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  CONSTRUCT concrete structures group BEST PRACTICE GUIDES FOR IN-SITU CONCRETE FRAME BUILDINGS... 1 Early age strengthassessment of concrete on site 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 of procurement, 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 quality of these structures. ... FROM THE EUROPEANCONCRETE BUILDING PROJECT This Guide provides recommendations for determining thestrength of concrete on site at early ages (less than three days)Key messages Knowledge of concrete strength at an early age: ã Allows significantly increased efficiency of in-situ concrete frame construction. ã Enables early striking of formwork and its economic re-use. This is furtherexplained in a companion Best Practice Guide, Early striking for efficient flat slab construction. ã Enables early prestressing with safety. ã Can give an indication of long-term strength, enabling early confirmation of the quality of the concrete as placed. Best practice ã Use pull-out inserts cast into the concrete to determine early age strength. ã Horizontally cast members (e.g. slabs) - locate inserts, using a floating cup,on the top surface of the slab near the end of the pour. ã Vertically cast members (e.g. walls, columns) - locate inserts on the formwork, with provision for early access before striking. Figure 1: the Lok-test jack in position.  2 Recommended test method  The principle behind the test method isthat the force required to pull an insertout of concrete can be correlated withthe concrete’s compressive strength.  The test equipment most commonlyused in the UK at the current time isthe Lok-test system. Load is appliedthrough a manually operated jack(Figure 1) that screws into the stem of the insert. The jack bears against theconcrete surface through a reactionring, typically of 55 mm internaldiameter. From the peak tensile forcerecorded by the jack, and by using anempirical correlation chart (Figure 2),the equivalent concrete cube strengthcan be estimated. Details of the actualcorrelation chart to be used, togetherwith the appropriate confidence limits,are available from the manufacturer of the Lok-test equipment. (See below for address.) Two main types of insert are used(Figures 3 and 4). For horizontally castconcrete there is a plastic buoyancycup that floats on the top surface of theconcrete, while for vertically castconcrete the insert is attached directlyto the formwork. The floating cupinserts are particularly easy to use sincethey do not require any pre-planning,but care must be taken to avoid thereinforcement. They are placed byhand and it is not essential that theyremain perfectly vertical. One key feature of this method is thatsmall changes in compressive strengthare easily detected. Also concretevariables such as aggregate type do notsignificantly affect the correlation. In the absence of more specific tests for correlation, the manufacturer’srecommended strength correlation forthese pull-out tests may be relied upon.When used in combination with thesuggested locations for sampling the concrete and appropriate confidence limits, this will give a lower bound strength. Site planning and practical issues ã Check that localised test damage tothe concrete surface finish will beacceptable. ã Position inserts 50 mm clear of reinforcement. ã Use regularly calibrated equipment. ã Use a trained operator. ã Base strength assessment on anaverage of at least four results. Figure 2:  Illustrative correlation between strengths determined by theLok-test and by conventional cube testing. Figure 3:  Lok-test inserts. Figure 4:  Lok-test inserts for formwork (left) and floating cup (right)    M  e  a  s  u  r  e   d  c  u   b  e  c  o  m  p  r  e  s  s   i  v  e  s   t  r  e  n  g   t   h   (   N   /  m  m    2    ) 100806040200 Force measured by Lok jack (kN)010203040506070 Screw |<25mm>| Floating cupSteel plate with holesScrew with washer StemStemDiscDisc Plate nailed to formworkCone of failureon testingCone of failureon testing (a) Fixed to formwork(b) Floating type 95% confidence limitManufacturer’srecommendedcorrelation  3 In general, due to relative differences in curing and compaction, concrete atthe top of a pour is less strong than thatat the bottom, so strengths derived fromtesting the top surface may beconsidered to be conservative. If it isnecessary to test vertically-castmembers, access to the test concretewill need to be provided, e.g. byproviding small removable panels in the formwork. Background Most concrete delivered to site iscovered by quality schemes. However,many users need to have specific dataon the strength of the concrete asplaced in their structure, e.g. for earlystriking of formwork or early pre-stressing for safety. This has led tosignificant benefits in terms of processefficiency and overall speed of construction.  The disadvantages of cube testing forearly-age strength assessment are: ã Generally, the results of testing come too late to allow economicalremedial action to be taken should a problem occur. ã Despite best intentions, the samplesare not necessarily representative of the concrete in situ. ã  The making, storing, transporting,testing and cleaning of cubes andcube moulds is a time-consumingand non-productive process, and willusually be impractical for very earlyage strength measurements.  These shortcomings are eliminated if measurements of concrete strength aremade in situ. The determination of formwork striking time or prestressingtime is a two-stage process. The firststage is to calculate the strengthrequired for the concrete to resist thedead load of the structure plus theestimated construction load. For form-work striking this is further explained in the companion Guide, Early strikingfor efficient flat slab construction . Thesecond stage is to determine a lowerbound estimate of the concrete’sstrength as recommended in this Guide.  