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Current Trends in the Australian Concrete Industry in 2013

Is concrete a fully mature product offering?

David Hocking, Technical Manager, Boral Concrete, Sydney, Australia

Introduction

Concrete has been in use for over 2000 years, so as such is a mature product and

industry, in which further innovation may be viewed as being limited. Based on the

ongoing research and development into concrete, and a passion by the industry to

continuously improve, there is much more to offer in concrete as a building material.

This continues to be the case with competition in the form of composite materials,

timber and steel energy intensity improvements, and recycled materials gaining

momentum.

In Australia, there are several areas of current key interest as listed below:-

1.) Manufactured Sands development

2.) Durability Assessment (Chloride diffusion coefficient versus Rapid Chloride,

Permeability/Sorptivity)

3.) Fly ash Availability/Quality LoNox burner upgrades

4.) Admixture Technology Slump Retention / Keepers

5.) Super Workable Concrete / Self Compacting Concrete (versus high slump concrete)

6.) Slag cement (GGBFS) Neat milled slag as a Supplementary cementitious material (SCM)

versus pre-blended GB cements

7.) Greenstar – Activated cements for improved early age, high durability and low shrinkage –

Envisia/ZEP Technology

This paper seeks to explore how some of the above key focus areas were considered

and successfully incorporated into the signature project – Port Botany Expansion,

Sydney (Australia). Also covered are the new technology aspects of Concrete

Maturity assessment and activated slag cements as ongoing development and

innovation in concrete supply.

1.) Manufactured Sands

As for other major industrialised countries, natural fine aggregate/sands resources are

being depleted requiring increased use of manufactured sand. Australia is keenly

watching, and utilising the very good work that has been done within the New

Zealand Manufactured sand space, and in particular the use of New Zealand Flow

Cone (NZFC) test to assess suitability of fine aggregate sources.

Alternative materials such as air cooled blast furnace slag fine aggregate, and recycled

glass sand blends have been used to supplement natural and manufactured sands.

Much work is going on to fully understand and implement controls for grading, shape

and surface texture, amongst other things, with manufactured sands.

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Increasingly specifications are considering a requirement for a range of air voids (%)

and flow time (seconds) limits for a manufactured (and natural) sand to ensure the

material will be acceptable for mixing, placing, compacting, screeding and finishing

processes. Where these two properties achieve the required combination of air voids

to flow time, sands generally will be of acceptable quality. As fine aggregates, and in

particular manufactured sands move to lower percentage voids, as well as faster flow

time, the optimal packing, water reduction, and cement efficiency is possible.

Historically in Australia manufactured sands are assessed individually, and if they fail

to flow due to bridging in the cone apparatus, the top size fractions are sieved out to

enable a flow time and voids result to be determined. This approach is not ideal as it is

not the actual product that is utilised in the concrete mix. Subsequently more work is

being done on combined fine and coarse sands (as a composite fine aggregate) to

ensure the NZFC test is representing all size fractions in the concrete, which is the

actual case in practice.

Ongoing development work is being explored in the use of real time capture of final

fine aggregate grading and shape characteristics in particular to pursue the European

experience of vision sizing techniques to real time link in line grading/aspect ratio

with feedback mechanisms to then adjust tertiary crushing plant.

Figure 1 – Individual and combined fine aggregate (sands) NZFC data

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For the above flow cone plots in Figure 1, the intention is to blend a

• coarse and fine sand,

• intermediate plus fine sand or

• all up intermediate sand

to achieve optimum % voids and flow time.

This can be observed with the combination of decreasing air voids and reducing flow

time limits applied in the specification to move towards the lower left corner of the

graph.

In the case study of Port Botany Expansion project, the competing priority of high

workability to consolidate the concrete (at 130mm slump) in a vertical lift with an

open bottom formwork required high cohesion to minimise heave of the horizontal

concrete surface at the bottom of the precast mould.

In contrast, the buttress pours required a 3 metre free fall of concrete, and high

compaction being required due to congestion and therefore internal compaction

impractical.

With the concrete mix design utilising a manufactured sand, workability

considerations were paramount to achieving both specification requirements, as well

as contractor buildability criteria of off form finish and shape.

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2.) Durability Assessment

(Chloride diffusion coefficient versus Rapid Chloride) Permeability/Sorptivity)

There is an increasing trend to use qualitative methods to determine service life of a

structure versus more screening test type measures, such as rapid chloride

permeability, sorptivity, and volume of permeable voids methods.

