resistant coatings, such as two-component aliphatic urethanes. For floors exposed to high levels of chemicals, consider using a chemical- resistant coating instead of a liquid floor hardener.

Types of Liquid Densifier

Chemical Hardeners (Densifiers) include three basic categories of chemicals: silicates, silicinates and silica:

A. Silicates - penetrate and harden. They are not good sealers. Disposal of the waste material is currently an issue.

A.1 The oldest is Magnesium Fluorosilicates, which have been around since 1905. This type of product requires multiple applications with varying rates of dilution.

A.2 Sodium Silicates Developed initially in Germany in the 1930’s. Application of the product requires that it be applied at an average of 200 square feet per gallon, spread and worked until the surface tension is broken, mist with water, allowed to gel a second time and then rinsed and wet vacuumed to remove.

A.3 Potassium Silicates. The main difference between the sodium silicates and potassium silicates is sodium is

more prevalent in the North American and potassium is predominate in Europe.

A.4 Lithium Silicates. Lithium silicates were developed to combat Alkali Silica Reaction (ASR). ASR is more prevalent in exterior applications where there is a constant source of water. Lithium silicates are less susceptible to solubilization than sodium or potassium. One of the byproducts of this particular silicate is its ability to reduce sweating on slabs.

B. Silicinates - excellent sealer, poor hardening characteristics. Real world typical life expectancy is 18 to 24 months, and then it should be reapplied. Disposal of the waste material is currently an issue.

- Silicinates are applied the same way silicates, spray, scrub, mist, rinse, and vac.

- Silicinates can offer increased abrasion resistance over silicates in the short term due to the coating effect of the silicinates.

- Silicinates are either potassium or sodium.

C. Silicas – are the newest and most promising of the chemical hardeners:

- Silicas are applied simply by spraying them on the surface of the slab and allowing them to dry. The surface should be clean and void of any curing compound. Application rates are between 400 to 600 square feet per gallon.

Some Photographs of Densified concrete floor (Reference: Densification for High Performance Floors, By Steven H. Miller, Concrete Tech. Today – Densifying Concrete)



- Silicas increase abrasion resistance over silicates or silicinates by up to twice as much

- Silicas do not contribute to ASR

- Silicas do not raise the pH of the concrete since the product is neutral 6.5.

- Silicas have the highest increase in abrasion resistance

- Silicas have reduced application and labor costs

- Silicas have no hazardous waste to remove or dispose

- Silicas do not contribute to silicosis and are not carcinogenic unlike silicates which do contribute to silicosis and are carcinogenic

- Silicas will not contribute to sweating or efflorescence

- Silicas performance is not contingent on dwell time unlike silicates or silicinates

In a nut shell there are features and benefits to each of these types of chemical hardeners. The upside for the silicates is that they harden better than silicinates, Silicinates seal better than silicates. Silicates have been directly linked to silicosis. Silicates and silicinates have been tagged as carcinogens. Silicates and silicinates must be disposed of as hazardous material. There is significant research that documents the ill effects of sodium, potassium silicates and silicinates on reactive aggregate in concrete. Currently the best technology for chemical densifiers is amorphous silica.

Advanced Densifiers

A new generation of densifiers based on colloidal silica is replacing older densifiers based on chemical compounds called silicates. Colloidal silica performs the same functions, but makes the densification process itself faster, easier, more affordable, and more sustainable. It is competitive in material cost, and reduces time and labor expenses for densification by as much as 75%. Colloidal silica has also been used to rescue slabs that have not responded to silicate treatments.

The first generation of concrete densifiers was generally made from silicate compounds such as sodium silicate and potassium silicate; this range of silicate based compounds was later broadened to include lithium silicate. All these silicate compounds are highly caustic, with a pH of 11 to 12, similar to the alkalinity of lime itself. Sodium- or potassium-silicate application is also time consuming and labor-intensive—the chemical has to be worked with a broom for an hour to help precipitate silica and scrub it into the slab surface. The process leaves behind a caustic, gelatinous slurry that has to be scrubbed off thoroughly and disposed of. (Sodium hydroxide, also known as lye, is a by-product of sodium silicate densifiers and is sometimes present in their residue.) Applicators often try to neutralize the slurry with other chemicals before disposal, costing additional time and money. Many silicate-based densifiers require an overnight curing period before polishing

can begin. Inadequate removal of sodium- and potassium-silicates often leads to concrete discoloration by salt deposits – a problem known in the industry as “whiting.” The only way to remove whiting is to regrind the slab, an expensive and time consuming process. Whiting can also be a danger with lithium silicates if the material is over-applied, tempting some applicators to under-apply the product and ultimately fail to adequately densify the floor. The high pH of silicate materials also makes them dangerous and unpleasant for applicators to handle, especially during the removal process.

With increasing restrictions on disposal of caustic substances and construction waste, properly getting rid of the silicate slurry has become an issue for applicators to deal with. The new water-borne colloidal silica product, available from Lythic Solutions (www.lythic.com) under the trade name LythicTM Densifier, eliminates these problems. It is made from 5-nanometer diameter amorphous silica particles, mechanically suspended in water rather than chemically tied up in a compound. Colloidal silica is more immediately available for reaction in concrete than a silicate compound – the molecule has more chemically reactive sites, and the greater pH difference between colloidal silica and lime makes the reaction begin quickly, within minutes after application. The particles consist of nearly pure silica, and the colloid’s extremely low sodium content eliminates the danger of whiting. There is no danger of over-application. The simplified application process reduces risks and cost, and only minimal training is required. There is no removal step (as required with silicates) and consequently no caustic slurry to dispose of – only a small amount of dry powder residue that can be boomed off, or will be vacuumed up during polishing.

Conclusion

Concrete is one of the world’s oldest construction products, but it is continually advancing to meet the needs of the times. Improved densifiying technology is a classic example of this process: taking advantage of the basic nature of the material, densifiers make a concrete slab more durable, sustainable, maintainable, and aesthetically versatile. Densification makes the concrete slab an ideal flooring solution for the economic and environmental requirements of our age. The newest densifiying technology, colloidal silica, carries those sustainability and cost advantages to the next level, and adds new performance levels to the densification process.