Techniques for assessing the in-situstrength of concrete have improvedgreatly over the past 20 years and thein-situ concrete building at Cardingtonprovided an ideal opportunity to assesswhat could be achieved using the bestpossible practices. A structuredprogramme of tests conducted byLiverpool University and Queen’sUniversity of Belfast enabled therelative merits of different techniques tobe effectively assessed and compared. It should be stressed that many of thesetechniques are not new and are dealtwith extensively in relevant parts of BS 1881 (1) . Details of results from Cardington  The results are summarised below.Further information about the workcarried out on the in-situ concretebuilding at Cardington and backgroundreferences can be found in the mainresearch report. ã For the Lok-tests, the combinedcorrelation for all concretes wasfound to be very close to themanufacturer’s correlation (Figure 5). This is particularly encouraginggiven the very diverse range of concretes at Cardington, with theirdifferent strengths, cement types andworkabilities (see Best PracticeGuide, Concreting for improvedspeed and efficiency  ). Each of the 30points plotted on the graph is anaverage of four test results. ã  The correlation curves for the Lok-test were found to be applicable tothe Capo test (see Table 1). ã  The value of in-situ tests such as theLok-test was clearly demonstrated asa means of verifying that therequired strength for early striking(as early as 19 hours after placingthe final concrete in a slab) hadindeed been achieved. ã Where minimal damage to thesurface of the finished concrete isacceptable, Lok-tests carried out insufficient number and at theappropriate locations can replaceinformation from temperature-matched cubes ã Air-cured cubes will tend to give alower bound estimate of the in-situstrength (i.e. the results areconservative). This means that theywill not allow the full benefits of early striking to be derived,particularly in cold weatherconditions. ã In-situ tests such as the Lok-test canbe carried out quickly and easily asrequired, whereas there are logisticaldifficulties in transporting cubes to atesting house and having staff available at short notice to test thespecimens. ã A single correlation curve could notbe derived for the Limpet pull-off test (as opposed to the pull-out testsrecommended) or for maturitymeasurements. ã 28 day in-situ concrete compressivestrengths could be predicted fromthree day Lok-test results with areasonable degree of accuracy.  This finding should, however, betreated with caution since it willdepend on the temperature history of the concrete up to three days afterpouring and subsequently. ã Acceptance of concrete andappropriate concrete quality controlprocedures need to be viewed in the context of the type of concretespecified and emerging British and European standards. These are moving towards avoiding thenecessity for site testing. For theforeseeable future, formalcompliance is likely to continue to be based on cube or cylinderstrength measurements at 28 days.  The Lok-test equipment can be obtained from theDanish manufacturers:Germann Instruments A/S Tel +45 39 67 71 17Fax +45 39 67 31 67e-mail Figure 5: Correlation of Lok-tests and cube tests carried out at Cardington.    M  e  a  s  u  r  e   d  c  u   b  e  c  o  m  p  r  e  s  s   i  v  e  s   t  r  e  n  g   t   h   (   N   /  m  m    2    ) 100806040200 Force measured by Lok jack (kN)010203040506070 Manufacturer’srecommendedcorrelationCardingtoncorrelationããããããããããããããããããããã  BEST PRACTICE GUIDES FOR IN-SITU CONCRETE FRAME BUILDINGS 4 Comparison of test methods  The use of pull-out tests involving pre-planned inserts is the methodrecommended in this Guide. Howeverthere may be situations where suchtesting is not appropriate e.g. wherespecial finishes are used, where theplanned inserts have not been installed,or where increased confidence isrequired from using a range of techniques. An outline description of available test methods is given in Table 1.Whatever techniques are employed, itis important that an attempt is made todetermine a lower bound estimate of the concrete’s strength. References 1. BSI. Testing concrete . London BS 1881.Parts 5-209, 1970-1998. Best Practice Guides in this series ã Improving concrete frame constructionã Concreting for improved speed and efficiencyã Early age strength assessment of concrete on site ã Improving rebar information and supplyã Early striking for efficient flat slabconstructionã Rationalisation of flat slab reinforcementFurther Guides are planned Research partners for this Guide Liverpool University The Queen’s University of BelfastBuilding Research Establishment LtdRMC Readymix Limited97.503First published 2000ISBN 0 7210 1554 9Price group A©BCA, BRE Ltd, Construct, RCC, DETRPublished by the British Cement Associationon behalf of the project partners.British Cement AssociationCentury House Telford AvenueCrowthorne, Berkshire RG45 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.  This Best Practice Guide is based onresearch report, Early age acceptance of concrete (Improved quality management), by J.H. Bungey, A.E. Long, M.N. Soutsosand G.D. Henderson. BRE Report 387, published by CRC Ltd. *N.B. Water-cured cubes should not be used for early age strength assessment, because the curing environmentis not related to that of the concrete in the structure. Table 1:  Comparison of test methods TechniqueDescriptionAdvantagesDisadvantages Pull-out test involvingpre-planned insertseither fixed toformwork or floatingon top surface.Pull-out test involvingdrilling of concreteafter hardening. Same jack andcorrelation curveused as for Lok-test.Direct measurementof temperature-timehistory of concrete. Compressive testingof cubes made on sitein accordance withBS 1881 and storedadjacent to thestructure in ambientconditions.Compressive testingof cubes made on site and stored undercontrolled conditionsin accordance withBS 1881: Part 130:1996.Pulling off a metaldisc together with a surface layer of concrete bonded to the disc.Quick, inexpensive,provide goodcorrelation.Small changes instrength can bedetected.Correlation is not mix specific.Less preplanningrequired, inexpensiveand provide goodcorrelation. Correlation is not mix specific. Useful forsupplementary tests. Relatively inexpensive.Less expensive thantemperature-matchedcubes.Provides lower boundvalue.Provides accurateresultsInexpensive to perform.No pre-planningrequired.Superficial damageonly.Some preplanningneeded. Some local damage.Slower than Lok-test.Some local damage. Surface preparationmay be needed.Correlation withstrength is mixspecific.Costs of making,transporting, testing etc.Not testing concreteactually in thestructure.Relatively expensive.Disadvantages as forair-cured plus extraequipment costs. Impracticality of use on site.Correlation is mix specific. Time needed forbonding to surface.Surface preparationneeded.Poor correlationbetween pull-off force and concretestrength.Lok-tests Capo tests MaturitymeasurementCube testing*(a) Air-curedcubes(b) Temperaturematched cubesLimpet pull-off test


Aug 1, 2017
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