混凝土搅拌车搅拌实验系统仿真设计英文原文.docx

混凝土搅拌车搅拌实验系统仿真设计英文原文.docx

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混凝土搅拌车搅拌实验系统仿真设计英文原文

PropertiesofFreshConcrete

EditedbyH.-J.Wierig

Freshconcreteisamixtureofwater,cement,aggregateandadmixture（ifany）.Aftermixing,operationssuchastransporting,placing,compactingandfinishingoffreshconcretecanallconsiderablyaffectthepropertiesofhardenedconcrete.Itisimportantthattheconstituentmaterialsremainuniformlydistributedwithintheconcretemassduringthevariousstagesofitshandlingandthatfullcompactionisachieved.Wheneitheroftheseconditionsisnotsatisfiedthepropertiesoftheresultinghardenedconcrete,forexample,strengthanddurability,areadverselyaffected.

Thecharacteristicsoffreshconcretewhichaffectfullcompactionareitsconsistency,mobilityandcompactability.Inconcretepracticetheseareoftencollectivelyknownasworkability.Theabilityofconcretetomaintainitsuniformityisgovernedbyitsstability,whichdependsonitsconsistencyanditscohesiveness.Sincethemethodsemployedforconveying,placingandconsolidatingdaconcretemix,aswellasthenatureofthesectiontobecast,mayvaryfromjobtojobitfollowsthatthecorrespondingworkabilityandstabilityrequirementswillalsovary.Theassessmentofthesuitabilityofafreshconcreteforaparticularjobwillalwaystosomeextentremainamatterofpersonaljudgment.

Inspiteofitsimportance,thebehaviourofplasticconcreteoftentendstobeoverlooked.Itisrecommendedthatstudentsshouldlearntoappreciatethesignificanceofthevariouscharacteristicsofconcreteinitsplasticstateandknowhowthesemayalterduringoperationsinvolvedincastingaconcretestructure.

13.1Workability

Workabilityofconcretehasneverbeenpreciselydefined.Forpracticalpurposesitgenerallyimpliestheeasewithwhichaconcretemixcanbehandledfromthemixertoitsfinallycompactedshape.Thethreemaincharacteristicsofthepropertyareconsistency,mobilityandcompactability.Consistencyisameasureofwetnessorfluidity.Mobilitydefinestheeasewithwhichamixcanflowintoandcompletelyfilltheformworkormould.Compactabilityistheeasewithwhichagivenmixcanbefullycompacted,allthetrappedairbeingremoved.Inthiscontexttherequiredworkabilityofamixdependsnotonlyonthecharacteristicsandrelativeproportionsoftheconstituentmaterialsbutalsoon

（1）themethodsemployedforconveyanceandcompaction,

（2）thesize,shapeandsurfaceroughnessofformworkormouldsand（3）thequantityandspacingofreinforcement.

Anothercommonlyaccepteddefinitionofworkabilityisrelatedtotheamountofusefulinternalworknecessarytoproducefullcompaction.Itshouldbeappreciatedthatthenecessaryworkagaindependsonthenatureofthesectionbeingcast.Measurementofinternalworkpresentsmanydifficultiesandseveralmethodshavebeendevelopedforthispurposebutnonegivesanabsolutemeasureofworkability.

Thetestscommonlyusedformeasuringworkabilitydonotmeasuretheindividualcharacteristics（consistency,mobilityandcompactability）ofworkability.However,theydoprovideusefulandpracticalguidanceontheworkabilityofamix.Workabilityaffectsthequalityofconcreteandhasadirectbearingoncostsothat,forexample,anunworkableconcretemixrequiresmoretimeandlabourforfullcompaction.Itismostimportantthatarealisticassessmentismadeoftheworkabilityrequiredforgivensiteconditionsbeforeanydecisionistakenregardingsuitableconcretemixproportions.

13.2MeasurementofWorkability

Threetestswidelyusedformeasuringworkabilityaretheslump,compactingfactorandV-Bconsistometertests（figure13.1）.ThesearestandardtestsintheUnitedKingdomandaredescribedindetailinBS1881:

Part2.TheiruseisalsorecommendedinCP110:

Part1.Itisimportanttonotethatthereisnosinglerelationshipbetweentheslump,compactingfactorandV-Bresultsfordifferentconcretes.Inthefollowingsectionsthesalientfeaturesoftheseteststogetherwiththeirmeritsandlimitationsarediscussed.

SlumpTest

ThistestwasdevelopedbyChapmanintheUnitedStatesin1913.A300mmhighconcretecone,preparedunderstandardconditions（BS1881:

Part2）isallowedtosubsideandtheslumporreductioninheightoftheconeistakentobeameasureofworkability.Theapparatusisinexpensive,portableandrobustdandisthesimplestofallthemethodsemployedformeasuringworkability.Itisnotsurprisingthat,inspiteofitsseverallimitations,theslumptesthasretaineditspopularity.

