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BPG_StGeorge_SpecialConcrete

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Concrete Best Practice Guides
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  CASE STUDIES ON APPLYING BESTPRACTICE TO IN-SITU CONCRETE FRAME BUILDINGS Special concretes Introduction Figure 1: The improved finish from the use of self-compacting concrete for vertical elements Specifying and using innovative concreting materials canimprove the speed of the construction process and enhancethe quality of the resulting concrete. Key points This Case Study discusses the experiences and benefits of using self-compacting concrete and the feasibility of using CRC JointCast 1 ãThe use of self-compacting concrete resulted in an improved quality of surface finish. If self-compacting concrete were used more widely forvertical elements the reduction in the amount of making good requiredcould outweigh the material cost premium. ãSelf-compacting concrete also provides wider environmental and health and safety benefits by eliminating the need for poker vibrators.This reduces noise and cuts the risks from hand-arm vibration (HAV).ãCRC JointCast* showed potential to speed up the construction of verticalelements by joining precast components and to greatly reduce the crane(hook) time required for this activity. *CRC JointCast is an ultra high strength jointing material that may be used to create monolithicconstruction using precast elements. St George Wharf Case Study The European Concrete Building Projectat Cardington was a joint initiative aimedat improving the performance of theconcrete frame industry. It led to thepreparation of a series of Best PracticeGuides, giving recommendations forimproving the process of constructing in-situ concrete frame buildings.  As part of a programme to disseminateand apply what has been learnt fromCardington, BRE has subsequentlyworked directly with those involved in St George Wharf, a high-profile, 100,000 m 2 mixed-use phased development on theRiver Thames.BRE worked jointly with the developers,St George (South London), theirengineers, White Young Green, and specialist concrete contractors,Stephenson, to develop and implementprocess improvements tailored to the St George Wharf site.This work has led to a series of innovations being trialled, the results of which are summarised in this series of Best Practice Case Studies. 1 www.cementindustry.co.uk www.stephenson-ssc.co.ukwww.wyg.comwww.bre.comwww.dti.gov.uk www.construct.org.uk www.stgeorgeplc.comwww.concretecentre.com  Introduction A number of options for using innovativeconcreting materials were explored in theconstruction of the reinforced concreteflat slab frame structures at St GeorgeWharf. Two particular materials wereidentified as offering potential benefits: ãSelf-compacting concrete (SCC) ã CRC JointCastTwo further options were considered, but were thought to be inappropriate for this project:ãThe use of high strength concretes(above 60 N/mm 2  ) to further reduceand rationalise column dimensionswas not seen to provide a significantadditional benefit.ã The use of a ‘Superstriker’concrete toallow earlier striking of the floor slabswas also considered but was regardedas unnecessary for the phaseinvestigated, principally because of the restrictions imposed on the floorcycle by the construction of the liftshaft walls. Use of self-compacting concrete Self-compacting concrete offers potentialadvantages in terms of improved qualityof finish, reduced noise, and health andsafety benefits [1]. The opportunity hasbeen taken to use it at St George Wharf in limited areas to compare costs and thequality of finish achieved with those of conventional concrete construction, andto investigate the ease of specifying andobtaining the material. Other countries are further ahead in theuse of SCC than the UK. However, thematerial is beginning to be used morewidely by the UK precast industry withfactory batching plants allowing greatercontrol over the whole process. Some of the larger UK precast plants currentlyuse SCC for approximately 65% of theirproduction, with this proportionincreasing all the time.Increasingly ready-mixed concrete plantsare offering the material; that used for 2 Figure 2: Proposed location for use of CRC Jointcast 0Traditional     R   e    l   a   t    i   v   e   c   o   s   t    (    %    ) Precast panels using CRC JointCast Jump-forming 20406080100120140160 Figure 3: Relative costs to the contractor of constructing cores using different methods Plan of Core  the St George Wharf site was suppliedlocally with all the materials required keptin stock. 48 hours notice was required forthe concrete supplier, principally to allowthem time to provide a technician on site.Self-compacting concrete was used inlimited areas on this development: on two floors in lift shaft walls, upstandbeams and columns, together with theprecast stairs constructed on site. A total of approximately 13 m 3 was used –all placed by skip. Pumping was not costeffective for the small volumes requiredfor these elements.One less operator was needed during concreting operations, as vibration wasnot necessary. Coupled with skipping of the concrete, this elimination of noise from poker vibration could havemade evening/weekend working a real possibility.No particular additional precautions weretaken to deal with formwork pressuresother than closer attention to sealing of joints, and no particular problems wereexperienced. However this does notnecessarily reflect experience elsewhere,where more stringent measures havebeen taken to control formworkpressures. The pressure resulting fromthe height of the fresh concrete is animportant factor to be considered.The quality of surface finish achieved wasfound to be superior to that forconventional in-situ concrete, with muchless requirement for making good. Thisimprovement is of particular benefit forvertical elements, which tend to requiremore making good, but use relativelysmall volumes of concrete, thus incurring a lower cost premium for the SCC. Cubes made from the self-compacting concrete showed a high early strength gain.At the time this study was carried out, the cost of the self-compacting concretewas about twice that for conventionalconcrete. This was due in part to the lowvolumes supplied, the additionaltechnician support on site and thesupervision required at the batching plant. The cost of self-compacting concrete is generally reducing, hencemaking the economics of its use muchmore attractive. At the time of writing,the cost premium over ‘normal’structuralgrade concrete is about 50% and falling. Use of CRC Jointcast Constructing vertical