外文翻譯-高性能混凝土的耐久性特點(diǎn)：一綜述

本科畢業(yè)設(shè)計(jì)英文翻譯院〔系部〕土木工程學(xué)院專業(yè)名稱土木工程專業(yè)年級(jí)班級(jí)道橋07-4班學(xué)生姓名周鵬指導(dǎo)老師程朝霞河南理工大學(xué)土木工程學(xué)院二○一一年六月十日Thedurabilitycharacteristicsofhighperformanceconcrete:areviewAbstractDurabilityproblemsofordinaryconcretecanbeassociatedwiththeseverityoftheenvironmentandtheuseofinappropriatehighwater/binderratios.High-performanceconcretethathaveawater/binderratiobetween0.30and0.40areusuallymoredurablethanordinaryconcretenotonlybecausetheyarelessporous,butalsobecausetheircapillaryandporenetworksaresomewhatdisconnectedduetothedevelopmentofself-desiccation.Inhigh-performanceconcrete(HPC),thepenetrationofaggressiveagentsisquitedifficultandonlysuperficial.However,self-desiccationcanbeveryharmfulifitisnotcontrolledduringtheearlyphaseofthedevelopmentofhydrationreaction,therefore,HPCmustbecuredquitedifferentlyfromordinaryconcrete.FieldexperienceintheNorthSeaandinCanadahasshownthatHPCs,whentheyareproperlydesignedandcured,performsatisfactorilyinveryharshenvironments.However,thefireresistanceofHPCisnotasgoodasthatofordinaryconcretebutnotasbadasissometimeswritteninafewpessimisticreports.Concrete,whateveritstype,remainsasafematerial,fromafireresistancepointofview,whencomparedtootherbuildingmaterials.AuthorKeywords:

Curing;Durability;Fire-resistance;Freezingandthawing;HighperformanceconcreteArticleOutline1.Introduction2.Volumetricchanges3.Curingconcrete4.Durability4.1.Generalmatters4.2.Durabilityinamarineenvironment4.2.1.Natureoftheaggressiveaction4.2.2.Chemicalattackonconcrete4.2.3.Abrasionresistance4.3.Freeze–thawresistance5.FireresistanceofHPC5.1.Thechanneltunnelfire5.2.TheDüsseldorfairportfire5.3.Spallingofconcreteunderfireconditions5.4.TheBrite–EuramHITECOBE-1158researchproject6.ConcludingremarksReferences1.IntroductionTherecentdevelopmentsinthefieldofhigh-performanceconcrete(HPC)representagiantsteptowardmakingconcreteahigh-techmaterialwithenhancedcharacteristicsanddurability.Thesedevelopmentshaveevenledtoitbeingamoreecologicalmaterialinthesensethatthecomponents––admixtures,aggregates,andwater––areusedtotheirfullpotentialtoproduceamaterialwithalongerlifecycle.Bethatasitmay,weknowthatconcretewillneverbeaneternalmaterialwhenmeasuredagainstageologicaltimeframe.Anyconcrete,ifwelookfarenoughintothefuture,willenditslifecycleaslimestone,clay,andsilicasand,whicharethemoststablemineralformsofcalcium,silica,iron,andaluminumintheearth’senvironment.Therefore,allwecandoasengineersorscientistsistoextendthelifecycleofthisartificialrockasmuchaspossible.Theconcretethatwasknownashigh-strengthconcreteinthelate1970sisnowreferredtoasHPCbecauseithasbeenfoundtobemuchmorethanjuststronger:itdisplaysenhancedperformancesinsuchareasasdurabilityandabrasionresistance.Althoughwidelyused,theexpression“HPC〞isveryoftencriticizedasbeingtoovague,evenashavingnomeaningatall.SincethereisnosinglebestdefinitionforthematerialknownasHPC,itispreferabletodefineitasalowwater/binderconcretewhichreceivesanadequatewatercuring.HPCcanbemadewithcementaloneoranycombinationofcementandmineralcomponents,suchas,blastfurnaceslag,flyash,silicafume,metakaolin,ricehuskash,andfillers,suchaslimestonepowder.Ternarysystemsareincreasinglyusedtotakeadvantageofthesynergyofsomemineralcomponentstoimproveconcretepropertiesinthefreshandhardenedstates,andtomakehighperformanceconcretemoreeconomicalandecological.Fig.1

representsschematicallythefundamentalmicrostructuraldifferencebetweencementpasteshavinga0.65and0.25water/cementratio.Ina0.25