Figure13.1Apparatusforworkabilitymeasurement:

（a）slumpcone,（b）compactingfactorand（c）V-Bconsistometer

Thetestprimarilymeasurestheconsistencyofplasticconcreteandalthoughitisdifficulttoseeanysignificantrelationshipbetweenslumpandworkabilityasdefinedpreviously,itissuitablefordetectingchangesinworkability.Forexample,anincreaseinthewatercontentordeficiencyintheproportionoffineaggregateresultsinanincreaseinslump.Althoughthetestissuitableforquality-controlpurposesitshouldberememberedthatitisgenerallyconsideredtobeunsuitableformixdesignsinceconcretesrequiringvaryingamountsofworkforcompactioncanhavesimilarnumericalvaluesofslump.Thesensitivityandreliabilityofthetestfordetectingvariationinmixesofdifferentworkabilitiesislargelydependentonitssensitivitytoconsistency.Thetestisnotsuitableforverydryorwetmixes.Forverydrymixes,withzeroornear-zeroslump,moderatevariationsinworkabilitydonotresultinmeasurablechangesinslump.Forwetmixes,completecollapseoftheconcreteproducesunreliablevaluesofslump.

Figure13.2Threemaintypesofslump

Thethreetypesofslumpusuallyobservedaretrueslump,shearslumpandcollapseslump,asillustratedinfigure13.2.Atrueslumpisobservedwithcohesiveandrichmixesforwhichtheslumpisgenerallysensitivetovariationsinworkability.Acollapseslumpisusuallyassociatedwithverywetmixesandisgenerallyindicativeofpoorqualityconcreteandmostfrequentlyresultsfromsegregationofitsconstituentmaterials.Shearslumpoccursmoreofteninleanermixesthaninrichonesandindicatesalackofcohesionwhichisgenerallyassociatedwithharshmixes（lowmortarcontent）.wheneverashearslumpisobtainedthetestshouldberepeatedand,ifpersistent,thisfactshouldberecordedtogetherwithtestresults,becausewidelydifferentvaluesofslumpcanbeobtaineddependingonwhethertheslumpisoftrueorshearform.

Thestandardslumpapparatusisonlysuitableforconcretesinwhichthemaximumaggregatesizedoesnotexceed37.5mm.Itshouldbenotedthatthevalueofslumpchangeswithtimeaftermixingowingtonormalhydrationprocessesandevaporationofsomeofthefreewater,anditisdesirablethereforethattestsareperformedwithinafixedperiodoftime.

CompactingFactorTest

Thistest,developedintheUnitedKingdombyGlanvilleetal.（1947）,measuresthedegreeofcompactionforastandardamountofworkandthusoffersadirectandreasonablyreliableassessmentoftheworkabilityofconcreteaspreviouslydefined.Theapparatusisarelativelysimplemechanicalcontrivance（figure13.1）andisfullydescribedinBS1881:

Part2.Thetestrequiresmeasurementoftheweightsofthepartiallyandfullycompactedconcreteandtheratioofthepartiallycompactedweighttothefullycompactedweight,whichisalwayslessthan1,isknownasthecompactingfactor.Forthenormalrangeofconcretesthecompactingfactorliesbetween0.80and0.92.Thetestisparticularlyusefulfordriermixesforwhichtheslumptestisnotsatisfactory.Thesensitivityofthecompactingfactorisreducedoutsidethenormalrangeofworkabilityandisgenerallyunsatisfactoryforcompactingfactorsgreaterthan0.92.

Itshouldalsobeappreciatedthat,strictlyspeaking,someofthebasicassumptionsofthetestarenotcorrect.Theworkdonetoovercomesurfacefrictionofthemeasuringcylinderprobablyvarieswiththecharacteristicsofthemix.IthasbeenshownbyCusens（1956）thatforconcreteswithverylowworkabilitytheactualworkrequiredtoobtainfullcompactiondependsontherichnessofamixwhilethecompactingfactorremainssensiblyunaffected.Thusitfollowsthatthegenerallyheldbeliefthatconcreteswiththesamecompactingfactorrequirethesameamountofworkforfullcompactioncannotalwaysbejustified.Onefurtherpointtonoteisthattheprocedureforplacingconcreteinthemeasuringcylinderbearsnoresemblancetomethodscommonlyemployedonthesite.Asintheslumptest,themeasurementofcompactingfactormustbemadewithinacertainspecifiedperiod.Thestandardapparatusissuitableforconcretewithamaximumaggregatesizeofupto37.5mm.

V-BConsistometerTest

ThistestwasdevelopedinSwedenbyB

hrner（1940）（seefigure13.1）.Althoughgenerallyregardedasatestprimarilyusedinresearchitspotentialisnowmorewidelyacknowledgedinindustryandthetestisgraduallybeingaccepted.Inthistest（BS1881:

Part2）thetimetakentotransform,bymeansofvibration,astandardconeofconcretetoacompactedflatcylindricalmassisrecorded.ThisisknownastheV-Btime,inseconds,andisstatedtothenearest0.5s.Unlikethetwoprevioustests,thetreatmentofconcreteinthistestiscomparabletothemethodofcompactingconcreteinpractice.Moreover,thetestissensitivetochangeinconsistency,mobilityandcompactability,andthereforeareasonablecorrelationbetweenthetestresultsandsiteassessmentofworkabilitycanbeexpected.

Thetestissuitableforawiderangeofmixesand,unliketheslumpandcompactingfactort