CONSTRUCTconcrete structures group

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BEST PRACTICE GUIDES FOR IN-SITU CONCRETE FRAME BUILDINGS ...

Concreting forimproved speedand efficiency

Introduction

The European Concrete Building Project is a joint initiative aimed atimproving the performance of theconcrete frame industry.

The principal partners in the world’smost ambitious concrete researchprogramme are:

British Cement AssociationBuilding Research Establishment LtdConstruct - the Concrete StructuresGroupReinforced Concrete CouncilDepartment of the Environment,Transport and the Regions

The programme involves theconstruction of a series of full-sizedconcrete structures in the LargeBuilding Test Facility at Cardington,where they are being subjected tocomprehensive testing of the buildingprocess and of their performance.

With support from the DETR and theEngineering and Physical SciencesResearch Council, the first of thesebuildings, a seven-storey in-situ flatslab concrete frame, was completedin 1998. The results of investigationsinto all aspects of the concrete frameconstruction process are summarisedin this series of Best Practice Guides.

These Guides are aimed at all those involved in the process ofprocurement, design and constructionof in-situ concrete frames. Theyshould stimulate fundamental changein this process in order to yieldsignificant improvements in the cost,delivery time and the qualityof these structures.

... FROM THE EUROPEANCONCRETE BUILDING PROJECT

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Figure 1: Organisations and facilities involved in the concrete procurement process

Designer/Specifier

Contractor/Purchaser

Supplier/Producer

Concretesupply

Concreteproduction plant

SiteConcreting

This Guide provides recommendations for improvingthe efficiency of the concreting process whilstmaintaining quality

Key messagesAchieving optimum efficiency and quality of the concreting process on site requiresconsideration of:

• The concrete procurement process as a whole (Figure 1).

• Interactions between concreting and inter-related construction processes (Figure 2).

• The use of performance-based concrete specification.

Best practice When planning the concreting process, the following aspects should be considered:

• Specified properties of hardened and fresh concrete.

• Selection of the concrete supplier.

• The method of handling / transporting the concrete on site.

• Reinforcement densities and congestion of reinforcement.

• Appropriate compaction, curing and finishing methods.

• Pour sizes and construction joints.

• Use of special concretes and innovative methods.

• Implications of interactions with other construction activities that may share resources.

• The overall construction programme.

• Appropriate communication methods.

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SpecificationConcrete specifications should be inaccordance with BS 5328: 1997,Concrete, Parts 1 and 2 as referencedby the National Structural ConcreteSpecification for Building Construction,(NSCS, Reference 1). The specificationdocuments should be concise andcomplete as given in NSCS Part 2. Thisenables the supplier to immediatelyconsider the specification data withouthaving to first interpret a new format.

Specifications should be performance-based. They should state the requiredproperties of the hardened and freshconcrete, but should otherwise be freeof unnecessary restrictions. This meansthat much of the responsibility forensuring that these qualities are achievedlies with the supplier. This is appropriate,since the concrete supplier is producingconcrete on a daily basis and thereforeis likely to have much greater expertiserelating to concrete production than anyother party in the construction process.Under BS 5328, performance specifiedconcretes are categorised as eitherdesigned or designated mixes.

The workability specified for theconcrete must be appropriate for theintended placing method. The additionof water on site should be avoidedwherever possible and must not be seenas a substitute for specifying the correctworkability in the first place.

Where there are many separate concretesspecified within a project, it may be moreefficient to rationalise these by mergingthose with similar performancerequirements. This will result in reducedrisk of confusion and possibly economiesof scale in batching.

Concrete specification can be mademore efficient by using software tools,e.g. ConSpec (for more information seethe Ready-mixed Concrete Bureau’swebsite at www.rcb.org.uk).

Concrete supplyTo achieve the most efficient concreting,potential concrete suppliers must besupplied with the concrete specificationas well as additional information suchas the proposed volumes, pour rates andplacing method. They may request furtherinformation or may make suggestions forimprovement (e.g. further rationalisation)before providing a quotation and mixdesign information.

When selecting a concrete supplier, theprimary issue other than cost is the abilityto supply at the required rate, particularlyfor large pours. The provision of a back-up supply should be considered by thepurchaser in case the primary plant fails.