W/Cratiocementpaste,therearemorecementgrainsandconsequentlylesswaterperunitvolumesothatcementgrainsaremuchclosertoeachotherthanina0.65

W/C

cementpaste.Thismajordifferenceresultsinacompletelydifferenttypeofhydratedcementpaste.A0.65

W/C

ratiocementpasteisveryporousandrichincrystallizedouterhydrationproductsformedthroughasolution–precipitationprocess,whilea0.25

W/C

ratiocementpasteisverycompactandessentiallycomposedofinnerhydrationproductsresemblingageldevelopedthroughadiffusionprocess.

Fig.2

and

Fig.3

illustratethemajordifferenceexistingbetweenthemicrostructureofahighandlow

W/C

ratiocementpaste.Thisessentialmicrostructuraldifferenceresultsinamajordifferenceinthemechanicalanddurabilitybehaviorofboththecementpasteandthetransitionzonebetweenthepasteandtheaggregates.

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(13K)Fig.1.Schematicalrepresentationofthemicrostructureoftwocementpasteshaving

W/C

ratiosof0.65and0.25.Full-sizeimage

(71K)Fig.2.Microstructureofhighwater/cementratioconcrete:(a)highporosityandheterogeneityofthematrix,(b)orientatedcrystalofCa(OH)2

onaggregate(AG),(c)CHcrystals.Full-sizeimage

(52K)Fig.3.MicrostructureofaHPC:lowporosityandhomogeneityofthematrix:(a)absenceoftransitionzonebetweentheaggregateandcementpaste;(b)densecementpasteinanairentrainedhighperformanceconcrete.Inparticular,inHPC,thecoarseaggregatecanbetheweakestlinkinconcretewhenthestrengthofthehydratedcementpasteisdrasticallyincreasedbyloweringitswater/binderratio.Insuchcases,concretefailurecanstarttodevelopwithinthecoarseaggregate.Asaconsequence,therecanbeexceptionstothewater/binderratiolawwhendealingwithHPC.Insomeareas,decreasingthewater/binderratiobelowacertainlevelisnotpracticalfromamechanicalpointofviewbecausethestrengthoftheHPCwillnotsignificantlyexceedthecompressivestrengthoftheaggregate.Whenthecompressivestrengthislimitedbythecoarseaggregate,theonlywaytogethigherstrengthistouseastrongeraggregate.Butalthoughthecompressivestrengthisnotincreasedwhendecreasingthe

