PropertiesofFreshConcreteEditedbyH.-J.WierigFreshconcreteisamixtureofwater,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.13.1WorkabilityWorkabilityofconcretehasneverbeenpreciselydefined.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.13.2MeasurementofWorkabilityThreetestswidelyusedformeasuringworkabilityaretheslump,compactingfactorandV-Bconsistometertests(figure13.1).ThesearestandardtestsintheUnitedKingdomandaredescribedindetailinBS1881:Part2.TheiruseisalsorecommendedinCP110:Part1.Itisimportanttonotethatthereisnosinglerelationshipbetweentheslump,compactingfactorandV-Bresultsfordifferentconcretes.Inthefollowingsectionsthesalientfeaturesoftheseteststogetherwiththeirmeritsandlimitationsarediscussed.SlumpTestThistestwasdevelopedbyChapmanintheUnitedStatesin1913.A300mmhighconcretecone,preparedunderstandardconditions(BS1881:Part2)isallowedtosubsideandtheslumporreductioninheightoftheconeistakentobeameasureofworkability.Theapparatusisinexpensive,portableandrobustdandisthesimplestofallthemethodsemployedformeasuringworkability.Itisnotsurprisingthat,inspiteofitsseverallimitations,theslumptesthasretaineditspopularity.Figure13.1Apparatusforworkabilitymeasurement:(a)slumpcone,(b)compactingfactorand(c)V-BconsistometerThetestprimarilymeasurestheconsistencyofplasticconcreteandalthoughitisdifficulttoseeanysignificantrelationshipbetweenslumpandworkabilityasdefinedpreviously,itissuitablefordetectingchangesinworkability.Forexample,anincreaseinthewatercontentordeficiencyintheproportionoffineaggregateresultsinanincreaseinslump.Althoughthetestissuitableforquality-controlpurposesitshouldberememberedthatitisgenerallyconsideredtobeunsuitableformixdesignsinceconcretesrequiringvaryingamountsofworkforcompactioncanhavesimilarnumericalvaluesofslump.Thesensitivityandreliabilityofthetestfordetectingvariationinmixesofdifferentworkabilitiesislargelydependentonitssensitivitytoconsistency.Thetestisnotsuitableforverydryorwetmixes.Forverydrymixes,withzeroornear-zeroslump,moderatevariationsinworkabilitydonotresultinmeasurablechangesinslump.Forwetmixes,completecollapseoftheconcreteproducesunreliablevaluesofslump.Figure13.2ThreemaintypesofslumpThethreetypesofslumpusuallyobservedaretrueslump,shearslumpandcollapseslump,asillustratedinfigure13.2.Atrueslumpisobservedwithcohesiveandrichmixesforwhichtheslumpisgenerallysensitivetovariationsinworkability.Acollapseslumpisusuallyassociatedwithverywetmixesandisgenerallyindicativeofpoorqualityconcreteandmostfrequentlyresultsfromsegregationofitsconstituentmaterials.Shearslumpoccursmoreofteninleanermixesthaninrichonesandindicatesalackofcohesionwhichisgenerallyassociatedwithharshmixes(lowmortarcontent).wheneverashearslumpisobtainedthetestshouldberepeatedand,ifpersistent,thisfactshouldberecordedtogetherwithtestresults,becausewidelydifferentvaluesofslumpcanbeobtaineddependingonwhethertheslumpisoftrueorshearform.Thestandardslumpappa