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外文翻译--新拌混凝土的性能.doc

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外文翻译--新拌混凝土的性能.doc

PropertiesofFreshConcreteEditedbyH.J.WierigFreshconcreteisamixtureofwater,cement,aggregateandadmixtureifany.Aftermixing,operationssuchastransporting,placing,compactingandfinishingoffreshconcretecanallconsiderablyaffectthepropertiesofhardenedconcrete.Itisimportantthattheconstituentmaterialsremainuniformlydistributedwithintheconcretemassduringthevariousstagesofitshandlingandthatfullcompactionisachieved.Wheneitheroftheseconditionsisnotsatisfiedthepropertiesoftheresultinghardenedconcrete,forexample,strengthanddurability,areadverselyaffected.Thecharacteristicsoffreshconcretewhichaffectfullcompactionareitsconsistency,mobilityandcompactability.Inconcretepracticetheseareoftencollectivelyknownasworkability.Theabilityofconcretetomaintainitsuniformityisgovernedbyitsstability,whichdependsonitsconsistencyanditscohesiveness.Sincethemethodsemployedforconveying,placingandconsolidatingdaconcretemix,aswellasthenatureofthesectiontobecast,mayvaryfromjobtojobitfollowsthatthecorrespondingworkabilityandstabilityrequirementswillalsovary.Theassessmentofthesuitabilityofafreshconcreteforaparticularjobwillalwaystosomeextentremainamatterofpersonaljudgment.Inspiteofitsimportance,thebehaviourofplasticconcreteoftentendstobeoverlooked.Itisrecommendedthatstudentsshouldlearntoappreciatethesignificanceofthevariouscharacteristicsofconcreteinitsplasticstateandknowhowthesemayalterduringoperationsinvolvedincastingaconcretestructure.13.1WorkabilityWorkabilityofconcretehasneverbeenpreciselydefined.Forpracticalpurposesitgenerallyimpliestheeasewithwhichaconcretemixcanbehandledfromthemixertoitsfinallycompactedshape.Thethreemaincharacteristicsofthepropertyareconsistency,mobilityandcompactability.Consistencyisameasureofwetnessorfluidity.Mobilitydefinestheeasewithwhichamixcanflowintoandcompletelyfilltheformworkormould.Compactabilityistheeasewithwhichagivenmixcanbefullycompacted,allthetrappedairbeingremoved.Inthiscontexttherequiredworkabilityofamixdependsnotonlyonthecharacteristicsandrelativeproportionsoftheconstituentmaterialsbutalsoon1themethodsemployedforconveyanceandcompaction,2thesize,shapeandsurfaceroughnessofformworkormouldsand3thequantityandspacingofreinforcement.Anothercommonlyaccepteddefinitionofworkabilityisrelatedtotheamountofusefulinternalworknecessarytoproducefullcompaction.Itshouldbeappreciatedthatthenecessaryworkagaindependsonthenatureofthesectionbeingcast.Measurementofinternalworkpresentsmanydifficultiesandseveralmethodshavebeendevelopedforthispurposebutnonegivesanabsolutemeasureofworkability.Thetestscommonlyusedformeasuringworkabilitydonotmeasuretheindividualcharacteristicsconsistency,mobilityandcompactabilityofworkability.However,theydoprovideusefulandpracticalguidanceontheworkabilityofamix.Workabilityaffectsthequalityofconcreteandhasadirectbearingoncostsothat,forexample,anunworkableconcretemixrequiresmoretimeandlabourforfullcompaction.Itismostimportantthatarealisticassessmentismadeoftheworkabilityrequiredforgivensiteconditionsbeforeanydecisionistakenregardingsuitableconcretemixproportions.13.2MeasurementofWorkabilityThreetestswidelyusedformeasuringworkabilityaretheslump,compactingfactorandVBconsistometertestsfigure13.1.ThesearestandardtestsintheUnitedKingdomandaredescribedindetailinBS1881Part2.TheiruseisalsorecommendedinCP110Part1.Itisimportanttonotethatthereisnosinglerelationshipbetweentheslump,compactingfactorandVBresultsfordifferentconcretes.Inthefollowingsectionsthesalientfeaturesoftheseteststogetherwiththeirmeritsandlimitationsarediscussed.SlumpTestThistestwasdevelopedbyChapmanintheUnitedStatesin1913.A300mmhighconcretecone,preparedunderstandardconditionsBS1881Part2isallowedtosubsideandtheslumporreductioninheightoftheconeistakentobeameasureofworkability.Theapparatusisinexpensive,portableandrobustdandisthesimplestofallthemethodsemployedformeasuringworkability.Itisnotsurprisingthat,inspiteofitsseverallimitations,theslumptesthasretaineditspopularity.Figure13.1Apparatusforworkabilitymeasurementaslumpcone,bcompactingfactorandcVBconsistometerThetestprimarilymeasurestheconsistencyofplasticconcreteandalthoughitisdifficulttoseeanysignificantrelationshipbetweenslumpandworkabilityasdefinedpreviously,itissuitablefordetectingchangesinworkability.Forexample,anincreaseinthewatercontentordeficiencyintheproportionoffineaggregateresultsinanincreaseinslump.Althoughthetestissuitableforqualitycontrolpurposesitshouldberememberedthatitisgenerallyconsideredtobeunsuitableformixdesignsinceconcretesrequiringvaryingamountsofworkforcompactioncanhavesimilarnumericalvaluesofslump.Thesensitivityandreliabilityofthetestfordetectingvariationinmixesofdifferentworkabilitiesislargelydependentonitssensitivitytoconsistency.Thetestisnotsuitableforverydryorwetmixes.Forverydrymixes,withzeroornearzeroslump,moderatevariationsinworkabilitydonotresultinmeasurablechangesinslump.Forwetmixes,completecollapseoftheconcreteproducesunreliablevaluesofslump.Figure13.2ThreemaintypesofslumpThethreetypesofslumpusuallyobservedaretrueslump,shearslumpandcollapseslump,asillustratedinfigure13.2.Atrueslumpisobservedwithcohesiveandrichmixesforwhichtheslumpisgenerallysensitivetovariationsinworkability.Acollapseslumpisusuallyassociatedwithverywetmixesandisgenerallyindicativeofpoorqualityconcreteandmostfrequentlyresultsfromsegregationofitsconstituentmaterials.Shearslumpoccursmoreofteninleanermixesthaninrichonesandindicatesalackofcohesionwhichisgenerallyassociatedwithharshmixeslowmortarcontent.wheneverashearslumpisobtainedthetestshouldberepeatedand,ifpersistent,thisfactshouldberecordedtogetherwithtestresults,becausewidelydifferentvaluesofslumpcanbeobtaineddependingonwhethertheslumpisoftrueorshearform.Thestandardslumpapparatusisonlysuitableforconcretesinwhichthemaximumaggregatesizedoesnotexceed37.5mm.Itshouldbenotedthatthevalueofslumpchangeswithtimeaftermixingowingtonormalhydrationprocessesandevaporationofsomeofthefreewater,anditisdesirablethereforethattestsareperformedwithinafixedperiodoftime.CompactingFactorTest

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