W/B

ratio,thecompactnessofthematrixisincreasedandthedurabilityofHPCisimproved.2.VolumetricchangesAswithanyothermaterial,thevolumeofconcretechangesasitstemperaturechanges.Likeanyothermaterialconcretecreeps.Butitisnottheonlyvolumetricvariationsexertingitselfonconcrete.Dependingonitscuringcondition,concretepresentsvolumetricvariations,itusuallyshrinksbutsometimesitswells.Inthispaper,swellingofchemicalorigin,suchassulfateorthaumasiteattackoralkaliaggregatereactionwillnotbeconsidered,theonlyvolumetricvariationtakenintoaccountwillbeplasticshrinkage,autogenousorisothermalshrinkage,anddryingshrinkage[2].Carbonationshrinkagewillnotbeconsideredbecauseitisaveryslowprocessthattakesplacemuchlater.Inallcasesthatwillbeconsideredinthispaper,theoriginofthevolumetricvariationisthesame,theappearanceoftensilestressesinthemeniscicreatedinthefreshconcreteasitisdrying(plasticshrinkage)orinthehardenedconcreteduetoself-desiccation(autogenousshrinkage)andduetodying(dryingshrinkage).Autogenousshrinkageisaconsequenceofthechemicalcontractionoccurringinthecementpastewhenwaterhydratescementparticles.Infact,theabsolutevolumeofthehydratesformedissmallerthanthesumoftheabsolutevolumeofthecementparticlesandthewaterthathavereacted.Hydrationcreatessome8%voids,asfoundbyLeChatelierandPowers[3].Thisveryfineporositydrainswaterfromthecoarsercapillarieswherewaterisnotasstronglybonded.Consequently,ashydrationprogressesitisobservedthatthecoarsecapillariesarebeingemptied(asinthecaseofdryingshrinkage)butwithoutanymassloss.Thisphenomenoniscalledself-desiccation.Self-desiccationisduetothemovementofthewaterthatismovingfromthepreexistingcoarsecapillariestowardstheveryfineporositycreatedbycementhydration.Dryingshrinkageoccurswhenconcretedriesindryair,asconcreteloosessomeofitsinternalwater;menisciappearwithinthecoarsesuperficialcapillaries.Inthecaseofdryingshrinkagethereisamassloss.Inordinaryconcretewith

W/C

ratiogreaterthan0.50,forexample,thereismorewaterthanrequiredtofullyhydratethecementparticlesandalargeamountofthiswateriscontainedinwellconnectedlargecapillariessothatthemeniscicreatedbyself-desiccationappearinlargecapillarieswheretheygenerateonlyverylowtensilestresses.Therefore,thehydratedcementpastebarelyshrinkswhenself-desiccationdevelops(40–60microstrains)[4].InthecaseofHPCwitha

W/B

ratioof0.35orless,significantlymorecementandlessmixingwaterhavebeenused,sothattheinitialporenetworkisessentiallycomposedofveryfinecapillaries.Whenself-desiccationstartstodevelop,assoonashydrationbegins,meniscirapidlydevelopintosmallcapillariesifnoexternalwaterisadded.SincemanycementgrainsstarttohydratesimultaneouslyinHPC,thedryingoftheveryfinecapillariescangeneratehightensilestressesthatshrinkthehydratedcementpaste.Thisearlyshrinkageisreferredtoasautogenousshrinkage.Ofcourse,autogenousshrinkageisaslargeasdryingshrinkageobservedinordinaryconcretewhenthesetwotypesofdryingdevelopincapillariesofthesamediameter[2].But,whenthereisanexternalsupplyofwater,thecapillariesdonotdryoutaslongastheyareconnectedtothisexternalsourceofwater[5].Theresultisthatnomenisci,notensilestress,andnoautogenousshrinkagedevelopwithinaHPCthinelementhavinga

W/C

ratioof0.35thatisconstantlywatercuredfromthemomentofitssetting.Butwhenthe

W/C

ratioislowerthan0.35oratthecenterofalargeconcreteelementmadewitha0.35

W/C

ratioHPC,concretemicrostructurecanbesodensethatwaterpenetrationcanbestoppedandself-desiccationcandevelopincertainpartsofconcrete.Infact,whencementparticlesarehydratingwithwatercomingfromanexternalsourcethereisanincreaseintheabsolutevolumeofthecementthatleadstothefillingofsomeporesandcapillaries.Inthiscase,itwouldbemoreappropriatetospeakofisothermalshrinkageratherthanautogenous,sinceautogenousshrinkagereferstotheshrinkageofaclosedsystem.Thus,theessentialdifferencebetweenordinaryconcreteandHPCisthatordinaryconcreteexhibitspracticallynoautogenousshrinkage,whetheritiswatercuredornot,whereasHPCcanexperiencesignificantautogenousshrinkageifitisnotwatercuredduringthehydrationprocess.AutogenousshrinkagedoesnotdevelopinHPCaslongastheporesandcapillariesareinterconnectedandhaveaccesstoexternalwater,but,whenthecontinuityoftheporeandcapillarysystemsisbroken,then,andonlythendoesautogenousshrinkagestarttodevelopwithinthehydratedcementpasteofaHPC,asshownin

Fig.4.

