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1、 Cement&ConcreteComposites292007397?401/0./locate/cemconcompMechanicalbehaviourofconcretemadewith?nerecycledconcreteaggregatesL.Evangelista a,J.deBritob,*a InstitutoSuperiordeEngenhariadeLisboa,R.ConselheiroEm?dioNavarro,1,1950-062Lisboa,Portugalb InstitutoSuperiorTecnico, Av.RoviscoPais,1049-001Lis
2、boa,PortugalReceived28December2005;receivedinrevisedform14December2006;accepted15December2006Availableonline10January2007AbstractThispaperconcernstheuseof?nerecycledconcreteaggregatestopartiallyorgloballyreplacenatural?neaggregatessandintheproductionofstructuralconcrete.Toevaluatetheviabilityofthisp
3、rocess,anexperimentalcampaignwasimplementedinordertomon-itorthemechanicalbehaviourofsuchconcrete.Theresultsofthefollowingtestsarereported:compressivestrength,splittensilestrength,modulusofelasticityandabrasionresistance.Fromtheseresults,itisreasonabletoassumethattheuseof?nerecycledconcreteaggre-gate
4、sdoesnotjeopardizethemechanicalpropertiesofconcrete,forreplacementratiosupto30%.2007ElsevierLtd.Allrightsreserved.Keywords: Sustainableconstruction;Concretewaste;Finerecycledaggregates;Structuralconcrete1.Introductionandsuperplasticizerswereused,hasproducedresultsthatcontradict these initial percept
5、ions. This paper presentsthe main results of the research concerning compressiveand split tensile strength, modulus of elasticity and abra-sionresistanceanddrawssomeconclusionsontheviabilityof using ?ne recycled concrete aggregates in structuralconcrete, namely by comparing them with conventionalcon
6、cretes with exactly the same compositions with theexceptionof?nefraction.Concrete demolition waste has been proved to be anexcellentsourceofaggregatesfornewconcreteproduction.Therearemanystudiesthatprovethatconcretemadewiththistypeofcoarseaggregatescanhavemechanicalproper-ties similar to those of co
7、nventional concretes and evenhigh-strengthconcreteisnowadaysapossiblegoalforthisenvironmentallysoundpractice1?3.However, the ?ne fraction of these recycled aggregateshasnot beenthe subject ofthorough similar studies sinceitisbelievedthattheirgreaterwaterabsorptioncanjeopar-dizethe?nalresults.Theresu
8、ltsofseveralstudiespresentedinthepasthavecausedtheexistingcodesconcerningrecy-cledaggregatesforconcreteproductiontostronglylimittheuseoftheseproducts4?6.2.ExperimentalresearchprogramThe ?ne recycled concrete aggregates that were usedduring the entire research program were obtained froman original co
9、ncrete OC, of standard composition andproperties, which was made in laboratorial conditions,solely for the purpose of being crushed afterwards. Byusing this procedure, it was possible to fully controlthe concretes composition and to determine its mainTheinvestigationconducted,inwhichtherecycled?neag
10、gregates were obtained from laboratory grade concrete* Correspondingauthor.Tel.:+351218419709;fax:+351218497650.E-mailaddress:jb/.ptJ.deBrito.0958-9465/$-seefrontmatter2007ElsevierLtd.Allrightsreserved.398L.Evangelista,J.deBrito/Cement&ConcreteComposites292007397?401properties, which, if not known,
11、could become an addi-tional variable, when analysing and concluding about theachieved experimental results. The OC composition canbeobservedinTable1.Theaveragecompressivestrengthof the OC, after a 28day period of wet curing, was29.6MPa.Table2PropertiesofFNAandFRAFNAFRADryspeci?cdensitykg/m3254425641