Reference

1. Look at Liquid. Floor Hardeners. By John Gill and Cyler Hayes, (www.concreteconstruction.net/.../A%20Look%20at%20Liquid%20Floor... )

2. Comparison of Concrete Chemical Hardeners (Densifiers). By Roger Allbrandt, B.A. Environmental Biology, (www.absolute polishing.com / A -brief -overview -of -Concrete -and -Chem...)

3. Densification for High Performance Floors, By Steven H. Miller, Concrete Tech. Today – Densifying Concrete



Factors Affecting the Selection, Economics Involved in Formwork

Formwork is a die or a mould, including all supporting structures, used to shape and support fresh concrete until it attains sufficient strength to carry its own weight. It should be capable of carrying all imposed dead and live loads apart from its own weight.

A formwork system is defined as “the total system of support for freshly placed concrete including the mould or sheathing

Sameer S. Malvankar Dy. Manager - Engineering, Gammon India Ltd.

Formwork

Appropriate selection of a formwork system is a crucial factor in successfully completing most building projects. However, in practice, selection of an appropriate formwork system has traditionally depended mainly on the intuitive and subjective opinion of practitioners with limited experience. This paper, discusses the guidelines on how to choose formwork, factors affecting the selection, economics involved in formwork and the present scenario of formwork in India. This article can assist engineers to determine the appropriate formwork system at the inception of future projects.

which contacts the concrete as well as supporting members, hardware and necessary bracing”. However, “System” implies a fully compatible arrangement of formwork with a minimum of individual components with reusable elements intended to solve each forming task thereby rationalizing the forming work.

Formwork system is among the key factors determining the success of a construction project in terms of speed, quality,


A large proportion of the cost of formwork is related to formwork labour cost. Significant cost savings could be achieved by reducing labour cost. An exemplary comparison reveals that the additional concrete use of up to 15% is economical than the handling of angular forming areas, since their assembly is rather time-consuming and the cost per square meter is higher than that for a straight surface.

B) An Integrated Formwork/concrete Life Cycle

The process of providing formwork and concrete is highly integrated. In the figure 2, the left circle represents the formwork life cycle, while the right circle represents the concrete construction life cycle. The two intersection points represent the beginning and the end of concrete construction life cycle. It should be noted that the phases ‘cure concrete’ and ‘stripping of formwork’ are interchangeable depending on the type of structural element. For example, columns and walls are cured after stripping the forms while slabs and beams are cured before and then stripped.

C) Economy of Formwork and Significance

1. Economy in design of a concrete structure

The architect or design engineer can also contribute much to reduce formwork cost by keeping the requirements of formwork economy in mind when one is designing the structure. At the time of design, consideration of the materials, methods that will be required to make, erect and remove the formwork. Avoid varying sizes in columns and beams; Usage of same sizes to the possible extent economizes the design permitting the reuse of formwork without alteration.

2. Economy in design, planning and building formwork

In designing, planning and building formwork, the contractor should aim for maximum economy without sacrificing quality and safety. Short cuts in design or construction th at endanger quality or safety may be false economy. For example, if forms do not produce the specific surface finish, much hand rubbing of the concrete may be required; if forms deflect excessively, bulges in the concrete may require expensive rectification measures.

cost and safety of works. Nowadays, most projects are required by the client to complete in the shortest time possible as a means to minimise costs. For high-rise buildings, the most effective way to speed up works is to achieve a very short floor cycle — to have the structure of a typical floor completed in the shortest time. On the other hand, aiming purely at speed often contradicts the achievement of other quality standards. Problems such as misalignment, misplacement, deflective concrete or holding up other works causing serious interruption can result.

The basic parameters of formwork are:

- Quality: in terms of strength, rigidity, position, and dimensions of the forms

- Safety: of both workers and the concrete structure- Efficiency: in operation, the ease of handling, erection and

dismantling, number of repetitions within the optimal limits- Economy: the least cost, consistent with quality and safety

A) share of Formwork Cost

In a typical multi-storey reinforced concrete building, formwork cost is the largest cost component. Formwork cost accounts for nearly 20-40% of cost of concrete and involves more than 60% cost of time. Overall formwork related cost have significant share ie.10% in the total construction cost.

Figure 3 - Categories of formwork classificationFigure 2 - Integrated formwork/concrete lifecycle

Figure 1 – Main cost type in a typical building project

Formwork


- Limit the size of formwork panels or systems to largest possible that can be handled at site.

- Standardize formwork making, erection and stripping to maximum possible extent

- Minimise the amount of components and accessories like nut, bolts, nails etc. to avoid risk of losing

- Create a material cost awareness cost of material con-sciousness in the personnel

- Control reuse and repetitions. Specially plywood and timber- Standardize various formwork systems

coordination and cooperation between engineers, contractors are necessary to achieve the goals. Saving depends on inventiveness and knowledge of the contractor. Judgment in the selection of the materials and equipment, in planning, fabrication and erection procedures, and in scheduling reuse of formwork, will expedite the job and cut costs.