Figure 2: Issues and interactions affecting optimisation of concreting process for in-situconcrete frames. Key influences are highlighted

Table 1: Influence of pour size on aspects of slab construction for the ECBP in-situ frame

Expertise from suppliers

Architecturalrequirements (e.g finishes)

Individualmix design

Testingrequirements

Construction jointsand detailing

Method of compaction

Method of curing

Requiredfinish

Structural requirements(e.g. grade, durability,

striking)

Regulatoryframework

Availabilityof materials

Concretespecification

Method of placing

Labouravailability

Optimised numberof mixes

Pour size

Overallsite constraints

Requirements of overall construction

programme and striking

Geometry ofstructure

Reinforcementfixing

Access, on-site transport

Formworkavailability and re-use

Limits onconcrete delivery

Single pour Two pours on Four pours onseparate days separate days

Each pour achievable within a Just Yes, easily Yes, easilyworking day

Supporting columns required to be All Over half Over a quartercomplete before slab pour

Can proceed with next level of No Yes Yescolumns before slab completed

Columns (above) poured at same No Possible Possibletime as slab (if same specification)

Formwork required Full slab area Two-thirds of One-third ofslab area slab area

Construction joints None One Three

Efficient use of pump hire Yes Less Much less

Need for additional safety provisions None Some Considerable(e.g. edge protection)

More even use made of site resources No Yes Yes (closer to a

(plant, operatives, materials etc.) continuous manu-facturing process)

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Handling, transporting andplacing concrete on siteAccess for the ready-mixed concretetruck should be provided as close aspossible to the required location in thestructure. It may be possible todischarge concrete directly from thetruck into foundations and columnbases, but for other components ameans of transporting the concrete fromthe truck discharge point to its requiredlocation in the frame will be required.

The two methods of transportingconcrete that are most applicable tomulti-storey frame construction are:

1. Pumping. A truck-mounted (oroccasionally static) concrete pump issited at ground level with its deliveryboom and/or hoses deployed so thatconcrete can be pumped directly towhere it is required in the frame.The input hopper of the pump istopped-up from truck-mixers.

2. By skip. A site crane hoists a skipfrom ground level (where it is filledwith concrete from a truck-mixer) tothe required location in the frame.

A typical in-situ reinforced concreteflat-slab frame has two primary structuralcomponents: columns and slabs (withor without downstands). The volume ofthese components is significantly differentand they are best considered separately.For example, the European ConcreteBuilding Project frame at Cardingtonconsists of seven flat slabs each with 20 columns. Each floor has a volume of about 165 m3, whereas the columnsfor each floor have an overall volume of only 9 m3.

Slab concretingThe volume of concrete required is amajor factor affecting the planning ofslab concreting operations. Daily pouringrates and hence slab concreting times canbe determined from considering the limitson the supply, handling on site, andplacing and finishing requirements. It maybe possible to pour an entire slab in oneday, but it could be more cost-effective tosplit the pour across two or more days.

Pumping is an attractive method forplacing concrete in slabs, since arelatively high rate of almost continuousplacing is possible. The pump can belocated where two truck mixers candischarge simultaneously into itshopper, resulting in a zero changeovertime. Little or no use is made of sitecranes, which are therefore available towork elsewhere on the site. Breaks inplacing are required only to connect ordisconnect pipework as the placingfront moves across the slab.

However, using a skip for slab poursshould still be considered. This will makecontinuous use of a site crane, but if thecrane would otherwise be idle duringthe concreting operation it would be anefficient use of this resource. The rate ofplacing is likely to be significantly lessthan that possible by pumping since theplacing is not continuous. This can bealleviated by using the largest capacityskip that can be hoisted by the crane orby using additional skips. As the framerises, the time taken to hoist the skip tothe slab will increase.

Both pump and skip placing will benefitfrom a planned progression across theslab. In general, pumping should bestarted in the corner most distant fromthe pump and worked backwardstoward it in a swathe of a convenientwidth, reducing the length of thepipeline as required. For skip placing,the swathes should be aligned in such a way that the most frequent movementsof the crane are rotational.