12、5170.819132165123413.12.38Surfacedryspeci?cdensitykg/m3Drybulkdensitykg/m3Waterabsorption%Theconcretewascrushedonthe35thday,usingasmalljaw crusher, which produced aggregates with a imumnominal size of 38.1mm. The aggregates were then sepa-ratedaccordingtotheirdimension,bymechanicalsieving,and only t
13、he fractions between 0.074mm and 1.19mmwere used, so the particle size range for both ?ne naturalaggregates FNA and ?ne recycled aggregates FRAwould be the same. In spite ofthis,the grading curves ofthenaturalandrecycledweredi?erentFig.1,andthere-fore,itwasnecessarytoadjustthelattertomatchthefor-mer
14、toachieveasimilar?nenessmodulus Toaccomplishthis, it was necessary to separate the recycled aggregatesaccordingtotheirdi?erentparticlesizes.Afterseparation,the aggregates were stored in tight containers to avoidmoisture exchanges with the environment. Although thistypeofprocedureistoodi?cultforpract
15、icalapplication,it enables comparisons between mix compositions withthesameparticlesizedistribution,eventhoughthereplace-mentratiosdi?er.Finenessmodulus2.38The di?erent mixes compositions were designed usingFaurys method 8, with a common target slump of80 10mm. The mix design was primarily conceived
16、 forthe reference concrete RC, made only of natural aggre-gates.Itwasthenadaptedfortheremainingmixes,takingintoaccountthedi?erentwater/cementratios,expectedtoincrease along with the recycled aggregates replacementratio.TheincreaseofwaterhastodowithFRAsgreaterabsorption and to the greater need of mix
17、ing water, onaccountofthegreaterparticlefrictionthatrecycledaggre-gatesgenerate7.ToestimatethewaterthattheFNAwouldabsorbdur-ingthemixing,therelationshipproposedbyLeite3wasconsidered, which established the FRA water absorptionthroughtime.Theauthorconcludedthatduringtheperiodof10?30minofmixing,theFNAw
18、aterabsorption stabi-lizes,reachingaround50%ofitsimumcapacity.Leitealso proposed, based on Nevilles observations 9, thatafter introducing the binder in the mixture, the recycledaggregates absorption was signi?cantly reduced, becauseitsealstheaggregatespores,limitingwaterexchanges.Theexperimentalrese
19、archwasdividedinthreedistinctstages: in the ?rst stage, di?erent concrete compositionswerestudiedandtestedinorderto?ne-tunethemixpropor-tionstocomplywiththestipulatedworkability;thegoalofthesecondstagewastoperformapreliminaryevaluationofthe concrete mixes, basedon parameters both mechanicalcompressi
20、vestrengthandshrinkageanddurability-relatedwater absorption; the third stages main purpose was toevaluate,asthoroughlyaspossible,themixesthatpresentedthemostpromisingresultsatthesecondstage.The main properties of the ?ne natural and recycledaggregatesaftercorrectionofitsgradingcurvewerestud-iedandar
21、edisplayedinTable2.FRApresentalowerden-sity than FNA, due to its greater porosity, that leads tomuchhigherwaterabsorption.Table1OriginalconcretecompositionOCCEMII32.5Ncementkg/m33626157174781880.52Riversandkg/m3Coarseaggregate1kg/m3Coarseaggregate2kg/m3Waterl/m3w/cratio100%90%80%70%60%50%40%30%20%10
22、%TocheckthesuitabilityofLeitesproposal,twodi?erenttechniquesformixingthe?neaggregatesbothnaturalandrecycled with water were used. In the ?rst technique,applied at the ?rst stage of the campaign, the ?ne aggre-gateswereinsertedintowater2/3oftherequiredmixingwater, plus the waterthat was estimatedto b
23、e absorbed,and were mixed during a period of 10min, after whichtheremainingconstituentswereplaced.Inthesecondtech-nique,usedatbothsecondandthirdstages,thesamemix-ingprocedurewasused,exceptthatthedurationofmixingwasextendedto20min.FNAFRAIt was expected that the replacement of FNA with thecorresponden