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(12K)Fig.4.Influenceofcuringconditionsontheoccurenceofautogenousshrinkage.Dryingshrinkageofthehydratedcementpastebeginsatthesurfaceoftheconcreteandprogressesmoreorlessrapidlythroughtheconcrete,dependingontherelativehumidityoftheambientairandthesizeofcapillaries.Dryingshrinkageofordinaryconcreteisthereforerapidbecausethecapillarynetworkiswellconnectedandcontainsopencapillariesatthesurfaceoftheconcrete.DryingshrinkageinHPCisslowbecausecapillariesareveryfineandsoongetdisconnected.Anothermajordifferencebetweendryingshrinkageandautogenousshrinkageisthatdryingshrinkagedevelopsfromthesurfaceinwards,whileautogenousshrinkageishomogeneousandisotropic,insofarasthecementparticlesandwaterarewelldispersedwithintheconcrete.Thus,thereareconsiderabledifferencesbetweenordinaryconcreteandHPCwithrespecttotheirshrinkagebehavior.Thecementpasteofanordinaryconcreteexhibitsrapiddryingshrinkageprogressingfromthesurfaceinwards,whereasHPCcementpastecandevelopasignificantisotropicautogenousshrinkagewhennotwatercured.Thisdifferenceintheshrinkagebehaviorofthecementpastehasveryimportantconsequencesforconcretecuringandconcretedurability.Althoughtheshrinkageofahydratedcementpasteisaveryimportantparameterwithrespecttoconcretevolumetricstability,itisnottheonlyone.Akeyparameteristheamountofaggregate,and,morespecifically,thequantityofcoarseaggregate.Toooftenitisforgottenthataggregatesdomorethansimplyactasfillersinconcrete.Infact,theyactivelyparticipateinthevolumetricstabilityofconcretewhentheyrestraintheshrinkageofthehydratedcementpaste:concreteshrinkageisalwaysmuchlowerthanthatofacementpastehavingthesame

W/C

ratio.Itiscommonknowledgethatconcreteshrinkagecanbeeasilyreducedbyincreasingthecoarseaggregatecontent;butitmustnotbeforgottenthattheshrinkageofthehydratedcementpastestaysthesame,itissimplymorerestrainedandthereislesscementpaste,sothatthevolumetricstabilityoftheconcreteisincreased.Restrainingtheshrinkageofhydratedcementpastebymodifyingthecoarseaggregateskeletonmayormaynotproduceanetworkofmicrocracks,dependingontheintensityofthetensilestressesdevelopedbythisprocesswithrespecttothetensilestrengthofthehydratedcementpaste.3.CuringconcreteHPCmustbecuredquitedifferentlyfromordinaryconcretebecauseofthedifferenceinshrinkagebehaviordescribedabove,asemphasizedin

Fig.5.IfHPCisnotwatercuredimmediatelyfollowingplacementorfinishing,itispronetodevelopsevereplasticshrinkagebecauseitisnotprotectedbybleedwater,andlaterondevelopssevereautogenousshrinkageduetoitsrapidhydration.Whilecuringmembranesprovideadequateprotectiontoordinaryconcrete(whichisinsensitivetoautogenousshrinkage),theycanonlyhelppreventthedevelopmentofplasticshrinkageinHPCbuthavenovalueininhibitingautogenousshrinkage.