D) Various Formwork Systems

Formwork can be classified according to a variety of categories as follows: (Refer Figure - 3)

Classification according to sizes

- Small-sized formwork- Operation by workers manually Wooden and aluminium

formwork- Large-sized formwork- Crane facilities are required in the operation Reduce the

number of joints and to minimize the number of lift Stiffening components -studs and soldier

Classification according to location of use

Various elements in the structure have specific design and performance requirements in the use of formwork. Some systems are more adaptive for specific location of use, such as

- Irregular frame structure – Conventional traditional timber form.- Wall, Column – Girder form, Frame panel form, climb form

or jump form- Slab – Conventional timber form, Modular slab formwork,

primary-and-secondary beam method, Panel form, Drop head beam- panel system, table form

- Repeated regular section – tunnel form, modular aluminium form

- Core walls, shells- Climbing formwork, Jump form and slip-form- Precast structure- steel /aluminium mould forms

Classification according to materials of construction

- Timber: most popular formwork material -low initial cost -high adaptability to complicated shape-labour intensive and environmental unfriendly

- steel: hot-rolled or cold-formed sections heavy weight -suitable for large-sized panels

Figure 4 - Parties involved in formwork selection process

- Aluminium: stiff and light weight-higher material and labour cost-excellent finish

- Plastic: recyclable, tough, lighter weight- Sacrificial concrete panels- Left in place formwork

Classification according to nature of operation

- Crane independent- Manually handled formwork -Self-climbing formwork

- Crane-dependent formwork- Gantry, traveling and tunnel type formwork system

E) Evaluation / Selection Criteria for Formwork System

Earlier formwork was once built in-place, used once, and subsequently wrecked. The trend today, however, is towards increasing prefabrication, assembly in large units, erection by mechanical means, and repetitive use of forms. These de-velopments are in tune with the increasing mechanisation of production in construction sites and other fields.

Formwork planning includes detailed layouts, cycle plans, calculation of optimum amount of material for the site, observance of fixed schedules and selection of the most appropriate and the most economic formwork system to be used at the construction site.

Different Parties involved in formwork selection

The proper selection of the formwork systems to be used in concrete structure is concern to all involved parties.

The following selection criteria have to be considered:

1) Geometry of building / structure

Internal layout

Some buildings may have very simple layouts with few i n-situ walls and floor plates framed with regularly spaced columns, as seen in many commercial and office buildings. However, some developments feature very complicated load bearing internal walls that can make the casting process difficult.

Structural forms

Like internal layout, the structural form of buildings also affects the formwork options. For example, buildings with a structural core in the form of a vertical shaft limit the use of other formwork

Formwork


systems other than those of a self-climbing nature. Buildings in flat slab design make table forms or flying forms the most obvious choice. For buildings with regularly arranged shear wall designs, the best selection is large-panel type steel forms or other types of gang forms.

Consistency in building dimensions

Some buildings may have non-standardised dimensions due to the architectural design and layout or to fulfill other structural requirements. These include the reduction of sizes for beams, columns and walls in high-rise buildings as the structure ascends. Formwork systems like the climb-form or steel form, may be quite difficult to use in such situations, due to the frequent adjustments of the form to meet the changes in dimensions may eventually incur extra cost and time.

Headroom

Higher headroom increases the amount (height) of staging required and can also create accessibility and safety problems. It can also make the erection of formwork, ensuring formwork stability and the placing of concrete more difficult.

Building span

Large building spans also create problems similar to those with high headroom situations. In addition, long-span structures generally have larger beam sections, heavier reinforcement provisions, or accompany post-tension works. This will further complicate the formwork’s design and erection process.

Repetitive nature

High-rise block-shaped structures usually require highly repetitive cycles and this is favourable to the use of formwork. However, the degree of repetition in building with very large construction area like a podium or underground structures such as basements is limited and the use of formwork, as an expensive resource, becomes very critical.

2) Project Planning/speed of work

The over-all construction sequence must be planned to use formwork in efficient manner and to permit the optimum investment in formwork to meet schedule requirements. Contractor should plan formwork and job sequence at the time of making a bid. Project planning such as the phasing or sectioning arrangement, integration of the structure, site layout and set-up arrangements or hoisting provisions and concrete placing facilities are influencing factors when considering formwork selection and application.

When working with buildings with large construction areas and horizontal spread, projects can be expedited by the introduction of additional sets of formwork, to create more independent work fronts. This will, of course, increase the cost of construction. For high-rise buildings, increasing the number of formwork used cannot always expedite the project, for the critical path still depends on the floor cycle. However, a properly

selected, designed and arranged formwork system will increase work efficacy for each typical cycle. In some cases, adding half or a full set of formwork, especially for the floor forms, may help to speed up the cycle as the additional set can provide more flexibility when the form is struck at an earlier time.

3) Construction process, methods

For selecting formwork one must know the sequence of con-struction activities and methods to be followed. Construction method will always give idea about inter dependency of the activities, specifications and additional requirements in pour. This will enable us to workout appropriate system which fulfils the construction needs.

4) Site logistics

Exceptionally small or very large sites sloped or very crowded sites, proximity to sensitive structures, sites where other major activities are underway, or sites with many physical or con-tractual restrictions will increase the difficulty of working with formwork. There is no specific solution to improve the situation in general and problems are tackled according to individual circumstances.

Accessibility to work during the course of construction, accessibility problems may be created through segregation, temporary discontinuation, or blocking of the layout by the partially completed building or, in cases constructing a shaft-type core wall is constructed in an advanced phase, the shaft may stand independently for a long period of time before it is connected to the horizontal elements. Proper access to all components should be considered while planning a site layout.

5) Climate condition

Formwork systems are sensitive to weather conditions. Typically, in vertical forming systems, the newly placed concrete is supported by the wall already cast below it. The lower wall section must get the sufficient strength to support the fresh concrete above. The rate of strength gain of lower wall is influenced by the ambient temperature, moisture content, and the freezing and thawing cycles.

Another factor that affects the economy of the selected system is the effect of stopping formwork activity and concreting because of extreme weather conditions. In the case of a slip-form, the work is usually continuous, 24hrs around the clock. If the slip-form stops because of weather conditions, it may impact structure as well as cost.

6) Labour efficiency

Considering the availability and qualification of the work force, improving labour cost efficiency is a major factor, especially in markets experiencing a building boom. Here, the qualification of workers tends to be low in relation to ever higher demands posed by construction methods.

Formwork


7) Cost of formwork system

This is a vital factor for deciding formwork system as one must know the capital provision for formwork in the project. It is always beneficial to work out these details at the time of bid. Cost is influenced by three components;

Initial cost or make-up cost:

Includes cost of transportation, materials, assembly and erection.

