4 CH 原文本.docx
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4 CH 原文本.docx
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4CH原文本
Adesignconsiderationfordurabilityofhigh-performanceconcrete
P.K.Chang
a,Y.N.PengaandC.L.Hwangb
aCivilEngineeringDepartment,VanNungInstituteofTechnology&NationalChiaoTungUniversity,Hsinchu300,Taiwan,ROC
bConstructionEngineeringDepartment,NationalTaiwanUniversityofScienceandTechnology,Taipei106,Taiwan,ROC
Received22January2000;
accepted6December2000
Availableonline16August2001.
Abstract
Environmentalfactors,especiallytheclimate,havesignificantinfluenceonconcretestructure.Thispaperaimstoinvestigatetheharmfuleffectsofmaritimeclimateonthedurabilityofconcretestructuresbuiltincoastalareas.Singlyreinforcedbeamspecimensoftraditionaldesignandthoseofdensifiedmixturedesignalgorithm(DMDA)wereemployedtostudythepotentialproblemsofconcretestructure.Resultsindicatethatcracksontheconcretestructure,ifgounnoticed,maycausefailures.Thus,itisimportanttoknowthemethodologyofachievinghighstrengthanddurableconcreteinordertoavoidformationofcracksinthestructuralmember.
AuthorKeywords:
High-performanceconcrete;Highstrength;Durability;Densifiedmixturedesignalgorithm
ArticleOutline
1.Introduction
2.HPCmixproportioning
2.1.Workability
2.2.Safety
2.3.Durability
2.4.Economy
2.5.Ecology
3.Experimentalplan
3.1.Materials
3.2.Experimentalprocedures
4.Resultsanddiscussion
4.1.Mediumandlong-termdurabilityindex
4.2.Analysisoncorrosivebehaviorofbeamsunderrepeatedloadingandmarineenvironment
4.3.Analysisofconcretecrackingandreinforcementcorrosionrate
4.4.CorrosionbehaviorofreinforcementinHPC
4.5.Relationshipbetweenbeambehaviourandsteelcorrosionunderrepetitiveloading
5.Conclusionsandsuggestions
Acknowledgements
References
1.Introduction
ThecastatrophicearthquakethattookplaceinTaiwanon21September1999causedseriouscausalityandattractedmuchattentiontothequalityofconstructionintermsofitsstrength,ductilityanddurability.Poorqualityhasoftenbeenashortcomingoftraditionalconcrete.Itposesdangertothesafetyoftheusers'lifeandproperty.Comparativelyspeaking,greateremphasisisusuallypaidtothestrengthandworkabilityoftheconcretemixturewhiledurabilityhasneverbeengivenmuchattentioninthemixdesign.Consequently,twokindsofproblemsarise.Oneistheover-stiffnessofthemixturewhichlacksinworkability.Toremedysuchaproblem,waterisadded,leadingtofurtherdeteriorationinquality.Theotheristheseriousdryingshrinkagecrackingasaresultofexcessiveamountofcementused.Inthepast,ACI318-89usedtotakeaccountofthewatertocementratio(W/C)asasafetycriterion.Asaresult,deterioration,corrosion,bleeding,efflorescenceorcrackswerecommonlyfoundinmanybuildingswithinafewyearsafterconstruction.Theseproblemsaresignsofagingcausedbytheneglectofdurabilityintheconcretedesign.
Themixturedesignofhigh-performanceconcrete(HPC)reliesheavilyontheappropriateamountofpasteused.Inaddition,chemicaladmixtureandpozzolanicmaterialsaswellasthelowwatercontentandlowamountofcementpastewereusedtoachievehighdurability[1].ACI318-95[11]hasbeenrecentlyrevised,specifyingthatstructuralconcreteshouldhavehighdurability,aspresentedbythewatertobinderratio(W/B)[2].Along-termmonitoringdeviceshouldbemadeavailableforlargestructuressoastoestablishdurabilitydataandtoaccumulatefurtherexperienceinfollowinguponconcretequalitywhichcanserveasfeedbackforfuturedesign.Inthisstudy,thecorrosionbehaviorofsinglyreinforcedHPCbeamsunderrepeatedloadinginmarineenvironmentwasexamined.Theresults,thusobtained,mayshedlightonthedesignofhighwaystructureconcreteinhumidcoastalenvironments.
2.HPCmixproportioning
HPCdesignedaccordingtothedensifiedmixturedesignalgorithm(DMDA)hastobesafe,durable,workable,economicandecological.However,thiseugenicmixturelogicisdifferentfromthatofthetraditionalACImixturedesign[3].Thefollowingisacomparisonbetweenthetwomixturedesignswithrespecttoconsiderationsofworkability,safety,durability,economyandecology.
2.1.Workability
TheamountofwaterusedisemployedasacriterionbytheACI.Thewatertocement(W/C)ratiohasadirectinfluenceonthecementcontent.Generallyspeaking,alowW/Cratiorepresentsahighpastecontentindicatinghighstrength.Asaresult,theamountofwaterhasoftenbeenwronglyemployedasanindexforcontrollingtheslumpandworkabilityofconcrete.Infact,forHPCtoachievehighworkability,ithastobemostdenselypackedwithleastvoids.Inaddition,itrequirestheuseofsufficientamountofpaste,alongwithpozzolanicmaterialsandsuperplasticizers.