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(14K)Fig.5.Themostappropriatecuringregimesduringthecourseofthehydrationreaction.ThecriticalcuringperiodforanyHPCrunsfromplacementorfinishing,upto2or3dayslater,andthemostcriticalperiodisusuallybetween12and36h.Infact,theshorttimeduringwhichefficientwatercuringmustbeappliedtoHPCcanbeconsideredasignificantadvantageoverordinaryconcrete.ThosewhospecifyanduseHPCmustbeawareofthedramaticconsequencesofmissingearlywatercuring.Initiatingwatercuringafter24histoolate,becausemostofthetime,agreatdealofplasticandautogenousshrinkagehavealreadyoccurredand,bythistime,thecapillaryandporenetworkaredisconnectedinmanyplacesandthemicrostructureisalreadysocompactthatexternalwaterhaslittlechanceofpenetratingverydeepintotheconcrete.WaterpondingorfoggingisthebestwaytocureHPC;oneofthesetwomethodsmustbeappliedassoonaspossible,immediatelyfollowingplacementorfinishing.Anevaporationretardercanbeappliedtemporarilytopreventthedevelopmentofplasticshrinkage.If,foranyreason,waterpondingorfoggingcannotbeimplementedfor7days,thentheconcretesurfaceshouldbecoveredwithwetburlap(hessian)orpreferablyaprewettedgeotextile.Theburlaporthegeotextilemustbekeptconstantlywetwithasoakerhoseandprotectedfromdryingbyapolyethylenesheetinordertoensurethatatnotimeduringthecuringperiodistheconcreteallowedtodryandexperienceanyautogenousshrinkage[6].Moreover,itisobservedthatwhenanyconcreteiswatercuredduringsettingitdoesnotshrinkbutratherswell.

Fig.6

illustratestheeffectofearlywatercuringonthevolumetricchangeofconcrete.

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(6K)Fig.6.Lengthchangesaccordingtodifferentcuringregimesforthe0.35

