粗集料對普通混凝土滲透性、耐久性和微觀結(jié)構(gòu)的影響

1、ConstructionandBuildingConstructionandBuildingMaterials19(2005)682690MATERIALSInuenceofcoarseaggregateonthepermeation,durabilityandthemicrostructurecharacteristicsofordinaryPortlandcementconcreteL.Basheer*,P.A.M.Basheer,A.E.LongSchoolofCivilEngineering,QueenÕsUniversityBelfast,BelfastBT71NN,Nor

2、thernIreland,UKReceived12February2005;receivedinrevisedform12February2005;accepted22February2005Availableonline26April2005AbstractReinforcedconcretestructuresinmarineandsimilarenvironmentalconditionshavetowithstandsevereexposures.Duetothis,themixdesignprocedureshavetoberevisedtoincludedurabilityasap

3、rimarycriterionalongwithstrengthandworkability.Inlinewiththisneed,aninvestigationwascarriedoutatQueenÕsUniversityBelfastwiththenancialhelpoftheDepartmentofEnviron-ment,TransportandtheRegions,UK.Themainobjectiveoftheresearchwastounderstandtheinuenceofmixparametersonworkability,strength,durabilit

4、yandmicrostructureandtoproposeanewmixdesignprocedurewithinwhichdurabilityhasahigherpriority.Thepresentpaperreportspartofthisstudy,whichinvestigatestheeectofsizeandgradingofthecoarseaggregateonthedurabilityandthemicrostructurecharacteristicsofconcrete.Thedurabilitytestscarriedoutincludedairpermeabili

5、tytest,freezethaw/saltscalingresistancetestandanacceleratedcarbonationtest.Themicrostructurestudywascarriedoutusingascanningelec-tronmicroscope(SEM)andincludedimageanalysisoftheinterfacialtransitionzone(ITZ)aroundthecoarseaggregate.Theresultsindicatedthatastheproportionoflargersizeaggregateincreases

6、inthemix,thelocalporosityattheITZincreasesandtheoveralldurabilitydecreases.Ithasalsobeenfoundthatreducingtheaverageaggregatesizeinamiximprovestheporestructureanden-hancesthedurabilityoftheconcrete.Ó2005ElsevierLtd.Allrightsreserved.Keywords:Concrete;Durability;Microstructure;SEM;ITZ1.Introducti

7、onMostoftheimportantpropertiesofhardenedcon-cretearerelatedtothequantityandcharacteristicsofvarioustypesofporesinthecementpasteandaggregatecomponentsoftheconcrete.Forexample,theengineer-ingpropertiesofconcrete,suchasstrength,durability,shrinkageandpermeability,aredirectlyinuencedorcontrolledbythenum

8、ber,typeandsizeofporespresent1.Thetotalvolumeofpores,nottheirsizeorcontinu-ityaectthestrengthandelasticityofconcrete,whereas*Correspondingauthor.Tel.:+2890974027;fax:+2890663754.E-mailaddress:l.basherqub.ac.uk(L.Basheer).concretepermeabilityisinuencedbyporevolume,sizeandcontinuity.Itisbelievedthatca

9、pillaryvoidslargerthan50lm,referredtoasmacropores,aredetrimentaltostrengthandimpermeability,whereasvoidssmallerthan50lm,referredtoasmicropores,aremorerelatedtodryingshrinkageandcreep2.Themovementofgases,liquidsandionsthroughconcreteisimportantbecauseoftheirinteractionswithconcreteconstituentsandthep

10、orewaterwhichcanaltertheintegrityofconcretedirectlyandindirectly,leadingtothedeteriorationofstructures.Inreinforcedconcretethisdeteriorationismainlyduetothecorrosionofrein-forcement,freezethawattackorchemicalattack.Intherecentpastthecostofrepairofstructuresduetosuch0950-0618/$-seefrontmatterÓ20

11、05ElsevierLtd.Allrightsreserved.doi:10.1016/j.conbuildmat.2005.02.022L.Basheeretal./ConstructionandBuildingMaterials19(2005)682690683deteriorationhasbeenhigh.Thishasledtoeortsbyresearchersandengineerstounderstandconcretebettersothatdurableconcretecanbeproducedinthefuture.Concretehasahighlyheterogene

12、ousandcomplexstructure.Atthemacroscopiclevelconcretemaybecon-sideredtobeatwo-phasematerial,consistingofaggre-gateparticlesdispersedinamatrixofcementpaste.Atthemicroscopiclevel,athirdphasethetransitionzonemaybeidentied(2).Thistransitionzoneexistsasathinshell,calledtheinterfacialtransitionzone(ITZ),be