Through the use of chloride diffusion coefficient (CDC) from the normal Norwegian

test method NT443 or accelerated method NT492, Fick’s Law of diffusion can be

used to more accurately predict service life of the structure.

Port Botany project having a precast placement methodology had competing priorities

of early age strength for precast bed turnover, and high durability to achieve 100 year

design life, and therefore a high supplementary cementitious content using fly ash and

slag cement. Rather than use the pre-existing low permeability test methods such as

Rapid Chloride Permeability (RCP), Sorptivity or Volume of Permeable Voids

(VPV), using bulk diffusion service life can be determined with more certainty.

Through the use of either Chloride diffusion coefficient test methods (NT443 or

NT492), or both in combination, design life can be estimated at 28 days using test

method NT492. Alternatively, by simulating the actual curing regime of the insitu

concrete the test samples can be tested using NT443 test method which then more

accurately determines the service life via Fick’s Law of diffusion. Whilst this method

can take up to 4 months (56 days curing, 35 days salt water immersion, and up to 10

days chloride profiling and reporting), when used in combination with a well

established correlation to the 28 day NT492 method, early approval of mix designs

can be performed.

As a result of these more qualitative and repeatable methods (NT443) road authorities

are permitting mature ready-mix supply companies to use performance specification

requirements. This is in comparison to prescriptive limits where such expertise to

demonstrate service life requirements is not able to be readily performed.

Roads and Maritime Services (RMS) Bridge specification B80 Clause 3.2 is one such

example of the move to performance based specifications.

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3.) Fly ash – Low NOx burners

Government regulation in Australia for the (mostly depending on state government)

deregulated power industry requires any major works/upgrades to coal fired power

stations to implement Low-Nox burners. This technology adjusts the flame burn

temperature to result in lower energy inputs, and therefore also reduced CO2 output

and chemical emissions to air.

The net outcome of this lower temperature burn means that residual carbon content

increases the loss on ignition (LOI) results. This increase in LOI from 0.5-1.0 to

typically as high as 2.0-2.5% negatively impacts air entrainment of concrete and

requires closer monitoring of plastic properties, particularly where air entrainment is

required for freeze/thaw or slipform paving construction.

Whilst Australian Standards allow up to 4% LOI, state road authorities and asset

owners require LOI values to be less than 2.5%, and when silo logistics require only

one source of fly ash to be captured and despatched, all fly ash used in Australia is

therefore required to have an LOI of less than 2.5%. This results in large quantities of

fly ash being deposited to landfill and/or dams, which could be used in Normal class

concrete, compared to lower quantities of road paving/bridge specification concrete.

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Following is a short snapshot into one particular road project plant showing the

variation in loss on ignition testing due to amongst other reasons, variable power

demand and coal sourcing locations for power station burners.

Whilst only at the feasibility and development stage, power producers are attaching

value to the availability of classified fine grade fly ash as a real end use in concrete

manufacture, and are pursuing ways to minimise the variation and magnitude of LOI

in the despatch fly ash.

Technology to treat bulk quantities of fly ash into the silo or in individual delivery

truck loads is available using powder activated carbon treatment (PACT) or fly ash

activated carbon treatment (FACT) whereby a surfactant is used to treat the fly ash

and negate the impact of the free carbon which adversely impacts water reducers but

in particular air entraining admixtures.

This treatment has been successfully introduced in the North American concrete

market to good effect, improving the capture an utilisation of fly ash in particular in

freeze/thaw and air entrained concretes.

4.) Admixture Technology – Slump Retention / Slump Keepers

Of all the developments in concrete technology, admixtures continue to show ongoing

innovation with chemistry changes and improved rheology of concrete. This in part is

attributable to the significant competition in this industry, and the Research and

Development activity undertaken by these global companies.

Polycarboxylate ethers (PCE’s) continue to improve in function, availability, and cost

competitiveness as the preferred admixture base material. The combination of PCE

backbone polymers, and flexible side chain polymers means that the combination of

water reduction and slump retention properties can be optimised.

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Some PCE formulations can be made as a packaged product containing an individual

water reducer and slump retention admixture, but generally these do not suit all

applications through from high strength concrete requiring high water cut, but not

necessarily long slump retention through to piling mix methodology require lower

water reduction but long slump retention.

Dependent on batch plant bulk admixture handling combination water

reduction/slump retention admixtures can be utilised or separate products can be

batched separately to achieve the desired outcome of water reduction or slump-

keeping properties. This has fundamentally changed the concrete placer experience in

that previously slump retention was achieved by re-dosing beta naphthalene

sulphonate (BNS) type superplasticisers to impart additional electrostatic charge on

cement particles to maintain workability. The negative impact of this is set retardation

often occurs which may result in concrete below the surface retarded and plastic,

whilst surface moisture loss may render a crust or “jelly set” whereby the surface has

set whilst the underlying concrete is still plastic.