W/C

ratioconcrete.Watercuringcanbestoppedafter7daysbecausemostofthecementatthesurfaceofconcretehashydratedandanyfurtherwatercuringhaslittleeffectonthedevelopmentofshrinkage.After7daysofwatercuring,HPCexperiencesslowdryingshrinkageduetothecompactnessofitsmicrostructure,andthatautogenousshrinkagehasalreadydriedoutthecoarsecapillariespores.Eventhen,theoreticallythebestthingtodoistopaintHPCortouseasealingagentsothatthelastwaterthatremainsinconcretecanberetainedtocontributetohydration.Thereisnorealadvantageofpaintingorsealingaveryporousconcretebecauseitisimpossibletoobtainanabsolutelywaterproofcoating;paintingorsealingHPC,however,canbeeasyandeffective.Partialreplacementofcoarseaggregatebyanequivalentvolumeofsaturatedlightweightaggregatehasbeenusedtocounteractautogenousshrinkageinternally[7].Thesaturatedlightweightaggregateparticlesactassmallwaterreservoirsthroughoutthemassofconcrete;theycanfilltheveryfineporescreatedbyhydrationreactions.Therefore,thewaterofthelightweightaggregateparticlesisdrainedalongwiththatcontainedinthefinecapillariesoftheHPC.Themenisciwithinthecementpastearenotdevelopedinsmallcapillaries,whichmeanslowertensilestressandlowerautogenousshrinkage.Lightweightaggregatealsoreducescompressivestrengthandelasticmodulus.Shrinkagereducingadmixturecanalsobeused[8].Itiswellknownthatconcreteisnevercuredproperlyinthefield,despitethefactthatitisalwayswritteninthespecificationsthatcontractorshavetocureconcrete.Contractorsarenotcuringconcreteforaverysimplereason:theyarenotspecificallypaidforit,therefore,concretecuringisalwaysperceivedbythemasanunprofitableactivityorevenasourceofexpenseandthereforeawasteoftime.But,whencontractorsarespecificallypaidtowatercureconcretetheydoitastheywouldforanyotheritemthatispaidfor.Forthreeyearsnow,theCityofMontrealandtheQuébecMinistryofTransportationhaverequestedunitpricesforeachitemdirectlyrelatedtoearlywatercuring.Sincetheinitiationofthisnewpolicyontheearlywatercuringofconcrete,itisamazingtoseehowzealouscontractorscanbecomeinthematterofwatercuring.Forthemwatercuringisnowseenasasourceofprofit.Fromthefirstexperiencesinthatmatterithasbeenfoundthatthecostofanearlywatercuringisaboutonetenthof1%,averymodestpricewhenconsideringtheimproveddurabilityoftheconcretestructuresthatarebuiltthisway.Therefore,thebestwaytobesurethatHPCsareproperlyandefficientlycuredinthefieldistospecificallypaycontractorstocureconcrete[6].Thisverylongintroductoryremarksweremadetoemphasizethattwoimportantkeyparameterscontrolthepenetrationofanyaggressiveagentsinconcrete:thewater/cementorthewater/binderratio,andthecuringofconcrete.Specifyingalowwater/binderratioconcreteisanecessarycondition,butnotasufficientone..高性能混凝土的耐久性特點(diǎn)：一綜述摘要普通混凝土的耐久性問(wèn)題，與環(huán)境的嚴(yán)重程度和不適當(dāng)?shù)母咚冶认嗦?lián)系。具有水灰比在0.30和0.40之間的高性能混凝土通常比普通混凝土更耐用，不僅因?yàn)樗麄儦饪咨?，而且還因?yàn)樗麄兊拿?xì)管和毛孔網(wǎng)狀物有點(diǎn)不連通而導(dǎo)致自我枯燥的開(kāi)展。在高性能混凝土〔HPC〕，入侵性因子的滲透是相當(dāng)困難的，只有外表的滲透。然而，自枯燥非常有害，如果它不是在水化反響的早期開(kāi)展階段的控制，因此，高性能混凝土的養(yǎng)護(hù)必須完全不同于普通混凝土。遠(yuǎn)在北海和加拿大的經(jīng)驗(yàn)說(shuō)明，當(dāng)混凝土得到適當(dāng)設(shè)計(jì)和養(yǎng)護(hù)，即使在非常惡劣的環(huán)境中也令人滿意。然而，高性能的耐火不如普通混凝土，但有時(shí)并不像一些悲觀的報(bào)告中那樣寫的不好?；炷粒瑹o(wú)論其類型，相對(duì)于其他建筑材料而言，仍然是一個(gè)平安的材料。作者關(guān)鍵詞：固化；耐久性；耐火性；冷凍和解凍；高性能混凝土文章概要1．介紹2．體積變化3．混凝土養(yǎng)護(hù)4．耐久性4.1．一般內(nèi)容4.2．在海洋環(huán)境中的耐久性4.2.1．更強(qiáng)硬措施的性質(zhì)4.2.2．化學(xué)對(duì)混凝土侵蝕4.2.3．耐磨性4.3．耐凍融性5．高性能混凝土的耐火性5.1．英吉利海峽隧道內(nèi)火災(zāi)5.2．杜塞爾多夫機(jī)場(chǎng)的火災(zāi)5.3．火災(zāi)條件下的混凝土剝落5.4．Brite-EuramHITECOBE-1158的研究工程6．結(jié)束語(yǔ)參考文獻(xiàn)1、簡(jiǎn)介在高性能混凝土領(lǐng)域的最新開(kāi)展〔HPC〕代表一個(gè)巨大的進(jìn)步，并增加其耐久性與特性，使混凝土成為高科技材料。這些事態(tài)開(kāi)展，甚至導(dǎo)致它是一個(gè)在意義上更加生態(tài)材料的組件——外加劑，骨料，水——是用來(lái)實(shí)現(xiàn)其全部潛力，生產(chǎn)具有較長(zhǎng)的生命周期材料。雖然如此，可我們知道，從對(duì)地質(zhì)測(cè)量時(shí)間來(lái)說(shuō)，混凝土絕不會(huì)是一個(gè)永恒的材料。如果我們放眼未來(lái)足夠遠(yuǎn)，任何混凝土，將結(jié)束其生命周期，像石灰石，粘土和石英砂，這是鈣，硅，鐵，和鋁在地球環(huán)境中最穩(wěn)定的礦物形式。因此，作為工程師或科學(xué)家，我們所能做的就是將盡可能多的延續(xù)這些人工巖石的生命周期。在70年代末被認(rèn)為是高強(qiáng)度混凝土現(xiàn)在被稱為高性能混凝土，因?yàn)樗话l(fā)現(xiàn)不僅僅是強(qiáng)度大：它顯示為增強(qiáng)耐久性和耐磨性等方面性能。

