Key messagesEarly* striking of formwork and improved backpropping procedures increaseconstruction efficiency by:

• Allowing early access to the supporting floors by following trades.

• Enabling greater re-use of formwork.

• Reducing the quantity of formwork and falsework required.

*Early in this context is defined as concrete less than three days old.

Best practiceOptimise formwork use by:

• Determining backpropping loads during construction using empirical methods for simplerepetitive structures and a three-dimensional approach for special structures.

• Determining required striking concrete strength from serviceability criteria.

• Determining early concrete strength in-situ, using pull-out inserts. (See companionBest Practice Guide, Early age strength assessment of concrete on site.)

• Striking formwork in an agreed sequence after consideration of concrete pouring sequence.

• Controlling the loads on newly struck slabs until they have gained the required strength.

CONSTRUCTconcrete structures group

www.bca.org.uk www.bre.co.uk www.construct.org.uk www.rcc-info.org.uk www.detr.gov.uk

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

Early striking andimproved backproppingfor efficient flat slab construction

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 EstablishmentConstruct – 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 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 qualityof these structures.

... FROM THE EUROPEANCONCRETE BUILDING PROJECT

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Figure 1: Table forms at Cardington

This Guide provides new and improved recommendationsfor striking formwork and gives new procedures for backpropping reinforced in-situ concrete flat slabs (less than 350 mm thick).

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IntroductionThe use, installation and striking offalsework and backpropping* is a vitallyimportant part of the safe and economicconstruction of in-situ concretestructures. In order to strike a slab, theconcrete must be strong enough toavoid failure or undue cracking anddeformation of the slab. Recent researchduring the construction of the EuropeanConcrete Building Project (ECBP) in-situbuilding at Cardington has led to a newunderstanding of the constructionprocess. It is now possible to makesavings in construction time and ontemporary works equipment whilemaintaining site safety and theperformance of the constructed flat slabframe. This Guide summarises the newrecommendations for formwork striking and slab backpropping. For a detailedanalysis and a worked example see Guide to flat slab formwork and falsework (Ref. 1).

Planning the strikingsequenceThe striking of formwork and falseworkfrom a slab needs careful consideration.The constructor is responsible forcarrying out the striking process safely.It is the responsibility of the TemporaryWorks Co-ordinator (TWC) to managethe risks in early striking. This willinclude preparing detailed proceduresand method statements, which shouldbe approved by the safety officer andPermanent Works Designer (PWD).

The PWD’s role is to provide designinformation so that the constructionmethods do not adversely affect theperformance of the structure. All technical and managerial staff must be fully aware of the implicationsof the methods and procedures adopted.Flow charts for striking and back-propping procedures are given in Ref. 1.

Construction loadconsiderationsIn multi-storey in-situ concreteconstruction, the critical loadingcondition for a slab is not necessarilywhen it is struck and becomes self-

Table 2: Sequence of striking flat slabs

Figure 2: Diagrammatic representation of backpropping

Table 3: Load distribution by backpropping

Notes

1. Assumes lower and supporting floors have been struck, have taken up their deflectedshape and are carrying their self-weight

2. Floor loading from imposed loads and self-weight is not considered

3. The strength of particular slabs to carry applied loads will have to be considered separately

4. All floors are suspended floors

5. Figure 2 gives location of loads wp, wb1, and wb2

*Backpropping is defined aspropping installed at levels belowthe slab supporting the falsework.

It is done to distribute the loadapplied to the uppermost slab to suitable supports, such as lower slabs or foundations

•Obtain approval to strike

•Check the safety precautions

•Strike according to guidance given below and as illustrated.

◊ Strike reinforced concrete slabs in two stages:

1. Ease all the supports

2. Starting at midspan, remove supportsworking towards columns and walls.

◊ Post-tensioned slabs will tend to lift off the formwork on tensioning, but use method for rc above.

◊ On large slab areas, comprising internal and edge panels, strike internal bays first, followed by edge and corner bays.

◊ Where soffit form is part of cantilever, start removal from tip and work towards wall, beam or columns.

◊ Post-tensioned cantilever slabs may havespecial striking requirements due to deflection of adjacent spans.