13、-tweenaggregateparticlesandhydratedcementpaste(hcp).IthasbeenestablishedthattheITZisprofoundlydierentfromthebulkcementpasteintermsofmor-phology,compositionanddensity210.Therehavebeensuggestionsthatthestrengthofthecementaggre-gatebondmaybethelimitingfactorinthedetermina-tionoftheultimatestrengthofcon

14、crete11.Furthermore,astheporosityoftheITZishigherthanthatofbulkpaste610,theITZhasasignicanteectonthepermeabilityanddurabilityofconcrete.Thispaperreportsastudycarriedouttoascertaintheeectofcoarseaggregategradingandsizeontheperme-ationpropertiesanddurabilityofconcrete.Theinvesti-gationinvolvedastudyof

15、theporestructurecharacteristicsoftheITZ,withtheobjectiveoflinkingtheporestructurecharacteristicstothepermeationpropertiesandthedurabilityofordinaryPortlandce-mentconcretes.1.1.CharacterisationofinterfacialtransitionzoneBackscatteredelectron(BSE)imagingtechniquecanbeusedtocharacterisetheporestructure

16、oftheITZ.LowviscosityepoxyimpregnatedpolishedsamplesofconcreteareusedforBSEimaging.Theunderlyingprin-cipleisthatthecontrastoftheBSEimagesdependsonthemeanatomicnumberoftheconcreteconstituents.IntheBSEimagesanhydrousphasesoftheunreactedce-mentparticlesappearbright,thecalciumhydroxidelightgrey,theother

17、hydrationproductsasvariousshadesofdarkergreywiththeepoxyimpregnatedporesappearingblack8.Consequentlybyprocessingimagesofpolishedsectionsithasbeenpossibletodeterminethedistributionofthephasesinhydratedcementpaste12.Furthermore,theseimagescanalsobeutilisedtoascer-tainporositycharacteristicsoftheITZinc

18、ludingtheporositygradient10andtheamountofpenetrablepores8,13.Hence,thistechniqueisusedinthisworkforstudyingtheITZporestructure.2.Experimentalprogramme2.1.VariablesTable1givesthemixvariablesusedintheinvestiga-tion.Toobtainarangeofcoarseaggregategradingtheproportionof10and20mmaggregatewasvaried.Forthe

19、aggregatesizeeect,10and20mmsinglesizeaggre-gateswereused.Aconstantwatercementratioof0.6andaggregatecementratioof6wasusedinallthecases.Mixeswithtwodierentneaggregate-coarseaggregateratios(FA/CA)wereusedforboththeperme-ationanddurabilitystudies.Forthemicrostructuralstudyonly0.5FA/CAwasused.2.2.Materia

20、lsusedClass42.5NPortlandcement14,mediumgraded15naturalsandandcrushedbasaltcoarseaggregatewereusedformanufacturingtheconcretesamples.ThevariouspropertiesoftheaggregatesaresummarisedinTable2.Tocontrolthemoistureconditionoftheaggregate,allaggregateswereoven-driedatatemperatureof40°C.Thecoarseaggre

21、gatewasdriedfor1dayandtheneaggregatefor2days.Beforetheaggregateswereusedtomakeconcretetheywereallowedtocoolforatleast1day.Thesaturatedsurfacedry1-hourwaterabsorptionvalues16werealsomeasured.Thisallowedacorrectedwaterquantitytobecalculatedwhenbatch-ingthematerials,toensurethattheaggregateinthecon-cre

22、tewasinasaturatedsurface-drycondition.Table2PropertiesofaggregatesusedAggregatetypeFineaggregate(sand)10mmcrushedbasalt20mmcrushedbasalt1habsorption(%)10.50.5Specicgravity2.722.932.88Finenessmodulus2.65.927.04Table1VariableusedfortheinvestigationVariableFA/CACoarseaggregategradingandnotationLevels0.