Reuse cost of formwork system:

The formwork system cost goes on reducing as we increase reuse of same. The re-use for traditional timber formwork is usually limited due to the durability of the plywood sheathing. The optimum number of uses for timber form usually ranges from 12 to 14. Thus, it is still sufficiently economical to use timber formwork for high-rise buildings at heights in accordance to the multiple of the usual re-used times. Although the metal form can be re-used many times, the high initial cost of providing the form often discourages its selection, especially when there is no need to reuse them too many times, for example in a low-rise development. A careful balance between cost, speed, performance and the quality of output should be properly con-sidered when making the selection.

8) Maintenance & storage cost:

It includes cost of stripping, repair, storage, etc. Formwork materials are a valuable asset of company, If proper care is taken during handling and storage, much return is obtained on the investment. Formwork needs to be handled correctly, maintained, repaired if necessary and finally, cleaned regularly. Avoiding damage reduces costs incurred. Proper storage of formwork materials gives easy reconciliation, faster retrieval of material, better space management and avoid unnecessary expenditures, improve safety at work place.

9) Availability of lifting devices (Crane time)

Many factors should be considered before employment of a construction plan and the selection of the right formwork system. These include considerations of whether there will be lifting appliances provided for the erection of formwork; whether these appliances will be able to access the work spot to assist in the operation as the structural works proceed; whether any special equipment will be required for striking the forms; and how the removed formwork panels can be transported to other spot to continue work.

Characteristic to high rise building sites is the confined and congested space availability for working. Crane time and space is regularly limited. In general, reinforcing (rebar) activities are most critical, since lifting the reinforcement to building level is the most crane- time consuming job of all. Thus, the capability of formwork to rely less on or be used independently of crane time is critical in high rise construction.

10) Simple logic of the system

Formwork system ought to be self-explanatory to use, this automatically eases the usage for the engineers/supervisor and also the labour who are end users of the system.

11) Working safety

Formwork should be self-securing with safe access and working platforms. Thus, it is not left to the end user whether they takes safety measures or not. Creating a safe work environment for the entire work force involved in the construction process, has become the pivotal issue in emerging construction markets.

12) Special requirements on Concrete surface/finish

Fair-faced concrete demands very high quality formwork in terms of surface treatment of the panels, tightness of the formwork joints and in dimensional accuracy. Requirements are slightly relaxed where the concrete surface is to be finished at a later stage.

13) Area or volume of cast per pour

The optimum volume of cast per pour depends on the types of formwork used, the particular elements of structure to be placed, the actual scale of work, and different levels of provisions of plant facilities. Usually a volume of concrete ranging from 60-200 cubic meters per pour can be comfortably handled in most Figure 5 - Factors affecting selection of formwork system

Formwork


site environments. It also depends on whether the concrete to be placed is for the vertical elements only or also includes the beams and slabs, as a means of saving an additional phase in the overall work cycle.

14) Involvement of other construction techniques

Tensioning and prefabrication activities are often involved in construction. This may create certain impacts on the use of formwork, especially where precast elements are to be incorporated during the casting process. Provision should be made for temporary supports or slot spaces and box out positions in the formwork for the precast elements, or extra working space for placing stressing tendons and onward jacking.

15) Provision of construction joints in structures

Many a times a large number of construction joints are inevitable in a large structure because of the subdivision of works into effectively workable sizes. The provision of construction joints can challenge the output and affect the quality of the concrete. Careful selection should be made to ensure a particular formwork system can satisfactorily allow such arrangements.

16) Inventory- The fewer, the better

The most frequent time & cost consuming activity of formwork assembly is the loose and small components/accessories. The lesser inventories will help to reduce risk of losing parts and provide ease in construction.

F) The Current Situation in India

In the past, India had been lagging behind over the other advanced countries in applying advanced and safe concepts for formwork in reinforced concrete construction resulting in a poor surface quality, wastage and low productivity of the people involved in concrete construction. This unfortunate situation continued for a long time because of availability and use of very cheap unskilled labour and very few skilled personnel who have had professional training for formwork jobs.

With increasing demand and competition and reducing project completion times, there have been significant developments in the construction industry in terms of experience and mastering of the required managerial, construction or engineering skills for handling very large and complex projects. At the same time, the motivating factors highlighted above have created an eagerness and readiness within the industry to advance. From the building construction point of view, the use of better formwork systems is no doubt a very direct way for introducing innovative methods in the construction of buildings.

- Formwork labour cost is so immense that any innovative system resulting in a labour cost reduction is highly lucrative.

- Fulfillment of fast track construction schedule provides fewer choices, one of which is to adopt more innovative formwork systems.

- Traditional systems can hardly satisfy the tight quality standard that is required nowadays.

- Similarly, traditional systems can hardly satisfy current safety and environmental standards.

- The accumulation of experienced crews makes the application of more sophisticated formwork systems more reliable and economical.

- Many developers view the application of innovative technology in the construction process as a positive image-building factor.

G) Major Systems Dominate Today’s State of Art Formwork Approach in High Rise Construction

- Slab edge protection by screens, providing a safe working environment on the construction levels

- Modular slab formwork, operated independently of the crane time, adapted flexibly to different building geometries and floor layouts

- Undisturbed shoring for slab with drop beam systems- Frame formwork for columns and walls- Crane dependent climbing formwork for shear walls/mega

columns- Crane independent climbing formwork for core

H) The Potential and Limiting Factors of Innovative Technologies in the Built Environment of India Potentials

- The public’s expectation (government, developers, building users) are rising all the time.

- More stringent regulations have been imposed to control the performance of the construction industries.

- Accidents are costly, especially where human casualties are involved (both for the reasons of compensation, image and government records).

- The development or importing of advanced technology have become more common and market affordable.

- The industry is gradually accepting the production of higher performance buildings involving a more expensive resource input.

Limitations

- Insufficient research and development in most contracting firms or other supporting units.

- Lack of working space on construction sites (both on-site and off-site work areas).

- Training opportunity (including on-the-job training) is still limited for both the professionals and other workers.

- No guarantee of a consistent market environment for the development and continual application of innovative technology in construction (learned skill and experience will lose eventually).

- The extensive use of cross wall design especially in most residential buildings and small-scaled projects makes the use of more innovative formwork system less feasible.