PCE technology means that set retardation is no longer an issue, but dependent on

admixture technology slump increase may be a factor as PCE backbones adhere to

cement grains and steric hindrance disperses cement resulting in slump increase later

in time.

From the following figure it can be observed that at both 130mm and 200mm slump

that through the use of slump retention admixtures that Precast type PCE

superplasticisers can be used in conjunction with slump retention admixtures to

maintain a consistent level of workability. Where second generation PCE

superplasticers are characterised by linear slump loss, newer PCE technology enables

both water cut and slump retention in combination.

This means that innovation can be employed to tailor concrete plastic properties to

suit the extremes of precast concrete with high water reduction mixes with rapid

strength gain through to long slump retention and lower water cut piling mix

applications.

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5.) Self Compacting Concrete versus high slump concrete

Increasingly self compacting concrete (SCC) or more correctly super workable

concrete (SWC) is gaining market acceptance in modern concrete supply. Some

applications and methodologies are moving forward with this technology more so

than others, these being the piling and precast type industries.

Consistency of supply is of paramount importance with SCC/SWC concrete as

moisture content changes in aggregates can be significant enough to alter performance

to the point of potential mix segregation or insufficient compaction and poor surface

finish.

The use of viscosity modifying admixtures (VMA) to provide more robustness of the

mix to resist uncontrolled moisture changes, and aggregates susceptible to low shear

stress in the paste, are growing in use. With deep piles placed by centrifugal flight

auger (CFA), bleed trails and settlement are very much undesirable outcomes,

particularly as piles are excavated and left as the finished concrete surface.

Super workable concrete for such applications requires a mix design to be low in

bleed rate to minimise water and fines bleeding up the pile, particularly on the outside

face, and leaving an unsatisfactory surface finish. Therefore with or without the use of

viscosity modifying admixtures, the concrete must not be susceptible to bleed water

or consolidation to the point of segregating the coarse and fine aggregates from the

paste/mortar of the concrete. Through the optimal combined grading of the concrete

to reduce coarse aggregate content (to typically 800-900kg/m3 depending on density)

and over sanding the mix to allow for a more cohesive mix design, resistance to

bleed/segregation can occur. This is the reason moisture contents of fine aggregates is

important in SCC/SWC to reduce the likelihood of consolidation in the pile/structure.

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6.) Slag cement (GGBFS) –

Increasingly the use of supplementary cementitious materials are being used for

sustainability, marine/aggressive environments, or as a cement production extender to

tailor a cement blend to suit a particular concrete application.

Historically in Australia, Ordinary Portland Cement – OPC (GP or SL) has been pre

blended as an interground, or post blended cement type such as GB (60% SL / 40%

GGBFS) or Marine/Low Heat (60-65% GGBFS / 40-35% SL). These cements may

have been interground to improve reactivity or performance, but more recently have

been manufactured and distributed for use individually to better enable tailored

cement blends. This therefore enables ready-mix concrete companies to have

flexibility to blend cement types to produce a cement blend that has the optimal

binary, ternary or potentially quaternary combination.

Supplementary cementitious materials such as slag cement and fly ash offer

significant benefits in high durability concrete applications, but are not readily able to

achieve early strengths in precast marine concrete elements. Port Botany Seawall

expansion was such a case as this, where durability (100 year service life) in a marine

environment was able to be achieved as well as early age strengths required for

construction efficiency in a precast yard operation.

Through the use of improved techniques to assess early age strengths, using concrete

maturity methods, insitu concrete strength can be used to optimise cycle times versus

the empirical relationship of destructive cylinder testing (often performed offsite and

requiring cylinder transportation). Also using service life predictions such as chloride

diffusion coefficient in Fick’s Law modelling programs, means much more accurate

durability can be determined.

7.) Sustainability

Green Building Council of Aust.

Greenstar Rating Tool and Activated cements for improved early age and low

shrinkage – Envisia/ZEP

As per the work performed and reported by Flowers and Sanjayan, Monash

University 2007, the GHG Emissions factor of Ordinary Portland Cement (OPC) is

the highest of cementitious materials followed by slag cement by a factor of 1/6th less

and fly ash at 1/30th. Coarse aggregate being the second most intensive GHG

Emission factor is approximately 1/20th of OPC. Therefore there is much to be gained

in supplementary cementitious materials use as a reduction in clinker content of the

concrete overall.