23、5100%10mm(10mm)0.767%10mm+33%20mm(10/20=2)33%10mm+67%20mm(10/20=0.5)100%20mm(20mm)Note:FA/CAneaggregatecoarseaggregateratio.684L.Basheeretal./ConstructionandBuildingMaterials19(2005)6826902.3.ManufactureoftestspecimensSix250·250·100mmblocksandthree100mmcubeswerecastforeachofthevariablesinT

24、able1.Theblockswereusedforthepermeationanddurabilitystudiesandthethreecubeswereusedtoobtainthecom-pressivestrength(theslumpandcubecompressivestrengthvaluesaregiveninTable3).Onesamplefromeachmixwasusedforthemicrostructuralstudy.Foreachmixtherequiredquantitieswerebatchedbymass.Thesequantitieswerethenm

25、ixedtogetherusingapanmixerinaccordancewithBS1881:Part125:198617.Forbothblocksandcubestheconcretewascastintwolayers.Avibratingtablewasusedtocompacttheconcrete.Fullcompactionwasconsideredtohavebeenachievedwhenairbubblesstoppedappearingontheconcretesurface.Aftervibrating,theconcretesur-facewasnishedsmo

26、othusingametaloatandthencoveredwithapolythenesheettopreventevaporationofwaterfromtheconcrete.Approximately,6haftertheconcretehadbeencastthemouldswerecoveredwithadamphessian.Themouldswerestrippednextdayandthehardenedcon-cretesampleswereplacedinaconstanttemperature(±20°C)waterbathfor3days.Th

27、especimenswerethentransferredintoastoragelaboratorykeptataconstanttemperatureof18°C(±2°C)andarelativehumidityof60%(±5%).Whentheblockswere14daysoldtheywereplacedinadryingcabinetatatemperatureof40°Candarelativehumidityof20%.Thiswasdonetoremovemoisturefromthesurfaceoftheconcret

28、einordertoeliminatetheeectofmoistureontheperme-ationresults.After14daysofdryingthespecimenswerereadyfortesting.Forthistheyweretransferredtoatest-inglaboratory(keptataconstanttemperatureof20°C(±0.5°C)andrelativehumidityof50%(±5%)andtestedonthefollowingday.Thecubeswerecuredandcondi

29、tionedinthesamewayastheslabsinordertoen-surethatthecompressivestrengthandotherparametersweremeasuredusingsimilarconcrete.Thecubeswerethencrushedtodeterminetheconcretecompressivestrength.Table3Slumpand28-daycompressivestrengththestudyNo:CA-GFA/CASlump(mm)Strength(N/mm2)110mm0.53546.9210/20=2.05644.13

30、10/20=0.57944.2420mm10342.9510mm0.71850.1610/20=2.02845.2710/20=2.02345.4820mm6144.9Note:CA-Gcoarseaggregategrading.2.4.Testmethods2.4.1.AirpermeabilitytestTheairpermeabilityofconcretewasmeasuredonthepreconditionedblocksusingtheAutoclamPermeabilitysystem18.Thistesthasbeenusedinresearchformorethanade

31、cadeandhasprovedtobeveryreliableandeasytouse.Detailsofthetestcanbeobtainedin18.Thetestprovidesarelativemeasureoftheairperme-abilityofconcreteinLn(Bar)/min.2.4.2.SaltscalingtestThefreezethaw/saltscalingtestwascarriedoutaccordingtotheRILEMtestprocedure19.Thespec-imensforthetestwere100mmdiametercoresta

32、kenfromtheslabsusedfortheAutoclamairpermeabilitytest.Thecoresweredriedfor2weeksinafan-assistedcabinetmaintainedat40°Cand20%relativehumidity.Thecurvedsurfacesofthesecoreswerethencoatedwithanepoxyemulsion.Thesecoreswerethenkeptina3%sodiumchloridesolutionfor7days,withthetestsurface(mouldnishedface

33、)touchingthesolution.Thefaceofthecoresoppositetothetestsurfacewasthencoatedwiththeepoxyemulsion.Thesampleswerethentrans-ferredtotestcellsagainwiththetestsurfaceincontactwiththe3%saltsolution.ThecellswerethenplacedinanautomaticenvironmentalchamberandsubjectedtotestcyclesasshowninFig.1.Attheendofevery

34、twocycles,thespecimensweretakenoutofthechamberandthelooseparticlesremovedbymeansofanultra-sonicbath.Thelooseparticlescollectedweredriedat100°Cfor24handthenweighed.Thesemeasurementswerecontinuedfor28cycles.2.4.3.AcceleratedcarbonationtestThecarbonationresistanceofdierentmixeswasdeterminedbycarry

35、ingoutanacceleratedcarbonationteston100mmcoresremovedfromtheblocksusedforthepermeabilitytests.Thecoresusedforthetestweredriedinafan-assistedcabinetfor2weeksat40°Cand20%relativehumidity.Thesewerethencoatedwithepoxyemulsiononthecurvedsurfaceandthetrowel-nishedface.Whentheepoxywascured,L.Basheeret