- The exceptionally large scale and complex nature of projects in terms of the site condition as well as structural and building design confine the application of more advanced and sophisticated formwork system.

Formwork


I) The Industry or Individual Corporations May Consider for the Following Measures:

- Explore ways to streamline and re-engineer the work structure at both industrial and corporate levels.

- Invest steadily in the human resources development and to train competent and high quality staff with the required attitude and readiness to work in the new environment.

- Invest steadily in the research and development of technologies that are particularly suitable for the built environment of India

- Strengthen the linkages among government departments, developers, consultants and contractor firms in the promotion, development, cooperation and implementation of more innovative projects.

- Government or other institutions may consider providing funding to support research and development for the exploration, recommendation or setting up of guidelines and standards in the application of newer technology and work systems in construction.

Conclusion

1. Selection of formwork system, is highly dependent on individual site/project environment

2. Economy of formwork can be achieved with seamless collaboration between owner, architect, designer teams and contractor. And it can aid in the effective use of advance formwork systems

3. The structural form of the building is one of the critical factors to determine the choice of formwork

4. System products can contribute much in the success of formwork application

Indian

IS 14687-1999 Falsework for concrete structure guidelines

IRC 87-1984 Guidelines for the design and erection of falsework for road bridges

IS 2750-1964(1995)

Specification for steel scaffoldings

International

ACI 347-04 Guide to formwork for concrete

ACI 347.2 R-05 Guide for shoring / reshoring of concrete multi-story buildings

ACI SP-4 Formwork for concrete

BS 5975-2008 British standard code for practice for temporary works procedures and the permissible stress design of falsework

DIN 4420 & 4421 German standard for formwork

DIN 18218 Pressure of fresh concrete on vertical formwork

CIRIA Report 108 Concrete pressure on formwork

Formwork Standards

References

1. ACI 347_04, “Guide to formwork for concrete”, American concrete institute, 2005

2. Hanna A. S. & Sanvido V.E. , “ An interactive knowledge based on formwork selection system for building”, Computer integrated construction, 1989

3. Hurd M. K. “Formwork for concrete”, American concrete institute, 1915, 6th edition

4. Raymond W. W. M. , “Application of formwork for high rise and complex building structures-Hongkong cases”, Division of building science & technology, city university of Hongkong.

Formwork


The use of Steel and Synthetic Fibres in Concrete under Extreme Conditions

The use of fibres to enhance the properties of construction materials can be traced back over 4000 years to the use of straw in bricks and horse hair in plaster. Fibres can reduce plastic cracking in fresh concrete and enhance the post-crack ductility of hardened concrete. The elimination of reinforcement fixing can have significant time, cost and safety benefits. Whilst the random orientation and dispersal of fibres means they are not as efficient as conventional reinforcement for dealing with predictable stresses, they are able to resist crack propagation under unforeseen stresses, particularly those arising close to the surface of elements during construction and in service, such as impact.

Fibres can be particularly beneficial under extreme environments, such as exposure to chlorides and fire. Fire and abrasion resistance are enhanced and the discrete nature of fibres means that the risk of corrosion and associated spalling is significantly reduced.

The mining and tunnelling industry makes extensive use of fibres in sprayed concrete linings for underground support. Fibres allow the lining to retain ductility, even under high deformation, which is critical for safety. Precast tunnel segments have utilised both steel and synthetic fibres for handling and improved fire resistance respectively (1). Marine works have used synthetic macro fibres to eliminate corrosion risk under exposure to seawater (2).

This paper provides a review of structural and durability design approaches based on published guidance. It should be noted that performance is dependent on the particular fibre type, including the manufacturer.

Fibre reinforced concrete

Fibre types

The principal fibre types in BS EN 14889 (3,4) are shown in Figure 1. The shape and surface texture of the fibres is important in determining their effect on concrete properties. Fibres should fail by gradual pull-out after disbondment from the cement matrix in order to provide ductility and features such as hooked ends,

Don Wimpenny1, Wolfgang Angerer2, Tony Cooper3 and Stefan Bernard4 1Principal Materials Engineer, Halcrow Pacific 2Senior Tunnel Engineer, Halcrow Pacific 3Consultant Elasto Plastic Concrete 4Consultant, TSE

Concrete: Fibres

crimping, twisting and embossing are intended to promote friction associated with the pull-out mode of failure.

Table 2 summarises the characteristics of two widely used fibre types and compares the attributes of concrete made with these fibres to conventional concrete.

Short term laboratory and field studies on the durability of uncracked steel fibre reinforced concrete (SFRC) indicate carbonation induced corrosion is restricted to those fibres immediately below the surface (5). Galvanic corrosion and spalling do not appear to occur (6), but there is a potential for corrosion at cracks (7). This is important, as it could ultimately lead to sudden failure of the concrete due to fibre breakage rather than ductile failure by fibre pull out (5). It is recommended that limiting crack width of 0.1 to 0.2mm be adopted, depending on the service conditions.

Corrosion of steel fibres will cause staining and where this would be unacceptable, galvanized steel is sometimes used (as the solubility of zinc is increased when chloride ions are present, stainless steel and synthetic macrofibres are more appropriate in a chloride-rich environment).

Structural synthetic fibres are an alternative to steel fibres for controlling handling damage and providing long-term ductility. They are not significantly affected by exposure to seawater (8) or sodium chloride at temperatures of 20-40°C (9). However, the maximum service temperature should ideally be limited to 60°C and exposure to some chemical agents, such as chlorine gas, should be avoided.

Field examples

Evidence of satisfactory performance of fibres in concrete elements is important in providing confidence to potential users. Unfortunately, there is relatively limited information on long-term durability of FRC in the field. For example, steel and synthetic macrofibres have only been employed in sprayed concrete since the 1970s and the 1990s respectively.

