外文翻译.docx

外文翻译.docx

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外文翻译

外文翻译

Concretetechnologyanddevelopment

Portlandcementconcretehasclearlyemergedasthematerialofchoicefortheconstructionofalargenumberandvarietyofstructuresintheworldtoday.Thisisattributedmainlytolowcostofmaterialsandconstructionforconcretestructuresaswellaslowcostofmaintenance.Therefore,itisnotsurprisingthatmanyadvancementsinconcretetechnologyhaveoccurredasaresultoftwodrivingforces,namelythespeedofconstructionandthedurabilityofconcrete.

Duringtheperiod1940-1970,theavailabilityofhighearlystrengthportlandcementsenabledtheuseofhighwatercontentinconcretemixturesthatwereeasytohandle.Thisapproach,however,ledtoseriousproblemswithdurabilityofstructures,especiallythosesubjectedtosevereenvironmentalexposures.

Withuslightweightconcreteisadevelopmentmainlyofthelasttwentyyears.

Concretetechnologyisthemakingofplentifulgoodconcretecheaply.Itincludesthecorrectchoiceofthecementandthewater,andtherighttreatmentoftheaggregates.Thosewhicharedugnearbyandthereforecheap,mustbesized,washedfreeofclayorsilt,andrecombinedinthecorrectproportionssoastomakeacheapconcretewhichisworkableatalowwater/cementratio,thuseasilycomoactedtoahighdensityandthereforestrong.Ithardenswithageandtheprocessofhardeningcontinuesforalongtimeaftertheconcretehasattainedsufficientstrength.

Abrams’law,perhapstheoldestlawofconcretetechnology,statesthatthestrengthofaconcretevariesinverselywithitswatercementratio.Thismeansthatthesandcontent（particularlythefinesandwhichneedsmuchwater）mustbereducedsofaraspossible.Thefactthatthesand“drinks”largequantitiesofwatercaneasilybeestablishedbymixingseveralbatchesofxkgofcementwithykgofstoneandthesameamountofwaterbutincreasingamountsofsand.Howeverifthereisnosandtheconcretewillbesostiffthatitwillbeunworkablethereforwporousandweak.Thesamewillbetrueifthesandistoocoarse.Thereforeforeachsetofaggregates,thecorrectmixmustnotbechangedwithoutgoodreason.Thisappliedparticularlytothewatercontent.

Anydrinkableandmanyundrinkablewaterscanbeusedformakingconcrete,includingmostclearwatersfromtheseaorrivers.Itisimportantthatclayshouldbekeptoutoftheconcrete.Thecementiffreshcanusuallybechosenonthebasisofthemaker’scertificatesoftensileorcrushingtests,butthesearealwaysmadewithfreshcement.Wherestrengthisimportant,andthecementatthesiteisold,itshouldbetested.

Thisstress,causingbreakage,willbeatensionsinceconcretesarefrom9to11timesasstrongincompressionasintension,Thisstress,themodulusofrupture,willberoughlydoublethedirecttensilebreakingstressobtainedinatensiletestingmachine,soaveryroughguessattheconpressivestrengthcanbemadebymultiplyingthemodulusofruptureby4.5.Themethodcanbeusedincombinationwiththestrengthresultsofmachine-crushedcubesorcylindersortensiletestpiecesbutcannototherwiseberegardedasreliable.Withthesecomparisons,however,itissuitableforcomparingconcretesonthesamesitemadefromthesameaggregatesandcement,withbeamscastandtestedinthesameway.

Extremecareisnecessaryforpreparation,transport,platingandfinishofconcreteinconstructionworks.Itisimportanttonotethatonlyabitofcareandsupervisionmakeagreatdifferencebetweengoodandbadconcrete.Thefollowingfactorsmaybekeptinmindinconcretingworks.

Mixing

Themixingofingredientsshallbedoneinamixerasspecifiedinthecontract.

HandlingandConveying

Thehandling&conveyingofconcretefromthemixertotheplaceoffinaldepositshallbedoneasrapidlyaspracticableandwithoutanyobjectionableseparationorlossofingredients.Wheneverthelengthofhaulfromthemixingplanttotheplaceofdepositissuchthattheconcreteundulycompactsorsegregates,suitableagitatorsshallbeinstalledintheconveyingsystem.Whereconcreteisbeingconveyedonchutesoronbelts,thefreefallordropshallbelimitedto5ft.（or150cm.）unlessotherwisepermitted.Theconcreteshallbeplacedinpositionwithin30minutesofitsremovalfromthemixer.

PlacingConcrete

Noconcreteshallbeplaceduntiltheplaceofdeposithasbeenthoroughlyinspectedandapproved,allreinforcement,insertsandembeddedmetalproperlysecurityinpositionandchecked,andformsthoroughlywetted（expectinfreezingweather）oroiled.Placingshallbecontinuedwithoutavoidableinterruptionwhilethesectioniscompletedorsatisfactoryconstructionjointmade.

WithinForms

Concreteshallbesystematicallydepositedinshallowlayersandatsuchrateastomaintain,untilthecompletionoftheunit,aplasticsurfaceapproximatelyhorizontalthroughout.Eachlayershallbethoroughlycompactedbeforeplacingthesucceedinglayer.