elements iscommonly believed to constrain potentialreductions in floor cycle completiontimes that can be achieved by adopting other innovations. Measurement of thetime and resources associated withconstructing lift shaft walls at St GeorgeWharf confirmed this, and highlightedthe potential benefits to be achieved byconsidering alternative approaches tothe construction of such walls. For the rapid construction of tallbuildings it is common practice to jump-form or slip-form the main verticalconcrete cores that provide lateralstability to the building. However there is usually a substantial cost premiumincurred by using these techniques,which needs to be offset against thesavings in time that can be generated. An alternative approach, which wasconsidered at St George Wharf, was theuse of precast elements joined with CRC JointCast* (Figure 2). This material * is anultra-high strength fine aggregateconcrete material with excellent bondproperties for ribbed bars. Its usepermits lapping of reinforcement oververy short distances, and allows theformation of monolithic constructionbetween precast elements by using verynarrow joints [2,3]. A preliminary study looked at thebenefits of using CRC JointCast and theassociated costs for use in panel-paneljoints. The material is expensive so thereis a desire to minimise the volumesused. In practice this means maximising the precast unit sizes (consistent withavailable crane capacity) and minimising the diameters of the bars projecting intothe joints. In this case, a wall panel thickness of 250 mm was assumed with a maximum weight of 9 tonnes and bar size of 16 mm. This restriction on panel weight gives awall area of 15 m 2 , and the bar size givesa joint width of 150 mm. Taking aneffective joint width of 75 mm all arounda typical panel, panel dimensions of 4 m x 3.75 m and costs of £1000/m 3 for CRC JointCast and £60/m 3 for the concrete inthe panel gives an overall concretematerial cost of £127/m 3 . To be added tothis is the additional cost of temporarilysupporting the wall units and mixing andplacing the JointCast. The total costwould then need to be offset againstsavings that should result fromincreasing the speed of the overallprogramme.The study considered CRC JointCastalongside other methods such as slip-forming and jump-forming and againsttraditional methods. A comparison ismade in Figure 3.The results of this exercise suggestedsome benefit in terms of speed of construction over traditional methodsand a considerable saving (approximately two-thirds) in crane time.This could have a major advantage infreeing up hook time for other activitiessuch as following trades, and reducing dependence on the crane during adverseweather conditions. Overall costs to the contractor of adopting the CRC JointCast solution,including additional items highlightedabove, were estimated to beapproximately 20% higher than adopting a traditional approach. This compareswell with jump-forming, which wasestimated to be 40% more expensivethan traditional methods for theparticular lift shafts considered. One advantage of using CRC JointCastwould be that it would allow all verticalelements to be precast and form amonolithic structure. Other forms of fast construction, such as slip-forming and jump-forming, can be used only forcores that have their own inherenttemporary stability.  3 * www.crc-tech.com  CASE STUDIES ON APPLYING BESTPRACTICE TO IN-SITU CONCRETE FRAMED BUILDINGS Conclusions andrecommendations 1. Measurement of the time andresources associated withconstructing lift shaft walls at StGeorge Wharf confirmed previousknowledge of the restriction theirconstruction imposes on anyreduction in floor cycle time. 2.The contractor and the client foundself-compacting concrete to be of ahigh quality and easy to use, withsavings to be made in manpower andtime. However, the cost of SCC stillmade it more expensive overall thanusing conventional concrete. 3. The contractor would be enthusiasticabout using SCC more generally if theprice were to continue reducing.4. More widespread use of SCC forvertical elements on the next phasesof St George Wharf may beconsidered, with additional materialcosts offset against those for making good.5. Self-compacting concrete offersparticular advantages in areas wherethere is highly congestedreinforcement or complicatedformwork.6. The health and safety benefits of using self-compacting concrete arenow increasingly recognised. This isbecause the requirements for record-keeping and limitation on exposuretime to percussive equipment arebecoming more stringent in efforts toreduce the risks from hand-armvibration injury.7. CRC JointCast showed potential forspeeding up the construction of liftshaft walls with considerable savingsin the amount of hook time required. The work undertaken and theconclusions reached in relation to theinnovations described above should beviewed in the context of the particularproject on which the innovations havebeen trialled. This Case Study is underpinned by a full report [4] giving the background and further information on the workundertaken. References 1.  Self-compacting concrete, ConcreteSociety Information sheet (publishedas a supplement to Concrete Vol . 35No.1 January 2001).2. Jointing in precast concrete buildings:developments and innovations, Concrete, October 2001, pp. 64 - 65.3. High-strength jointing methods, Concrete, February 2002, pp. 52 - 53.4. Best practice in concrete frameconstruction; practical application of at St George Wharf, by R. Moss. BREReport 462, 2003.  Acknowledgements The support of the DTI for this projectunder the Partners in Innovation schemeis gratefully acknowledged. Case Studies in this series of applying best practice:ã St George Wharf project overviewã Early age concrete strengthassessmentã Early age construction loading ã Reinforcement rationalisation andsupplyã Slab deflectionsã Special concretes Ref TCC/03/07First published 2004Price group AISBN 1-904818-07-2© The Concrete Centre 2004 Published by The Concrete Centre onbehalf of the project partners. www.concretecentre.com. For further copies of these Best PracticeCase Studies contact www.concretebookshop.com  All advice or information from The ConcreteCentre is intended for those who willevaluate the significance and limitations of its contents and take responsibility for itsuse and application. No liability (includingthat for negligence) for any loss resulting from such advice or information is accepted.Readers should note that all Centre publications are subject to revision from timeto time and should therefore ensure that they are in possession of the latest version. 4
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