Table 3 summarises a selection of projects where the concrete is exposed to extreme service conditions. Despite the many projects

The tunnelling and maritime environments can provide extreme conditions for reinforced concrete. Exposure to aggressive saline water can be combined with design lives of 100 years or more. In recent years, traditional carbon steel reinforcement has been substituted by the use of steel fibres and, more recently, by synthetic fibres for sprayed concrete and precast items, such as tunnel segments. This paper describes some of the factors which have prompted the change from rebar to fibres and includes a detailed consideration of the specific durability design aspects of fibre reinforced concrete use within desalination facilities.


utilising fibres, the duration in service is modest compared to the design lives of 100 years, or more, which are increasingly required.

Structural design

Design methodology

The design of an FRC section follows the same approach used for the design of reinforced concrete, with appropriate modifications for material properties in the tension zones. In reinforced concrete the properties of the concrete and reinforcement can be determined separately. For FRC the properties of the composite of fibre and concrete, such as tensile splitting strength, flexural strength at first crack and residual flexural strength, should be determined

concurrently. These properties can utilised in standard equations, which define the load capacity of the composite element.

Initial designs may use manufacturer’s data or data from previous projects to estimate the material properties. However, it is important that the validity of the data is checked and that the performance of the material is demonstrated by testing in pre-production trials and by regularly sampling throughout production.

Figure 2 shows the stress-strain diagram adopted within the Rilem design method. The values of stress and strain in the diagram

are defined in RILEM TC-162 TDF (10) and the design standards using this approach such as Eurocode 2 (11) or NZS3101 (12). Similar recommendations are given in guidance by the German

Characteristic Steel Fibres Synthetic micro-fibres Synthetic macro-fibres

Characteristic of Fibres

Shape/Texture Cold drawn hooked ends Straight smooth Continuously embossed

Collation Glued bundles Fibrillated Uncollated

Length (mm) 60 12 48

Diameter (mm) 0.75 0.02-0.03 0.5-1

Tensile Strength (MPa) 1050 30 550

Elastic Modulus (GPa) >200 2 10

Dosage (kg/m3) 25-35 1-2 6-10

Service temperature (°C) 300 60 60

Melting point (°C) >800 150 150

Base material Carbon steel Polypropylene Polyolefin (polypropylene, polyethylene)

Comparison with conventional concrete (Unreinforced except where indicated by*)

Workability Reduced Slightly reduced Slightly reduced

Plastic shrinkage cracking Unaffected Reduced Slightly reduced

Early-age thermal cracking Reduced Unaffected Reduced

Long-term shrinkage cracking Reduced Unaffected No data

Stray current corrosion Reduced Unaffected* Eliminated

Durability in chloride exposure* Increased Unaffected* Greatly Increased

Fire spalling resistance Slightly Increased Greatly Increased Increased

Compressive strength Unaffected Unaffected Unaffected

Residual flexural strength Increased Unaffected Increased

Impact strength Greatly Increased Unaffected Increased

Flexural toughness Increased Unaffected Increased

Abrasion resistance Increased Slightly increased Slightly increased

Freeze-thaw resistance Slightly increased Increased Increased

Flexural energy absorption Greatly Increased Unaffected Greatly Increased

Concrete permeability Slightly increased Slightly increased Slightly increased

Pump wear Increased Reduced Reduced

Safety*Hazard from handling and protruding

fibresIncreased Increased

Finishing Extra care during floating Exposed fibres soon abradeFibres may float and protrude in poorly

designed mixes

Table 2. Comparison of steel and synthetic fibres

Concrete: Fibres


Concrete Association and in Concrete Society Technical Reports 63 and 65 (13, 14).

Although codes and recommendations mainly provide information regarding steel fibre reinforced concrete, the design principles apply equally to other structural fibres. Testing may be required to confirm the validity of some of the design factors and allowance should be made for the difference in stress-strain behaviour, long-term performance and implications of exceptional load scenarios, such as fire.

The properties determined from specific tests will differ from the full-scale structure. The designer should verify that properties are based on representative concrete samples and provide adequate allowances for the variability of the test method, the pattern of loading and the difference in geometry between the

test specimen and the actual structure. Care should be taken in the choice of test method (eg. beam or panel) and specified limit. In particular, high residual flexural strength requirements at high deflection values may have no relevance to the design and prove unnecessarily difficult to achieve.

The full life of the structure should be considered in the structural deign (5). For example, bursting performance of tunnel segments during construction is based on tensile splitting and compressive strength. Permanent loading for the serviceability limit state is normally based on ensuring elastic un-cracked behaviour and as such relies on the flexural strength at first crack. The ultimate limit state allows for some plastic behaviour and utilizes the residual flexural strength. However, fibres are generally not relied upon as the primary reinforcement in plastic hinge regions. Structural designs for serviceability generally incorporate limits on stress, strain (eg. 10 microstrain in the tensile section) or crack width (eg. 3mm maximum for durable fibres).

Due to the random distribution of the fibres in a concrete matrix, there is no recognised method to calculate crack widths in fibre reinforced structures. A reliable first approximation of crack widths can be estimated in members subject to axial and bending forces by derivation of stress-strain diagrams in a linear elastic analysis. Crack widths can be therefore be predicted according to the

Project Country Application Fibre typeDate

entered service

Halsney Tunnel Project Norway Sprayed concrete lining to a sub-sea tunnel Macro-synthetic 2005

Atlantic Ocean Tunnel Project

Norway non-corrodible reinforcement in a sub sea tunnel Macro-synthetic 2009

E18 Motorway Norway Sprayed concrete lining to tunnel close to waterfront Steel 2009

E18 Motorway Norway non-corrodible reinforcement in ground with high sulphide levels Macro-synthetic 2009

Sydney Northside Storage Tunnel

Australia sprayed concrete lining under chloride exposure Macro-synthetic 2003

Docklands Light Rail Extension

United Kingdomtrack slab subject to stray currents and sulfate and chloride

exposureMacro-synthetic 2004

Quarry Bins United KingdomSprayed concrete repair to abrasion damaged aggregate storage

binsSteel 1998

Blackpool South Shore United Kingdom Precast revetment units subject to handling, abrasion and seawater Macro-synthetic 2006

Channel tunnel rail link United Kingdom precast segments subject to fire loading Steel and micro-synthetic 2003

Gold Coast Desalination Project

Australia Segmental lining for intake and discharge tunnels Steel 2008

Table 3. Examples of fibre reinforced concrete under extreme conditions.