Location Load No backprops One level of Two levels ofbackprops backprops

On slab On slab In props On slab In props

New slab being cast total 100% 100% 100%

Falsework/formwork wp 100% 100% 100%

On supporting slab (1) 100% wp 70% wp 65% wp

In backprops wb1 30% wp 35% wp

On lower slab (2) 30% wp 23% wp

In backprops wb2 12% wp

On lower slab (3) 12% wp

One level Two levelsof propping Load of propping

Slab to be cast

Falsework wp

Supporting slab (1)

Backprops wb1

Lower slab (2)

Backprops (when fitted) wb2

Lower slab (3)

REINFORCEDCONCRETE SLAB

POST-TENSIONEDCONCRETE SLAB

REINFORCEDCONCRETECANTILEVER

POST-TENSIONEDCONCRETECANTILEVER

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supporting but often when it is calledupon to support the weight of a freshlycast slab above.

Before concreting operationsTo cast the new slab, the supportingslab immediately below should havesufficient capacity to carry the loadsimposed on it during construction. The loads on the supporting slab are:

• Self-weight of supporting slab (at 24 kN/m3).

• Any construction load on thesupporting slab.

• Total construction load of the newslab, i.e.

◊ weight of concrete in new slab plus

◊ the self-weight of the temporaryworks plus

◊ construction live load on new slab.

If the supporting slab has sufficientcapacity to carry the loads imposed,backpropping is not required. If it doesnot then backpropping is required (see‘Design of backpropping’ on page 3).

Before they are loaded, the concretestrength required in backproppedsupporting slabs should be calculatedusing the determinations in Table 1. It should be noted that w will be muchhigher than for the new slab (see Table 3.) Determination 1 might be critical and this temporary conditionmight need to become a design loadcase agreed with the PWD.

Before strikingThe TWC will have assessed constructionimposed loads when determining therequired strength for striking but thismust be checked, by site inspection,against actual loading. Considerationmust be given to the timing of theloading of the struck slab, especiallywhere flying or tableform systems are quickly moved onto it.

Determining the requiredconcrete strength for strikingThe recommended method is based on serviceability considerations, as inpractice these are critical. The basis of this determination is fully detailed in ECBP research report BR394 (Ref. 2)

The required strength of a concrete slabfor striking depends on:

• Characteristic design service load.

• Construction load.

• Characteristic concrete designstrength.

• Actual concrete characteristicstrength at the time of striking.

The formula for calculating the requiredearly striking strength is given byDetermination 2 in Table 1. This givesthe characteristic strength of concrete fc required for early striking. It isrecommended that a minimum liveconstruction load of 0.75 kN/m2 beused for the slab. This load should be added to the slab self-weight tocalculate w. An alternative graphicalmethod of obtaining fc is given in Ref. 1.

Measurement of early ageconcrete strengthFor early striking, the in-situ strength of the concrete should be determined.The Best Practice Guide Early ageassessment of concrete on siterecommends the Lok test or similar as it tests the actual cast concrete. Thearea to be struck should be determinedin advance and the location of the mostcritical concrete (usually the leastmature) should be established and tested. The average result of four or more tests is used to estimate theexisting characteristic strength (fc) in Determination 2 for the area to be struck.

Striking sequenceThe order of striking soffit formwork is important. For early striking, thesequence should follow the order in which the concrete was poured.

However, the nature of the structuremay dictate the procedure adopted (see Table 2). The TWD, in co-operation with the PWD, shouldstipulate the method and procedure for the removal of the formwork.

After strikingAs the concrete gains strength withmaturity and approaches designstrength, the construction load can be increased. Early striking will allowmaximum re-use of formwork and falsework, and clearance of thesupporting floor and any lower levels of backpropping. The procedure shouldmake allowance for loading regimesafter early striking. It is good practiceonce a floor is struck to installbackpropping beneath it at the earliestopportunity; this will then helpdistribution of subsequent loads.

Deflection of slabssubjected to early strikingDeflection of concrete sections isclosely linked to the extent of crackingand the degree to which the crackingcapacity of the section is exceeded.Research at ECBP has shown that,provided the strength of the concrete at time of striking meets Determination 2in Table 1, there is no significant effecton the total deflection.

Ongoing research on long-term (3000 days) deflection indicates that,compared with striking at 7 days,striking at 24 hours increases long-termdeflection by 25% and, at 3 days, by15%. ECBP slabs struck at 19 hoursremain within design limits.

Traditionally, UK design codes haveplaced limits on the ratio of span toeffective depth. These have led to anacceptable serviceability condition formost flat slab structures withoutnecessarily addressing current needs for early-age loading. However, it canbe generally assumed that early strikingwill not significantly affect totaldeflection This is considered further in Section 6.3 of Ref. 1.

Design of backpropping The requirement for backpropping and its installation is not strictly part of striking formwork. It has beenincluded in this Guide as the recentresearch at Cardington (Ref. 2) haschanged previous understanding and re-emphasised the importance of backpropping in efficient flat slab construction.