雖然廣泛使用，但是“高性能混凝土〞還是很經(jīng)常被過(guò)于模糊的批評(píng)，甚至有沒(méi)有意義。

由于高性能混凝土沒(méi)有一個(gè)最正確的材料定義，更適合將其定義為一低水灰比并接收充足的水分固化的混凝土。高性能混凝土，可單獨(dú)與水泥或與水泥和任何礦物成分結(jié)合，如高爐礦渣，粉煤灰，硅粉，偏高嶺土，稻殼灰，石灰石粉填料。三元系統(tǒng)越來(lái)越多地用于采取一些礦物成分的協(xié)同作用，以改善混凝土在新鮮和硬化方面的性能，使高性能混凝土更經(jīng)濟(jì)和生態(tài)。圖1代表最根本的微觀結(jié)構(gòu)，在水灰比為0.65和0.25的水泥漿體之間的區(qū)別。在水灰比為0.25的水泥漿中的單位體積內(nèi)有更多的水泥顆粒和較少的水，使其水泥顆粒比水灰比為0.65水泥漿中的更緊密。這個(gè)主要的不同結(jié)果在于水泥漿水化的類型完全不同。水灰比為0.65的水泥漿會(huì)有很多孔和通過(guò)降解水的過(guò)程形成富含結(jié)晶的外部水化產(chǎn)物，而一個(gè)水灰比為0.25的水泥漿非常緊湊，在本質(zhì)上水泥水化產(chǎn)物組成的是一種凝膠中開(kāi)展類似的擴(kuò)散過(guò)程。圖2和圖3說(shuō)明了在高和低水灰比的水泥漿之間的主要顯微結(jié)構(gòu)的差異。在這個(gè)根本的微觀結(jié)構(gòu)差異的一個(gè)主要差異是水泥漿和過(guò)渡區(qū)的糊狀物和骨料的機(jī)械和耐久性。

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(13K)圖1.表示的是水灰比為0.25和0.65的水泥漿體的顯微結(jié)構(gòu)。Full-sizeimage

(71K)圖2微觀結(jié)構(gòu)的高水灰比的混凝土:(a)高孔隙度和儲(chǔ)層非均質(zhì)性的基質(zhì)(b)朝向晶體的Ca(OH)2的骨料(AG),(c)CH晶體。Full-sizeimage