Figure 2. Stress-strain diagram for fibre reinforced concrete

Figure 1. Fibre types to BS EN 14889

Concrete: Fibres


following equation:

w = b (h-x) (1)

where w is the crack width, b is the strain at the extreme tension fibre, h is the section thickness and x is the depth of the compression portion (Figure 3).

However, the values obtained from such assessments are often higher than the 0.1 to 0.2mm limit for steel fibres in extreme exposure conditions (Section 2.1). Furthermore, the design methods are not reliable enough to ensure these crack widths will be consistently achieved. A prudent design approach for SFRC exposed to chloride from seawater or de-icing salts is to prevent the section from cracking during serviceability loading.

Long-term performance of FRC is heavily influenced by the interaction between the fibres and the concrete matrix. Two aspects are particularly important. Firstly, the aim of the addition of fibres is to avoid the brittle failure associated with unreinforced concrete. In FRC the development of excess strength and hardness in the enveloping concrete matrix may result in a change from ductile behaviour to brittle behaviour, as fibres fracture instead of gradually pulling out of the matrix (15).

Secondly, the discontinuous nature of fibres means that they may exhibit slip within cracked concrete leading to creep under sustained loading. Recent research by Bernard on the creep of cracked FRC (16) indicates that creep depends on the fibre type, dosage and applied load (Figure 4). The results are specific to the particular fibre types tested. The designer needs to verify the influence of creep behaviour on the structure and provide appropriate contingencies in the design, such as limiting the tensile stresses.

Durability Design

Design Methodology

The durability design for steel fibre reinforced concrete involves the prediction of carbonation and chloride ingress and the associated corrosion. In conventionally reinforced concrete, corrosion damage to the concrete, such as spalling or cracking, would often be used as the serviceability limit state. In the case of FRC, the loss of cross-sectional area of fibres by corrosion determines the depth of concrete over which the contribution of fibres to tensile and flexural strength should be ignored by the structural designer. A limiting value of 20% loss is typically used.

The accuracy of the assumed exposure conditions is critical to the validity of the predictions. In the case of a desalination facility,

conditions during construction and any maintenance outages should be considered, as these may be more severe than those in service. Table 4 indicates the input parameters defining the service conditions and the characteristics of the concrete mix.

Figure 3. Calculation model for estimation of crack width in fibre reinforced members subject to axial compression and bending

Figure 4. Relationship between creep deflection at 100 days and imposed load for ASTM C1550 panel test (16).

The surface level of chloride will vary depending on the salinity in the environment and amount of contact the concrete has with the environment. For concrete in the marine environment the surface chloride level will typically vary from 1-2% for submerged and atmospheric exposure to 4% for wetting and drying. For desalination schemes where the salinity can be 50% higher than seawater surface chloride levels may be correspondingly increased.

The carbonation and chloride models used in durability design are discussed below.

Carbonation Model

The carbonation model commonly used is based on work by the Building Research Establishment (17) showing a good correlation between the carbonation rate and oxygen permeability. The latter is predicted based on the cement type, concrete strength and duration of curing. Gas permeability or diffusion testing should be carried out on proposed concrete mixes to validate the assumed oxygen permeability value.

The modelling is normally deterministic rather than probabilistic,

General input parameter Specific input parameters

Relative humidity (%) Temperature (°C) 28-day cube strength (MPa) Cementitious content (kg/m3) Tri-calcium aluminate level of cement (%) Slag, fly ash or silica fume level (%) Reinforcement diameter (mm) Fibre thickness (microns) Cover (mm) Allowable corrosion loss (%) Service life (years)

Carbonation: Carbon Dioxide Level (%) Intrinsic oxygen Permeability (m2) Curing (days)

Chloride: Background and surface chloride levels (%) Chloride diffu-sion value (m2/s) and age (days) Aging factor Exposure type (wetting/drying, submerged, atmospheric) Steel type (stainless, carbon) Additional protective measures (coatings, corrosion inhibitor)

Table 4. Input parameters to durability modelling

Concrete: Fibres


Concrete: Fibres

in that it adopts single values for each parameter rather than allowing for variation and then predicting the likelihood of different outcomes. The adopted values often reflect the worst likely conditions for temperature and carbon dioxide levels. The carbon dioxide level will typically lie between 0.04%, the average outdoor value, and 0.1% for poorly ventilated spaces (17).

Relative humidity is a key parameter affecting carbonation and a sensitivity analysis should be carried out to encompass the range of possible values during construction and in service.

Typical output from the model is shown in Figure 5.

Chloride Model

The chloride ingress by diffusion is predicted using a model based on published research (18). The model uses Fick’s second law of diffusion:

(1)

where Dc is the diffusion coefficient (m2/s), Cx is the chloride

concentration at depth x (m) after exposure time t (s), Csn is the notional surface chloride level (% by mass of binder) and erf is the error function. The diffusion coefficient will increase with temperature but reduce with age, dependent on the concrete mix and the exposure conditions. The model allow for these effects, as well for as the release of chlorides due to carbonation (18). The chloride diffusion coefficients assumed in the design should be verified by appropriate testing, such as the NTB 492 method (19).

The chloride levels increase until the threshold chloride level is surpassed and then corrosion is initiated. The threshold level depends upon the cementitious type, service temperature, presence of corrosion inhibitors and reinforcement type (eg. carbon steel, stainless steel).

The rate of corrosion of steel is assumed to increase as the chloride level in the concrete adjacent to the steel increases. For example, the tidal and splash zone in marine conditions:

(3)

where CR is the corrosion rate (microns/year) and Cx is the chloride concentration (% by mass of binder) at the depth of the steel reinforcement. The corrosion rate will depend on the temperature and relative humidity. An adjustment for temperature can be made using Arrhenius law (18).