36、al./ConstructionandBuildingMaterials19(2005)682690685thecoreswereplacedinanelectronicallycontrolledcar-bonationchamberinanenvironmentof5±0.1%car-bondioxideat20°±0.5°Cand65±1%relativehumidity.Attheendof3weeks,thesamplesweretakenoutandsplitlongitudinally.Thefreshlybrokensurfac

37、ewassprayedwithaphenolphthaleinindicatorsolution.After24hthedepthofthepinkcolorationwasmea-suredtothenearestmillimetreatsixdierentlocations.Anaverageofthesevalueswasusedforcomparingdif-ferentmixes.2.4.4.ElectronmicroscopicstudyoftheITZCoresof50mmdiameterwerecutfromtheblocksusedforthepermeationstudie

38、s.Thecoreswerethencutalongtheirdiameterandasliceofsize25mm·25mm·2mmwasremovedfromthemould-nishedfaceofeachcore.Thesliceswerethenwrappedinthinpolythenesheetstopreventanycarbonationtak-ingplace.Thesethinslicesweredriedundervacuumandthenimpregnatedwithanultralowviscosityepoxy.Aftertheepoxyhad

39、curedthespecimenswerepolishedwithdiamondgritofprogressivelynersizesdownto0.25lm.Thepolishedsampleswerethencoatedwithcarbonto10nmthickness.Thecarbon-coatedsampleswereplacedunderascan-ningelectronmicroscope(SEM)and20backscatterimagesfromaroundthecoarseaggregatepresentintheslicewerecollectedatrandom.Th

40、eimagemagnica-tionusedwas400timesandtheelectronbeamwaskeptatadistanceof15mmawayfromthetopofthespeci-meninallcases.Eachimageconsistedofasmallpartofaggregateonthelefthandsideofthepictureandtherestwasthematrixofcementpasteandmortar(Fig.2(a).Theoverallsizeofeachimagewas336·262lm.Theporositygradient

41、wasdeterminedusinganimageanalysiscomputersoftwareprogrammeNIHImage.Inordertodothis,thresholdingofthegreyscaletoidentifyporesfromtherestwasessential.Thiswasdonebytrialanderrorandbycarryingoutasensitivityanalysis.Oncethethresholdgreyscalewasdecided,thesoftwareen-abledthedeterminationofporosityinastrip

42、of10lmthicknessawayfromthefaceoftheaggregate(Fig.2(a).Thisstripwasmovedonanincrementalbasisby10lmwidthawayfromthefaceofthecoarseaggregateandeachtimetheporosityofthe10lmthickstripwasdeterminedasapercentageofthetotalareaofthestrip.Oncethestripreachedtheendoftheimage,theanalysiswasrepeatedonotherimages

43、collectedfromthesamesample.Therefore,foreach10lmincrementstripawayfromthecoarseaggregate,therewere20porosityvaluesandanaverageofthesevalueswasreportedastheporos-ityofthis10lmthickstripofconcrete.Theaverageporosityvalueswerethenplottedagainstthedistancefromthefaceoftheaggregatetodeterminethecharac-te

44、risticsoftheinterfacialzone(Fig.2(b).Fig.2.(a)Segmentationusinghistogram.(b)Typicalporositydistribuationcurve.3.Resultsanddiscussion3.1.SlumpandcompressivestrengthTheslumpand28-daycompressivestrengthresultsofeachmixarepresentedinTable3.Anincreaseinslumpandmarginaldecreaseincubecompressivestrengthcan

45、benotedwithanincreaseintheproportionof20mmaggregateinthemix.Thereductioninwaterdemandwithanincreaseinthesizeofcoarseaggregateiswelldocumented2.Inthisexperimentalprogrammewhentheproportionof20mmaggregateincreasedforagivenwatercementratio,thereductioninwaterde-mandledtotheincreaseinslump.However,asthe

46、watercementratiowasnotchanged,thestrengthdidnotvarysignicantly.3.2.AirpermeabilityTables4and5givetheresultsoftheairpermeabilitytestasAPindex18.Whentheeectofcoarseaggregategradingisconsidered,itcanbeseenthatasthepropor-tionof20mmaggregateincreasesintheconcreteairpermeabilityincreases,indicatingareduc

47、tionindurabil-ity20.ThisisseenforbothFA/CAratios.InTable5,686L.Basheeretal./ConstructionandBuildingMaterials19(2005)682690Table4Theeectofaggregategradingonairpermeability,saltscalingresistanceandcarbonationresistanceCoarseaggregategrading10/20210/200.520mm10/20210/200.520mmFA/CA0.5APindex(Ln(mBar)/m