Column concretingIn contrast to slab concreting, that forcolumns may involve small volumes ofconcrete. For example, at each level ofthe in-situ frame at Cardington, the totalvolume of concrete for the columns was9 m3. There were 20 columns and sevensets of formwork available, so theconcrete was supplied and placed onthree separate days, each delivery being3 m3 (i.e. half a truckload).

Overall speed of construction shouldbe balanced against other costs. Forexample, these columns could havebeen poured on two separate days if tensets of formwork had been available;in which case each concrete deliverywould have been 4.5 m3. One or twodays would possibly have been cut fromthe construction programme, butformwork costs would have increased.On the other hand, concrete costsmight have decreased due to fewerpart-load surcharges.

Pour sizes andconstruction jointsIdeally, each structural component of a building would be cast monolithically,but this is often not practicable. TheNSCS, Part 1, Table 1 (Reference 1)suggests limits on pour sizes for wallsand slabs, although these can beexceeded by agreement. In general, a pour should be achievable within aworking day. Above ground level, a pour for the floor slabs of a typicalframe can be over 500 m2 in area andup to 30 m in any dimension (the ECBPin-situ frame was 30 m by 22.5 m or 675 m2). Construction joints areacceptable provided they do not

compromise the performance of thestructure. Even when the use of aconstruction joint is not intended, thepossibility of an unexpected cessation of a pour should be planned for.

The size and number of pours for eachfloor slab of a multi-storey frame affectsthe overall progression of construction,particularly the processes related toformwork and reinforcement. Forexample, each slab of the ECBP in-situframe was cast in a single day, but thismajor pour could have been split intotwo or four pours, each carried out onseparate days. To determine whichapproach is most efficient it is necessaryto consider the cost implications of thepoints of comparison listed in Table 1.

CompactionAdequate compaction is essential toensure that the concrete performssatisfactorily in the completed structure.Under-compacted concrete will havereduced strength and /or durability, andmay be of unacceptable appearance.Appropriate compaction equipmentmust therefore be available when it isrequired and concrete-placing personnelshould be trained to use it correctly

The poker vibrator is likely to be mostappropriate compaction device forin-situ reinforced concrete frames.Beam vibrators or hand tamping canbe used to compact and finish the topsurfaces of slabs, but a poker vibratorwill still be required to ensure thatadequate compaction is achieved throughthe full thickness and at the edges.

FinishingThe finish required for a frame will bespecified within the overall specificationfor the structure. It will be at least Type A for formed finishes and U1 forunformed finishes (References 1 and 2).These will generally be adequate wherethe concrete surfaces are to be coveredby cladding or raised floors etc, as isoften the case for office buildings.If the concrete surface is to be visuallyexposed, or is to be directly fitted witha floor covering, then a higher qualityfinish may be required. This should bespecified in the NSCS, Part 2, Projectspecification.

CuringCuring involves preventing the loss ofmoisture from the concrete after casting.This can best be achieved by protectingexposed surfaces.

It may also be desirable, or even essential,to control the temperature of the concreteto ensure that cement hydration proceedsat an acceptable rate. In winter, the

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ambient temperatures may be so lowthat it is necessary to take measures toensure that the concrete temperature ismaintained at a suitable level during theinitial stages of curing. The cast concretecan be insulated against loss of heatgenerated by the hydration process; inaddition the concrete can be supplied atan elevated temperature.

Special concretesReady-mixed concrete suppliers haveextensive databases that will allow themto rapidly produce a mix design to meetthe requirements of most in-situ frames.In some cases, however, some mixdesign development will be necessary.Examples include high strength andself-compacting concretes.

High strength concreteHigh strength concrete is beingincreasingly used, particularly in highlystressed compression elements. Thecolumns on the lower three levels of theECBP frame were of grade C85 concrete,which permitted the same column sizeto be used economically over the fullheight of the structure. Polypropylenefibres were added to the high strengthconcrete columns at Cardington toreduce the potential for spalling in theevent of fire.

Self-compacting concretesSelf-compacting concretes are nowbecoming available. They can be placedmuch more rapidly with savings inlabour and plant costs, which will offsetthe higher material cost. Their useresults in reduced noise on site andpermits placing of concrete incircumstances which would otherwisebe difficult or impossible with conven-tional concrete (e.g. highly congestedreinforcement, pumping from below,and greater lift heights up to 10 m).