Table 1: Determination of strengthrequired for early striking of flat slabs

These formulae apply for flat slabs

300 mm thick or less

where

wser is the total unfactored designservice load, kN/m2

w is the total unfactoredconstruction load on the slabconsidered, kN/m2

fc is the required characteristicconcrete strength to be able to strike the flat slab, N/mm2

fcu is the characteristic strength of the concrete,N/mm2

Determination 1

w

wser

≤ 1.0

Determination 2

fc ≥ fcuw 1.67

wser( (

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Determining thebackpropping requirementThe construction load is determined as described on page 2 under ‘Beforeconcreting operations’. If the supportingslab has sufficient capacity to carry theloads imposed, no backpropping isrequired, but if not then backproppingis required.

Figure 2 gives a representation of typical backprops over differentnumbers of floors.

The number of floors to be backproppeddepends on the load on the slabimmediately below that being cast not exceeding the design service load.In practice there will be little benefit in having more than one level ofbackpropping, as can be seen fromTable 3. The assumptions made inarriving at the percentages in Table 3 are outlined below and are explained in detail in Section 6.5.2 of Ref. 1. They are:

• The slabs behave elastically.

• An imposed construction operationsload is applied at all times.

• The density of normal-weightconcrete is 24 kN/m3.

• The backprops between the floorslabs and the falsework shoringsupporting the soffit formwork are notrigid, i.e. they change in length withload. (Props of different material e.g.steel and aluminium, MUST NOT be mixed. The percentages given in Table 3 are for aluminium props. If steel props are used, the load in the supporting slab may be reducedby 4%.)

• The load applied onto the supportingslab from the falsework is uniformlydistributed and, further, the back-propping generates a uniformlydistributed support system fromunderneath the slab. (This isreasonable if the backprops arespaced at least at one-third points along the supporting slab.)

• The load transferred throughsupporting slabs can be proportionedpro rata to the stiffness of the slabsconsidered. The percentages given inTable 3 are for slabs of equal stiffness.

• The backpropping is installed with zero pre-load. (Props are ‘finger tight’ to ensure stability in the unloaded condition.)

• The effects of temperature change are ignored.

References1. PALLETT, P. Guide to flat slab

formwork and falsework, CONSTRUCT,Crowthorne. To be published in earlysummer, 2001. 120 pp.

2. BEEBY, A.W. A radical redesign of thein-situ concrete frame process, Task 4:Early striking of formwork and forces in backprops, The University of Leedsand Building Research EstablishmentLtd. Report BR 394, CRC Ltd, London,2000, 106 pp inc 31/2" disk,

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 and improved

backpropping for efficient flat slab construction

• Rationalisation of flat slab reinforcement

• Prefabricated punching shearreinforcement for flat slabs

• Approaches to flat slab design

These guides are available for downloadingfree at www.rcc-info.org.uk/research

Research partners for this GuideLeeds UniversityBuilding Research Establishment

BEST PRACTICE GUIDES FOR IN-SITU CONCRETE FRAME BUILDINGS

This Best Practice Guide is based onGuide to flat slab formwork and falsework (Ref. 1).

97.505First published 2001ISBN 07210 1556 5Price 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.

Determining backpropping load The backprop load and hence the load in the supporting slabs can bedetermined by various methods usingboth two- and three-dimensionalanalysis. These are considered furtherin Ref. 1.

The simplest method, to be used for basic repetitive structures, is anempirical method based on thepercentage of load transferred in two-dimensional analysis. This isreproduced in Table 3, which sets out the recommendation from ECBPresearch (Ref. 2).

The three-dimensional approach shouldbe used for complicated structures, or where more precise calculations are required. An Excel spreadsheet for this is supplied with Ref. 1.

There is a requirement, summarised by Determination 1 in Table 1, that the design construction load should not exceed the design service load. On occasion during construction thisrequirement may not be met. Thiscould occur when the self-weight is a high proportion of the design load. In these circumstances, the permanentand temporary works designers shouldagree on the procedure to enable a safeand economical structure to be built. This is further considered in Ref. 1.

Striking of backproppingIn no circumstances should anybackpropping be removed until theformwork and falsework to the ‘new’slab have been struck.

Backpropping should be removed floorby floor in a top-down sequence. Eachfloor’s backpropping should be struckin the same sequence as for falsework(see Table 2). When special strikingsequences are required, which differfrom those set out above, detailedwritten procedures should be providedby the TWC for site staff, and theoperatives informed.

Backpropping should be installed assoon as possible after the ‘new’ slabhas been struck. This will assist indistributing the loads through thesupporting floors for the construction of the next slab.