SOURCE: Summary of CO2 Emissions From Concrete—Monash University (Flowers and Sanjayan, 2007)

Activity Emission Factor

Coarse Aggregates - Granite/Hornfels 0.0459 tCO2-e/tonne Coarse Aggregates - Basalt 0.0357 tCO2-e/tonne Fine Aggregates 0.0139 tCO2-e/tonne Cement 0.8200 tCO2-e/tonne Fly Ash (F-type) 0.0270 tCO2-e/tonne GGBFS 0.1430 tCO2-e/tonne Concrete Batching 0.0033 tCO2-e/m

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Concrete Transport 0.0094 tCO2-e/m3

On Site Construction Activities 0.0090 tCO2-e/m3

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In these times of sustainability and life cycle assessment of structures, from cradle to

grave irrespective of durability requirements, increased use of fly ash and slag

cements results in concrete with lower early age strengths, and generally higher

drying shrinkage with ground granulated blast furnace slag. Therefore much effort is

being expended on developing cements that have high supplementary cementitious

materials contents, but not to the detriment of early age strength or drying shrinkage.

This is being achieved by various means ranging from geopolymer concrete through

to activated cements.

One such example is Boral Cement Envisia concrete utilising ZEP activated cement

which has similar early age strength growth as straight cement mixes, but with the

added benefit of up to 60%-70% cement reduction (versus Green Building Council of

Australia – GBCA).

Also with the added benefit of drying shrinkage approximately 50% or greater of

equivalent conventional concrete, and also durability built in from the high SCM

content. This is a substantial shift from other high SCM mixes, in that typically

shrinkage is increased for high slag cement blends, and fly ash whilst favourable for

shrinkage cannot be used at 60-70% replacement. Even high volume fly ash mixes

with 50% reduction in OPC have drawbacks, such as low water cement ratio

additional requiring consideration.

The benefits derived from lower drying shrinkage are both risk minimisation due to

propensity to cracking, and the commercial advantage of reduced crack control

reinforcement. Post tensioned concrete application also has the benefit of loss of

prestress force due to relaxation of tendons on concrete shrinkage.

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Creep strain also has been shown to be approximately 40% lower than that of the

conventional sustainable mix designs. With the added benefit of low creep strain and

low drying shrinkage, vertical element size can be reduced for increased floor space,

or potentially less reinforcement may be required. Creep is quite often dictated by

aggregate quality and deformation under load, where Envisia concrete exhibits lower

creep irrespective of aggregate quality so has advantages over other high shrinkage,

relatively higher creep concrete.

One significant drawback of high supplementary cementitious material concrete mix

designs is the sacrifice in early age compressive strength, due to the lower reactivity

of the cements and slower hydration effects. Therefore cement blends containing in

excess of conventional 30% fly ash binary blends, or alternatively 60% OPC/40%

GGBFS slag cement mixes (with or without fly ash) have slow strength gain. These

early age strengths are even more problematic with Marine or Low heat cement

blends containing up to 65% GGBFS and 35% OPC.

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The significant advantage of activated cements is that early age strength is not

compromised to the degree of some high SCM concretes. Alternatively additional

total binder content is not required to achieve performance requirements, which may

compromise the intended sustainability measures. This can be observed in the

following illustration, where early age mixes such as post tensioned concrete

applications, or slip/jump form core wall structures can therefore utilise overall

cement (OPC) reductions of up to 60-70% when compared to GBCA (Greenstar

Rating Tool) reference cement contents.

Durability in marine or acid sulphate soils or aggressive exposure conditions usually

would require a Marine type cement blend comprising say 65% GGBFS and 35%

OPC or potentially a ternary blend comprising high slag/fly ash content (45% OPC,

30% GGBFS, 25% fly ash).

Where only two cement silos exist, potentially a 70% OPC and 30% fly ash may have

been used. The concern with these three options is there are compromises associated

with each concrete. The Marine cement has slow set times (particularly in winter),

and low bleed due to fineness of the slag cement means most probably an aliphatic

alcohol evaporation retarder should be used, and requires greater curing effort to

minimise the propensity to crack due to slow tensile strength gain and higher drying

shrinkage. The ternary blends sacrifices some chloride diffusion durability

performance, and still has relatively slower set time and bleed characteristics. Use of

an OPC/FA blend has the least desirable chloride diffusion coefficient, but has a set

time and strength gain profile that requires the lesser curing regime and protection

during placement.