24、t FRA would cause a large increase in thew/cratio10,11.Inordertokeepitatanacceptablelevelbelow 0.45 since, for a 100% replacement ratio, existingSieve size mmFig.1. GradingcurvesforFNAandFRA.L.Evangelista,J.deBrito/Cement&ConcreteComposites292007397?401399Table3Concretecomposition1m3RCC10RC20RC30RC5
25、0RC100R%ofreplacementCementCEMI42.5RkgWaterl010203050100380180.90.480.4505094003743884.9380155.80.410.4166804093823974.9380160.60.420.42598524073803954.9380165.40.440.435291034043783934.9380170.20.450.444601544023763904.9380175.60.460.453272544003743884.9w/cratiow/cefratioFNAkgFRAkgCoarseaggregate1k
26、gCoarseaggregate2kgCoarseaggregate3kgSuperplasticizerkg/m3literature predicted a huge increase in the water contentnecessary to keep the workability of the mix constant, amodi?ed carboxylate based superplasticizer was used,1.3% by weight of cement. A CEM I 42.5R Portlandcement 380kg/m3, and three di
27、?erent crushed limestonecoarse aggregates were used. Table 3 shows the mixturecompositionsforallconcretes,forallstages.Table4CompressivestrengthresultsFirststageD%SecondandD%dbetweenthirdstagesstagesRC59.462.258.461.360.861.0?59.359.057.357.158.854.8?0.25.32.06.83.210.2C10RC20RC30RC50RC100R4.70.63.4
28、3.70.87.61.73.12.32.7Compressivestrengthtestswerecarriedouton150mmcubes,accordingtoNPEN12390-512.Testedspecimenswere subjected to 28-day wet curing, for ?rst and secondstages, while for third one, 7, 28, and 56days wet-curedspecimens were evaluated. For split tensile strength andmodulus of elasticit
29、y tests, 150mm diameter cylinders300mmtall31dayswet-curedwereused,accordingtoNPEN12390-613andLNECE39714,respectively.Asforabrasion resistance, 71?71?40mmprisms were tested,usingagrindingwheel,accordingtoDIN52108115.A reasonable explanation for the maintenance of thecompressive strength with increasi
30、ngne aggregatesreplacement has been proposed by Katz 16, which con-cludedthatrecycledaggregateshavehighlevelsofcementbothhydratedandnon-hydrated,thatcanreachasmuchas25%ofitsweight,increasingthetotalamountofcementinthemix.Although the di?erences between the ?rst and second/thirdstagesweresmall,apossi
31、blecausefortheslightresis-tancelosshasbeenproposedbyPoonetal.17andBarradeOliveiraandvasquez18,thatconcludedthatthesatu-ration level of the recycled aggregates may a?ect thestrength of the concretes, since at higher saturation levelsthe mechanical bonding between the cement paste andthe recycled aggr
32、egates becomes weaker. Therefore, asin the second/third stages the mixing period was longerthaninthe?rstone,thatmayhaveledtoaweakerperfor-manceofthoseconcretes.ThevariationofcompressivestrengthwithtimeFig.2indicates that the reference concretes resistance, madeexclusivelywithnaturalaggregates,almost
33、stabilizedafter28daysofage.Inopposition,thecompressivestrengthoftheconcretemixesmadewith?nerecycledaggregatescon-tinuestoincreaseafterthatage.Thisresultsomehowcor-roborates the assumption that there is non-hydratedcement mixed with the ?ne recycled aggregates that con-tributes to the overall resista
34、nce. The reference concretewas made of high hydration speed cement CEM type I,whichwouldjustifytherapidstabilizationofitscompres-sivestrength.On thecontrary, theoriginalconcrete, usedtoproducetherecycledaggregates,wasmadewithnormalhydrationspeedcementCEMtypeII,thattakeslongerto3.Resultsanddiscussion
35、3.1pressivestrengthThecompressivestrengthresultsobtainedforthethreedi?erent stages the second and third stages joint resultswerepresented,sincetherewerenodi?erencesintheirmix-ing process, at 28days of age, are shown in Table 4.Strength variations of the various concrete mixes in rela-tion to the ref