2.2.Safety
TheW/CratioisalsotakenasanindicatorofsafetyforbothHPCandtraditionalconcrete.TheonlydifferenceisthatHPCemploystheW/Bratioasanadditionalcontrolcriterionformediumandlong-termstrength;aswellasthewatertosolid(W/S)ratioforitslong-termproperties.ThesafetyofHPCischieflybuiltupontheforcetransferredthroughaggregates.Thisisdifferentfromtraditionalconcretewhichreliesonpasteasitsmainsupport.
2.3.Durability
TraditionalconcreteachievesdurabilitybycontrollingmainlytheW/CratiowhileHPCusesboththeW/BandW/Sratiosascriteria.Thewatercontentisminimizedinordertoreducewaterchannels.Similarly,thecementcontentiskepttotheminimumsoastoreducetheyieldofcalciumhydroxidewhich,inturns,lowerefflorescence,andsulfateattack(SA)reaction.Inaddition,decreasingtheamountofalkalicanreduceaggregatesattackrisk(AAR).Increasingthedensityofwaterandresistivitycandecreaseinfiltrationofharmfulsubstancesandelectronmovementwhileincreasingtheadditionofpozzolanicmaterialscansealthecracksformed.Thiscanensurehomogeneityandstabilityinconcrete,thusleadingtocontinuousdevelopmentinstrength.
2.4.Economy
Traditionalconcretedoesnotconsidereconomyinitsmixturedesign.Theaverageefficiencyofconventionalcementcalculatedis0.07MPa/kg,whilethatofHPCis0.14MPa/kg[3].Asfarascementpasteisconcerned,voidvolume(Vv)canbereducedandharmfulalkalimaterials(CH,KH,NH)canbetransformedintobeneficialbindertoattainstrength.Consequently,thiscanreducethecontentofexpensivecementneeded.Anothermeanstoachieveeconomyistoemploythecheapindigenousresourcesandrecyclingmaterials,suchasflyashandslag.
2.5.Ecology
Tominimizethepossiblepollutiontotheenvironment,industrialwastesarefilledintothecracksandvoidsthroughdensifiedlogicinfiltrationwhichsolidifiesindustrialwasteswithoutdamagingtheconcrete.Insomecases,thisevenhelpstoachievebetterquality.Inaddition,reducingthecementcontentcanlowertheemissionofCO2,whichinturnreducesthegreenhouseeffectandcontributestoecologicalprotection.Anotherkeypointisprolongingthelifecycleofastructuresothatrepairsanddeteriorationcanbereduced.
3.Experimentalplan
ThedurabilityofbeamsmadeaccordingtotheHPCmixtureproportionalgorithmandthosefabricatedfollowingtheACI318-89[11]arecomparedwithrespecttothefollowing:
(1)optimumpackingdensityofmaterials;
(2)applicationofpozzolanicmaterials;(3)boundaryactivator.
3.1.Materials
ThecementusedwasPortlandCementTypeIproducedbyTaiwanCementCompany,Blast-furnaceslagwasprovidedbyChinaSteelCompany.ClassFflyashwasproducedbyTaiwanPowerShingtaStation.Superplasticizer,silicafumeandsteelfiberareavailablefromthelocalmarket.
3.2.Experimentalprocedures
Thepropertiesofthedifferentconstituentmaterialswerefirstdetermined.Theconcretestrengthwasdesignedaccordingtotheleastvoidmethod.Singlyreinforcedconcretebeamspecimens(12cm×20cm×80cm)asshowninFig.1weremadewithW/Bratiosof0.28,0.32and0.4.Allspecimenswerecuredinwaterfor120days,andthensubmergedin3%artificialseawatersolutiontosimulatemarineenvironment.RCcorrosion-relatedperformancetestsaswellasrepeatedloadingtests(10Hz,106cycles)wereconducted.Themostrepresentativespecimenwastestedforthecontentofchlorideandhydroxylioncontent.Thepermeabilityindexbeforeandaftercorrosionwasmeasuredtoevaluatetheanti-corrosionefficiencyofHPC.
Full-sizeimage(11K)
Fig.1.Singlyreinforcedconcretebeamspecimens(12cm×20cm×80cm).
ViewWithinArticle
4.Resultsanddiscussion
4.1.Mediumandlong-termdurabilityindex
AstheW/Cratiocanonlycontrolearly-agestrengthofconcrete[3],themajorinfluenceonlong-termstrengthdevelopmentliesintheW/Bratiobecauseittakesintoaccountthepozzolaniceffect.Amongtheexistingmixturespecifications,durabilityisguaranteedbycontrollingtheairentrained,theW/Bratioandthecontentofchloridecontentwithoutconsideringthenegativeeffectofphysicalandchemicalinteractions[1].Meanwhile,lowW/Bratiomayreducetheamountofcementrequiredandminimizeproblemscausedbyahighpastecontent.Thus,ACI318-95hasadoptedthesumofW/B,cementandpozzolanicmaterialsratio,tomakeconcreteofhighqualityandlowpermeability.Inaddition,W/Sistheratioofweightofwatertothatofthesolidcomponents.Fig.2showstherelationshipsbetweenallW/Sparametersandresistivityunderanytypeofcementpastesystems[4].ItcanbeseenthatthelowertheW/Sandlongertheage,higheristheelectricalresistivityefficiency.Inparticular,whentheW/Sislessthan8%,theresistiv
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