(52K)圖3高性能混凝土的微觀結(jié)構(gòu)和孔隙度低均勻的基質(zhì):(a)在水泥漿和骨料之間缺乏的過(guò)渡區(qū);(b)濃厚的水泥漿在一個(gè)空氣中產(chǎn)生高性能混凝土。在高性能混凝土中，特別是粗骨料在混凝土中最薄弱的環(huán)節(jié)時(shí)，通過(guò)降低其水灰比，使水泥漿的強(qiáng)度大幅增加。在這種情況下，混凝土破壞就可以開(kāi)始在粗骨料開(kāi)展。因此，在高性能混凝土處理的時(shí)候，可以有例外水灰比。在一些地區(qū)，減少了水灰比且低于一定水平的比例，實(shí)際上并不是從機(jī)械角度，因?yàn)楦咝阅芑炷恋膹?qiáng)度不會(huì)明顯超過(guò)了混凝土骨料的抗壓強(qiáng)度。當(dāng)其抗壓強(qiáng)度被粗骨料限制，唯一的方法只有增加骨料的強(qiáng)度，才能獲得較高的強(qiáng)度。不過(guò)，減少水灰比時(shí)雖然沒(méi)有增加抗壓強(qiáng)度，但基質(zhì)密壓實(shí)度的增大，也提高高性能混凝土的耐久性。2、體積變化與任何其他材料一樣，混凝土體積隨溫度變化而變化。像任何其他材料一樣具體的蠕動(dòng)。但它不是混凝土本身發(fā)生變化的唯一體積。根據(jù)其固化條件，混凝土出現(xiàn)體積變化，它通常會(huì)縮小，但有時(shí)它膨脹。在本文中，化學(xué)制品膨脹的來(lái)源，如硫酸鹽或碳硫硅鈣石攻擊或堿骨料反響等引起的膨脹將不予考慮，只有考慮塑性收縮，自體或等溫收縮，枯燥收縮等引起的體積變化。碳化收縮將不會(huì)被考慮，因?yàn)檫@是一個(gè)非常緩慢的過(guò)程，發(fā)生的晚得多。在所有被本文所考慮的情況下，體積變化的來(lái)源是相同的，拉應(yīng)力出現(xiàn)在新拌混凝土制造中，因?yàn)樗强菰铩菜芰鲜湛s〕或在硬化混凝土中因?yàn)樽钥菰铩沧允湛s〕和死亡〔枯燥收縮〕而引起的。自收縮是一種發(fā)生在水泥水化物水泥顆粒化學(xué)收縮的結(jié)果。事實(shí)上，固化劑的絕對(duì)體積小于水泥顆粒和參加反響的水總和絕對(duì)體積。水化能產(chǎn)生8％空隙，被LeChatelier和Powers發(fā)現(xiàn)。這很細(xì)的孔隙從較粗的毛細(xì)管那里吸水。因此，水化的進(jìn)展已觀察到，粗毛細(xì)管的水被掏空〔如在枯燥收縮的情況〕，但沒(méi)有任何質(zhì)量損失。這種現(xiàn)象被稱為自枯燥。自枯燥是由于水的運(yùn)動(dòng)正在從既存的粗毛細(xì)血管向很細(xì)的孔隙度由水泥水化。當(dāng)混凝土枯燥收縮在枯燥空氣中，混凝土?xí)斔鸵恍﹥?nèi)部水，半月板在粗糙毛細(xì)管外表出現(xiàn)。在枯燥收縮的情況是有質(zhì)量的損失。在普通混凝土中水灰比比值大于0.50，例如，有更多的水要被水泥顆粒充分水化，大量的水都包含在與其保持良好關(guān)系,以便狀大微血管由自枯燥出現(xiàn)于大毛細(xì)管的地方,他們只生成非常低的拉應(yīng)力。因此，水泥的水化時(shí)幾乎縮自枯燥的開(kāi)展。在高性能混凝土中水灰比為0.35或低于的情況下，顯然更多的水泥和少量水已被使用，因此，初始孔隙網(wǎng)絡(luò)根本上是由很細(xì)的毛細(xì)血管組成。當(dāng)自我枯燥開(kāi)始開(kāi)展，一旦水化開(kāi)始，如果沒(méi)有外部加水那么將迅速開(kāi)展成為小毛細(xì)管。由于許多水泥顆粒開(kāi)始在高性能混凝土中與水化合，非常細(xì)的枯燥毛細(xì)管可以產(chǎn)生高拉應(yīng)力。這種早期收縮被稱為自收縮。當(dāng)然，這兩種類型的枯燥開(kāi)展在相同直徑的毛細(xì)管中，在普通混凝土中可以看到自收縮和枯燥收縮的收縮率是一樣大的。但是，當(dāng)有外部提供的水，只要它們連接到這個(gè)水源，毛細(xì)管就不會(huì)枯燥。結(jié)果是，沒(méi)有半月板就無(wú)拉應(yīng)力，并且沒(méi)有自收縮開(kāi)展在一個(gè)水灰比為0.35且有不斷的水來(lái)養(yǎng)護(hù)的高性能混凝土中。但是，當(dāng)水灰比低于0.35或在該中心有一些大的混凝土構(gòu)件水灰比為0.35的高性能混凝土?xí)r，混凝土微觀結(jié)構(gòu)是如此密集，可以阻止水