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