The advantage of Envisia concrete is that it has the high slag cement component to

achieve low CDC values, but does not set as slowly as other high slag cement blends,

and with the relatively more rapid strength gain, is less prone to plastic or drying

shrinkage cracking.

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The table following shows typical improvement in concrete performance of relative

cement blends versus slag activated cements such as Envisia concrete.

50 MPa – high slump –

high durability mixes

Standard Port Botany

Expansion

Envisia

Grade 50 MPa 50 MPa 50 MPa

Cement Replacement NIL 57% 65%

Slump 200mm 200mm 200mm

Drying Shrinkage 650µs 450µs 300µs

Chloride Diffusion

(NT443 test, 35 days)

3.7 x 10-12 m2/s 2.8 x 10-12 m2/s 1.4 x 10-12 m2/s

Port Botany Case Study

(refer 1, 2,4,5 and 6)

The Port Botany Expansion Project comprised the additional 1.8 kilometres of sea

wall to accommodate 5 extra shipping berths as part of the Third Container Terminal

Operator. Rather than utilising the previous method of wall unit manufacture used for

Brotherson Dock in 1972, and railing the wall units into place in the water, the

process adopted by Baulderstone Pty Ltd was to construct a Precast yard and utilise

multi axle heavy unit transporters to transport, and then stage and cure the wall units

before a barge mounted shear crane was used to accurately place the units into the sea.

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Each of the 216 units comprised three pour segments (Base, Wall and twin Buttress

pours) and was manufactured using over 200m3 of concrete and weighing in excess

of 640 tonnes. The wall units were approximately 24 metres high, 8 metres wide and

12 metres deep.

Caisson Blocks were used for the sea wall at change in direction corners, and insitu

pours were utilised in the transition zone between the existing wharf and new section.

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A 2000 tonne Ringer crane was utilised to lift wall segments into position within the

Precast yard area, with the transporter unit used to transport walls to the interim

curing and storage area.

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The mix design having competing priorities of early age strength requirements for

efficient precast yard operation, was in conflict with the Sydney Ports Corporation

requiring a concrete that was high durability to achieve a minimum 100 year

design/service life, and therefore requiring a high supplementary cementitious

material (SCM) ternary blend with slow strength gain.

To minimise this conflict, rather than use traditional destructive physical test cylinder

samples to determine formwork striking and lifting strengths, the well established but

seldom used Concrete Maturity method of early age strength assessment was used.

This technique has been in use for many years, but was limited by the technology

needed to support it in the field. Historically the temperature capture process required

such equipment as site dataloggers, modems, computer, power/battery source to read

concrete temperatures and convert or transmit data back to a central location for

strength calculation.

Using newer technology such as RFID temperature tags, site antennae and a local

computer to remote server, the temperatures were more easily and efficiently captured

and strengths determined.

The following diagram essentially shows the typical sacrificial embedded temperature

tag used, and the associated wireless PDA hand held computer to manually capture

the insitu concrete temperature data.

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This manual system was able to be automated using an antennae system and cable

network to return concrete temperatures to a central computer system. Following is

typical of the Yagi antennae arrangement for capture and transfer of temperature data

to the computer network.

Further by using a SQL code, and Microsoft Outlook, client notification using an

automated and “out of hours” email process meant that the builder/client was able to

achieve a 30% reduction in total construction program against that forecast.

Therefore not only was construction cycle time more efficient, but a durable mix

using (53% OPC, 25% Fly ash and 22% GGBFS) resulted in a more environmentally

responsible mix, which culminated in the Cement Concrete and Aggregates

Association awarding the project the “Best Environmental Innovation Award” in

2011.

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Conclusion

The concrete industry, being one of the most used materials in the world, continues to

reinvent itself with innovative materials, technology and methodologies to solve

engineering challenges.

The previously mentioned areas of

• Manufactured Sands

• Durability assessment by chloride diffusion coefficient

• Fly ash manufacture by lower emissions technology such as LoNox burners

• Admixture Technology PCE Superplasticiser and slump retention admixtures

• Super workable/Self compacting concrete

• Slag cement – GGBFS

• Sustainability – Greenstar rating tools and slag activated cements

are one of many examples of innovation in a complex and competitive construction

industry.

It is clear that whilst the concrete industry faces challenges, there is also passion and

innovation not only within the concrete producer space, but also from suppliers such

as admixture, aggregates, and cement such that the construction materials industry is

in a healthy state.

Therefore it is evident that whilst there is a desire to improve the product offering, the

concrete industry truly is a sustainable and innovative material.