48、in)0.1140.1390.1810.060.1050.115Cumulativescalingwt(g)148.6406.9642.367.0253.0586.05Depthofcarbonation(mm)12.713.113.712.811.312.40.7Table5Theeectofaggregatesizeonairpermeability,saltscalingresistanceandcarbonationresistanceCoarseaggregatesize(mm)10201020FA/CA0.50.7APindex(Ln(mBar)/min)0.110.1810.07

49、60.115Cumulativescalingwt(g)46.2642.378.0586.05Depthofcarbonation(mm)11.113.711.212.4theeectofincreasingthesizeofcoarseaggregatefrom10to20mmistoincreasetheairpermeability.Therearetwopossiblereasonsfortheseresults.Whenthesizeofcoarseaggregateincreases,thereisareductioninthetortuosityoftheowpath,which

50、tendstoincreasethepermeability.Thesecondreasonisthatwhenlargersizecoarseaggregatesareused,thepossibilityofthebleedwatergettingcollectedbelowthecoarseaggregatein-creases,whichleadstoanincreaseinlocalwatercementratioandhencetheporosity.Anincreaseinporosityaroundtheaggregatewouldincreasethepermeability

51、oftheconcrete.However,Table6indicatesthatonlymoderatevariationsinairpermeabilityoccurredandallthesamplesusedforthisstudyfallintheregionofmediumriskconcretesintermsofdurability20.ItcanalsobeseenfromTables4and5that,astheneaggregatecoarseaggregateratiowasincreasedfrom0.5to0.7,theairpermeabilityofthecon

52、cretede-creased.ThisisquiteunexpectedbecausewhentheFA/CAincreases,theincreaseinsandvolumeforagivenaggregatecementratiowouldincreasetheareaofinter-facialtransitionzone,whichinturnreducethetortuos-ityoftheowpath.Thismeansthattheairpermeabilityshouldhaveincreased.InTable3,thereisareductioninslumpandani

53、ncreaseincompressivestrengthwiththeincreaseinFA/CA,whichwouldsuggestthatwhentheFA/CAincreased,therewasanassociateddecreaseinwatercementratio.Thismighthavebeentheresultofincreasedabsorptionofmixwaterbysandoverandabovethatwasaccountedinmixproportioning.Never-theless,itcanbeconcludedthatthereductionine

54、ectivewatercementratioassociatedwiththeincreaseinneaggregatecontentisresponsibleforthereductioninairpermeability.Fig.3showstheeectofaverageaggregatesizeontheairpermeability.(Theaverageaggregatesizewasob-tainedfromthesieveanalysisdata,usingstandardpro-cedures).Astheaverageaggregatesizeinthemixincreas

55、estheairpermeabilityindexincreases.Astraightline,witharegressioncoecientof0.87,canrepresentthisincrease.Thecombinedeectofaggregatesize,itsrelativeproportioninamixandthequantityofneaggregateinthemixonairpermeabilitywasobtainedbyplottingairpermeabilityagainsttheaveragesizeofaggregateinFig.3.Thegurewou

56、ldsuggestthatthepermeability(andhencethedurability)ofconcretecouldbeimprovedbyreducingtheaveragesizeofthecombinedneandcoarseaggregates.Table6RiskratingofdurabilitybasedonAutoclamairpermeabilityindex(20)Airpermeabilityindex(Ln(mBar)/min)Range<0.50.9>0.9RatingLowMediumHighVeryhighL.Bash

57、eeretal./ConstructionandBuildingMaterials19(2005)6826906873.3.SaltscalingThesaltscalingtestgaveresultssimilartothoseoftheairpermeabilitytest(Tables4and5).Asthepropor-tionofthe20mmaggregateincreasesinthemixthecumulativesaltscalingalsoincreases,indicatingade-creaseindurability(Table4).Asimilartrendcan

58、alsobeseeninTable5whentheresultsof10and20mmcoarseaggregatemixesarecomparedforboththeFA/CAratios.However,theeectofanincreaseinneaggregatecoarseaggregateratiofrom0.5to0.7istoimprovethesaltscalingresistanceoftheconcrete.Asinthecaseofairpermeability,itispossibletoconcludethatanincreaseintheproportionofs

59、mallersizefrac-tionsofaggregateinamixwouldimproveitssaltscalingresistancewhenthewatercementratioiskeptconstant.However,ithastobepointedoutthattheeectoftheaverageaggregatesizeisnotassignicantafactorasthewatercementratioofamix.Inapreviousstudy,alowerwatercementratio(0.45)concretewithstood100cyclesoffreezingandthawing21comparedto0.6