The model allows different corrosion mitigation measures to be assessed, including: changes to the concrete mix and the use of protective coating, integral waterproofers, silane, controlled permeability formwork and stainless steel reinforcement.

The cumulative corrosion from each stage of the structures life is calculated and used to estimate a depth over which the effect of the fibres should be ignored, in a similar way to the carbonation. Figure 6 shows typical graphical output from the modelling.

Conclusions

Fibres provide significant advantages over reinforced and plain concrete under extreme conditions, including exposure to fire, abrasion and seawater. Steel fibres and synthetic macrofibres

Summary of Input

Parameter units value

relative humidity % 85

temperature °C 25

carbon dioxide level % 0.03

28-day cube strength MPa 50

cementitious content kg/m3 400

ggbs level % 0

pfa level % 30

minimum cover mm 30

bar diameter mm 16

fibre thickness micron 750

acceptable loss of section % 20

curing days 3

oxygen permeability 10-6m2 1

service life years 100

Summary of Output (rebar)

corrosion initiation 52years

propagation of 0.1mm crackspropagation of 0.3mm cracks

3years15years

service life to 0.1mm crackscover for service life

55years41mm

service life to 0.3mm crackscover for service life

67years37mm

Summary of Output (fibres)

depth of loss of section of 20% 32mm

(a) Summary of input and output

(b) Graphical output showing loss of fibres during construction periodFigure 5. Typical output from carbonation model


increase post-crack ductility. This is an important consideration for use in ground support,

Steel fibres by virtue of their discrete nature and small diameter appear to eliminate galvanic corrosion and associated spalling damage compared to steel rebar and enhance resistance to chloride and carbonation induced corrosion. However, maximum crack width should be limited to 0.1-0.2mm depending on the service life and exposure conditions. Synthetic macrofibres are non-corrodible but will be affected by elevated temperature and some chemical agents.

Structural design methods are available for fibre reinforced concrete. These should use values of compressive, flexural and tensile strength based on performance tests on the FRC and should consider the whole life of the element. Serviceability limit state design should generally be based on elastic behaviour of an uncracked section using flexural strength to first crack data. Ultimate limit state design can allow for some plastic behaviour using residual flexural strength data.

The interaction between the concrete matrix and the discrete fibre is critical. SFRC may lose ductility when the concrete matrix becomes too strong leading to fracture rather than pull-out the fibres. Creep due to slip and elongation of fibres is also an issue. Deformation and creep in cracked FRC depends on fibre type, dosage and loading.

Durability design of facilities, such as desalination schemes, should assess the conditions during construction, operation and at times of outages. Carbonation and chloride ingress can be predicted from models based on published research. The loss of cross-section of fibres by corrosion can be used to estimate the depth of concrete over which any effect of fibres on flexural or tensile strength should be ignored.

References

1. Greenhalgh J., “Segmental linings – the future is steel-fibre-reinforced”, Concrete Magazine, October 2003, pp 19-20.

2. Perry B., “Synthetic macrofibres storm to the front of coastal defence innovation”, Concrete Magazine, November 2006, pp 72-73.

3. British Standards Institute, “BS EN 14889-1 Fibres for concrete, Part 1: Steel fibres- definition, specifications and conformity”, BSI, London, 2006.

4. British Standards Institute, “BS EN 14889-2 Fibres for concrete, Part 2: Polymer fibres- definition, specifications and conformity”, BSI, London, 2006.

5. King M. R., and Alder A. J.,”The practical specification of steel fibre reinforced concrete (SFRC) for tunnel linings”, Proceedings of Underground Construction 2007 Conference, London, Brintex Ltd.

6. American Concrete Institute, “ACI 544.4R Design considerations for steel fibre reinforced concrete”, 1996

7. Bernard E. S., “Durability of cracked fibre reinforced shotcrete”, 2nd International Conference on Engineering Developments in Shotcrete, Cairns, Australia, October 2004, In Shotcrete: More Engineering Developments (Bernard E S ed) Taylor and Francis, London, 204, pp 67-80

8. Hannant D. J., “The effects of age up to 18 years under various exposure conditions on the tensile properties of polypropylene fibre reinforced cement composite”, Materials and Structures, Vol 32, No 216, March 1999, pp 83-88.

9. Basell, “Prof-fax and Moplen Polyproplene Chemical Resistance”, Data sheet 08/02, 2002.

10. Rilem, “Final recommendations of TC162-TDF, Test and design methods for steel fibre reinforced concrete, design method”, Materials and Structures, Vol 36, 2003, pp 560-565.

11. British Standards Institution, “BS EN 1992, Eurocode 2, Design of concrete structures, Part 1-1, General rules for buildings”, BSI, London, 1994

12. NZS 3101, Concrete Structures, “Part 2- Commentary on the Design of concrete structures, Appendix C5A, Test and design methods for steel fibre reinforced concrete subject to montonic load”, 2006, pp 7-13

13. Concrete Society, “Guidance for the Design of Steel-Fibre-Reinforced Concrete”, Technical Report 63, 2007.

14. Concrete Society, “Guidance on the use of Macro-synthetic-fibre-reinforced Concrete”, Technical Report 65, 2007.

15. Bernard, E.S. 2008. “Embrittlement of Fibre Reinforced Shotcrete”, Shotcrete, ACI, Summer edition, pp 16-21.

16. Bernard E. S., “Creep deformation of cracked fibre reinforced shotcrete panels for Elasto-plastic concrete”, TSE Report Number 189, March 2008.

17. Quillin K. ”Modelling degradation process affecting concrete”, CRC Ltd, 2001.

18. Bamforth P. B., “Enhancing reinforced concrete durability, Guidance on selecting measures for minimising the risk of corrosion of reinforcement in concrete”, Technical Report 61, 2004.

19. NTB 492, “Concrete, mortar and cement based repair materials: chloride migration coefficient from non-steady migration experiments”, Nordtest method, 1999.

(a) chloride ingress 50mm depth (b) loss of section of fibres for outages of

Figure 6. Output from chloride model

Concrete: Fibres

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