Use of such special concretes, which is not currently routine, can affect theconstruction process. The supplier mayneed to develop and test an appropriatemix design. An extended lead-time willbe required for this and it will need tobe built into the overall constructionprogramme. Special arrangements mayalso be necessary if some of the mixconstituents are not routinely stored inhoppers at the ready-mixed plant.

If the use of such special concretes isproposed, discussions with the suppliershould be held as early as possible.

Early striking of formworkTo increase the speed of the constructionprocess and reduce formwork costs, itmay be desirable to strike formwork asrapidly as possible to make it availablefor early re-use. The NSCS states that itis the contractor’s responsibility toensure that the concrete has adequatestrength to support its own weight andany construction loads without sufferingshort- or long-term distress. This iscovered by a separate Guide in thisseries, Early striking for efficient flat slabconstruction. Concrete not required tocarry its own weight should still have acompressive strength of at least 5 N/mm2,to ensure that damage does not occurduring formwork stripping.

Planning for interactionswith other site operationsConcreting is but one component of acomplex construction process. Manyparameters affect its efficiency andthese will vary between projects. Majorplanning decisions concerningconcreting operations cannot thereforebe taken without considering inter-actions with other processes on site.This needs to be done within thecontext of the specific project and withreference to the erection drawings,which may preclude some economicallyattractive options. Consideration mustalso be given to the effect of the variousoptions on construction loadings andrequired concrete maturity.

Figure 2 identifies the main issues andinteractions in relation to the concretingof an in-situ frame.

CommunicationsCommunications are an important aspectof concreting, and indeed the wholeconstruction process. Everyone shouldbe aware of the need for informationflow to be concise, complete and timely.Clear specification is the key in thisprocess (Reference 3). All parties musthave the information they require inorder to play their part in placing thespecified concrete in the correctlocation in the frame, in an appropriatemanner. Information technology anddocument layouts that are standardisedacross the industry should be used more widely.

References1. CONSTRUCT, BCA, BRE and RCC

National structural concrete specificationfor building construction (NSCS).Crowthorne, BCA, 2000. Ref 97.378.

2. BSI. Structural use of concrete. Part 1:Code of practice for design andconstruction. London, BSI, 1997.BS 8110-1: 1997.

3. COLIN GRAY. In situ concrete frames.The University of Reading, 1995.

Best Practice Guides in this series• Improving concrete frame construction• Concreting for improved speed

and efficiency• Early age strength assessment of

concrete on site• Improving rebar information and supply• Early striking for efficient flat slab

construction• Rationalisation of flat slab reinforcement

Further Guides are planned

Research partners for this GuideImperial CollegeRMC Readymix LimitedBuilding Research Establishment Ltd

BEST PRACTICE GUIDES FOR IN-SITU CONCRETE FRAME BUILDINGS

This Best Practice Guide is basedon research report, Process efficientconcreting: improved speed and qualityby A. Pullen, J.B. Newman and P. Chana. BRE report published by CRC Ltd. (020 7505 6622).

97.502First published 2000ISBN 0 7210 1553 0Price group A© BCA, BRE Ltd, Construct, RCC, DETR

Published by the British Cement Associationon behalf of the project partners.

British Cement AssociationCentury HouseTelford AvenueCrowthorne, Berkshire RG45 6YSwww.bca.org.uk

For further copies of the Best Practice Guidesring the Concrete Bookshop on 01344 725704.

All advice or information from the British CementAssociation is intended for those who will evaluate thesignificance and limitations of its contents and takeresponsibility for its use and application. No liability(including that for negligence) for any loss resulting fromsuch advice or information is accepted. Readers shouldnote that all BCA publications are subject to revision fromtime to time and should therefore ensure that they are inpossession of the latest version.

Ready-mixed Concrete BureauCentury House, Telford AvenueCrowthorne, Berkshire RG45 6YSTel: 01344 725732 Fax: 01344 774976www.rcb.org.uke-mail: jhannah@rcb.org.uk