Compacting

Method.Concreteshallbethoroughlycompactedbymeansofsuitabletoolsduringandimmediatelyafterdepositing.Theconcreteshallbeworkedaroundallreinforcement,embeddedfixtures,andintothecomersoftheforms.Everyprecautionshallbetakentokeepthereinforcementandembeddedmetalinproperpositionandtopreventdistortion.

Vibrating.Whereverpracticable,concreteshallbeinternallyvibratedwithintheforms,orinthemass,inordertoincreasetheplasticityastocompacteffectivelytoimprovethesurfacetextureandappearance,andtofacilitateplacingoftheconcrete.

Vibrationshallbecontinuedtheentirebatchmeltstoauniformappearanceandthesurfacejuststartstoglisten.Aminutefilmofcementpasteshallbediscerniblebetweentheconcreteandtheformandaroundthereinforcement.Overvibrationcausingsegregation,unnecessarybleedingorformationoflaitanceshallbeavoided.

Theeffectspentoncarefulgrading,mixingandcompactionofconcretewillbelargelywastediftheconcreteisbadlycured.Curingmeanskeepingtheconcretethoroughlydampforsometime,usuallyaweek,untilithasreachedthedesiredstrength.Solongasconcreteiskeptwetitwillcontinuetogainstrength,thoughmoreslowlyasitgrowsolder.

Admixturesoradditivestoconcretearematerialsarematerialswhichareaddedtoitortothecementsoastoimproveoneormoreofthepropertiesoftheconcrete.Themaintypesare:

1.Acceleratorsofsetorhardening,

2.Retardersofsetorhardening,

3.Air-entrainingagents,includingfrothingorfoamingagents,

4.Gassingagents,

5.Pozzolanas,blast-furnaceslagcement,pulverizedcoalash,

6.Inhibitorsofthechemicalreactionbetweencementandaggregate,whichmightcausetheaggregatetoexpand

7.Agentsfordamp-proofingaconcreteorreducingitspermeabilitytowater,

8.Workabilityagents,oftencalledplasticizers,

9.Groutingagentsandexpandingcements.

Whereverpossible,admixturesshouldbeavouded,particularlythosethatareaddedonsite.Smallvariationsinthequantityaddedmaygreatlyaffecttheconcretepropertiesinanundesiraaleway.Anacceleratorcanoftenbeavoidedbyusingarapid-hardeningcementorarichermixwithordinarycement,orforveryrapidgainofstrength,high-aluminacement,thoughthisisverymuchmoreexpensive,inBritainaboutthreetimesascostlyasordinaryPortlandcement.Butintwenty-fourhoursitsstrengthisequaltothatreachedwithordinaryPortlandcementinthirtydays.

Aretardermayhavetobeusedinwarmweatherwhenalargequantityofconcretehastobecastinonepieceofformwork,anditisimportantthattheconcretecastearlyinthedaydoesnotsetbeforethelastconcrete.Thisoccurswithbridgeswhentheyarecastinplace,andtheformworknecessarilybendsundertheheavyloadofthewetconcrete.Someretarderspermanentlyweakentheconcreteandshouldnotbeusedwithoutgoodtechnicaladvice.

Asomewhatsimilareffect,milderthanthatofretarders,isobtainedwithlow-heatcement.Thesemaybesoldbythecementmakerormixedbythecivilengineeringcontractor.Theygiveoutlessheatonsettingandhardening,partlybecausetheyhardenmoreslowly,andtheyareusedinlargecastssuchasgravitydams,wheretheconcretemaytakeyearstocooldowntothetemperatureofthesurroundingair.IncountrieslikeBritainorFrance,wherepulverizedcoalisburntinthepowerstations,theash,whichisveryfine,hasbeenmixedwithcementtoreduceitsproductionofheatanditscostwithoutreducingitslong-termstrength.Uptoabout20percentashbyweightofthecementhasbeensuccessfullyused,withconsiderablesavingsincementcosts.

Incountrieswhereair-entrainingcementcementcanbeboughtfromthecementmaker,noair-entrainingagentneedstobemixedin.Whenair-entrainingagentsdrawintothewetcementandconcretesome3-8percentofairintheformofverysmallbubbles,theyplasticizetheconcrete,makingitmoreeasilyworkableandthereforeenablethewater|cementratiotobereduced.TheyreducethestrengthoftheconcreteslightlybutsolittlethatintheUnitedStatestheiruseisnowstandardpracticeinroad-buildingwhereheavyfrostoccur.Theygreatlyimprovethefrostresistanceoftheconcrete.

PozzolaneisavolcanicashfoundneartheItaliantownofPuzzuoli,whichisanaturalcement.Thenamehasbeengiventoallnaturalmineralcements,aswellastotheashfromcoalortheslagfromblastfurnaces,bothofwhichmaybecomecementswhengroundandmixedwithwater.PozzolanasofeithertheindustrialorthemineraltypeareimportanttocivilengineersbecausetheyhavebeenaddedtooridinaryPortlandcementinproportionsuptoabout20percentwithoutlossofstren