36、erence concrete designated by D and thestrength variations between ?rst and second/third stagespresented in the d between stages column are alsoshown.Itispossibletorealizethat,withineachstage,thestrengthresistancehadinsigni?cantvariationsandnovis-ible trend due to the FNA replacement with FRA. Whenc
37、omparingtheresultsofthedi?erentstages,thedi?erencesbetween them are also small, although the second/thirdstagesgenerallypresentslightlylowercompressivestrengthresistancesthanthe?rststage.1 RoughlyequivalenttoBSEN13892-4:2002.400L.Evangelista,J.deBrito/Cement&ConcreteComposites292007397?4016560555045
38、403530Table6SplittensilestrengthresultsfctmMPaD%RCC30RC100R3.853.652.95?5.230.5RCC30RC100RTable7ModulusofelasticityEcGPaD%RC35.534.228.9?7d28d56dC30RC100R3.718.5Age daysFig.2. Compressivestrengthvariation7,28and56days.Theconcretesmodulusofelasticityisdeeplyrelatedtothe sti?ness of the coarse aggrega
39、tes, the sti?ness of themortar,theirporosityandbond9,19.Therefore,forsmallreplacementratios,itispossiblethattheoverallsti?nessisnotsigni?cantlyin?uenced,becausethemortarsti?nessisonly one of several factors, while for total replacementthemortarwithstandssuchabigsti?nesslossthatthecon-cretesmodulusof
40、elasticityisconsiderablya?ected.Toestablisharelationshipbetweencompressivestrengthand modulus of elasticity of concrete mixes made withFRA,aregressionbasedonthemodelestablishedbyZilchandRoos20wasused.Theauthorssuggestthatthemod-ulus of elasticity is dependant not only on the concretescompressivestre
41、ngthfcbutalsoonitsdensityq: Table5CementtypewithinconcretemixesCEMI42.5RCEMII32.5N*OriginalconcreteOCReferenceconcreteRCConcreteswithFRAC10RtoC100R*fullyhydrate,andthereforea?ectsthewaythecompressivestrength of the concretes made with it develops throughtime.Table5summarizesthedi?erentcementtypeswit
42、hineach concrete, to better understand the in?uence overconcretescompressivestrength.qb2Ec?a?efct8T13e1Twherecoe?cientsaandbare9100and2400,respectively.For this investigation, a and b are regression coe?cientsdetermined to be 8917 and 2348, respectively, based onthe experimental results for concrete
43、s made with FRA.ThecorrelationfactorR20.85.TheconcretessplittensilestrengthispresentedinTable6, which shows a clear decrease of this property with theincrease of FNA replacement with FRA, as well as thestrength variations relative to the reference concrete, pre-sentedinDcolumn.AccordingtoCoutinho19,
44、thetensilestrength is not as a?ected by the cement content as thecompressivestrength,sothetensilestrengthdoesnotpar-ticularlybene?tfromtheadditionalcementthatisincorpo-ratedalongwiththeFNA.Therefore,itisperfectlynaturalthatadecreaseoccursasthereplacementratiorises,duetothemoreporousstructureoftherec
45、ycledaggregates.The abrasion resistance obtained for the di?erent con-crete types analysed is shown in Table 8 D1 and D beingrespectivelytheabsoluteandrelativethicknesslossofcon-cretemixesmadewithFRAincomparisontothereferenceconcrete.ItispossibletoconcludethattheconcreteswithreplacementofFNAwithFRAh
46、avegreaterabrasionresis-tance than the reference concrete. That may have to dowith the fact that abrasion resistance is deeply connectedwiththebondofthecementpastewiththe?neaggregates,Theresultsachievedforthemodulusofelasticityofthedi?erentconcretesarepresentedinTable7,whereDreferstovariationsinFRAconcretemixesmodulusofelasticitywhen compared with the reference concrete. As shown,there