广西左江崇左港多用途码头工程桥吊码头结构设计译文Word文档下载推荐.docx

广西左江崇左港多用途码头工程桥吊码头结构设计译文Word文档下载推荐.docx

《广西左江崇左港多用途码头工程桥吊码头结构设计译文Word文档下载推荐.docx》由会员分享，可在线阅读，更多相关《广西左江崇左港多用途码头工程桥吊码头结构设计译文Word文档下载推荐.docx（11页珍藏版）》请在冰豆网上搜索。

a、PlainConcrete

Concreteisastone-likematerialobtainedbypermittingacarefullyproportionedmixtureofcement,sandandgravelorotheraggregate,andwatertohardeninformsoftheshapeanddimensionsofthedesiredstructure.Thebulkofthematerialconsistsoffineandcoarseaggregate.Cementandwaterinteractchemicallytobindtheaggregateparticlesintoasolidmass.Additionalwater,overandabovethatneededforthischemicalreaction,isnecessarytogivethemixturetheworkabilitythatenablesittofilltheformspriortohardening.Concretesinawiderangeofstrengthpropertiescanbeobtainedbyappropriateadjustmentoftheproportionsoftheconstituentmaterials.Specialcements（suchashigh-early-strengthcements）,specialaggregates（suchasvariouslightweightorheavyweightaggregates）,andspecialcuringmethods（suchascream-curing）permitanevenwidervarietyofpropertiestobeobtained.

Thesepropertiesdependtoaverysubstantialdegreeontheproportionsofthemix,onthethoroughnesswithwhichthevariousconstituentsareintermixed,andontheconditionsofhumidityandtemperatureinwhichthemixismaintainedfromthemomentitisplacedintheformsuntilitisfullyhardened.Theprocessofcontrollingtheseconditionsisknownascuring.Toprotectagainsttheunintentionalproductionofsubstandardconcrete,ahighdegreeofskillfulcontrolandsupervisionisnecessarythroughouttheprocess,fromtheproportioningbyweightoftheindividualcomponents,throughmixingandplacing,untilthecompletionofcuring.

Thefactorswhichmakeconcreteauniversalbuildingmaterialaresopronouncedthatithasbeenused,inmoreprimitivekindsandwaysthanatpresent,forthousandsofyears,probablybeginninginEgyptianantiquity.Thefacilitywithwhich,whileplastic,itcanbedepositedandmadetofillformsormoldsofalmostanypracticalshapeisoneofthesefactors.Itshighfireandweatherresistanceareevidentadvantages.Mostoftheconstituentmaterials,withthepossibleexceptionofcement,areusuallyavailableatlowcostlocallyoratsmalldistancesfromtheconstructionsite.Itscompressivestrength,likethatofnaturalstones,ishigh,whichmakesitsuitableformembersprimarilysubjecttocompression,suchascolumnsandarches.Ontheotherhand,againasinnaturalstones,itisarelativelybrittlematerialwhosetensilestrengthissmallcomparedwithitscompressivestrength.Henceplainconcreteisusedonlyforfootingsandconcreteslabslaidontheground,andformassivestructuressuchasretainingwalls;

eventhenreinforcementisfrequentlyemployed.

b、PropertiesofConcreteandreinforcedConcrete

Concreteisaman-madeconglomeratestonecomposedofessentiallyfouringredients:

Portlandcement,water,sand,andcoarseaggregate.Thecementandwatercombinetomakeapastethatbindsthesandandstonestogether.Ideally,theaggregatesaregradedsothatthevolumeofpasteisataminimum,merelysurroundingeverypiecewithathinlayer.MoststructuralconcreteIsstoneconcrete,butstructurallightweightconcrete（roughlytwo-thirdsthedensityofstoneconcrete）isbecomingincreasinglypopular.

Concreteisessentiallyacompressivematerialhavingalmostnotensilestrength,soconcretesweaknessintensionalsocausesittobeweakinshear.Thesedeficienciesareovercomebyusingsteelbarsforreinforcementattheplaceswheretensileandshearingstressesaregenerated.Underload,reinforcedconcretebeamsactuallyhavenumerousminutecrackswhichrunatrightanglestothedirectionofmajortensilestresses.Thetensileforcesatsuchlocationsarebeingtakencompletelybythesteel“re-bars.”

Thecompressivestrengthofagivenconcreteisafunctionofthequalityandproportionsofitsconstituentsandthemannerinwhichthefreshconcreteiscured.（Curingistheprocessofhardeningduringwhichtimetheconcretemustbepreventedfrom“dryingout”,asthepresenceofwaterisnecessaryforthechemicalactiontoprogress.）Coarseaggregatethatishardandwellgradedisparticularlyessentialforqualityconcrete.Themostimportantfactorgoverningthestrength,however,isthepercentageofwaterusedinthemix.Sminimumamountofwaterisneededforproperhydrationofthecement.Additionalwaterisneededforhandlingandplacingtheconcrete,butexcessamountscausethestrengthtodropmarkedly.

Theseandothertopicsarefullycoveredinthebooklet,“DesignandControlofConcreteMixtures,”publishedbythePortlandCementAssociation.Thisisanexcellentreference,treatingbothconcretemixdesignandproperconstructionpractices.TheAmericanConcreteInstitutepublishesawidelyadoptedcodespecifyingthestructuralrequirementsforreinforcedconcrete.

Concreteisknownasthe“formable”or“moldable”structuralmaterial.Comparedtoothermaterials,itiseasytomakecurvilinearmembersandsurfaceswithconcrete.Ithasnoinherenttexturebutadoptsthetextureoftheformingmaterial,soitcanrangewidelyinsurfaceappearance.Itisrelativelyinexpensivetomake,bothintermsofrawmaterialsandlabor,andthebasicingredientsofPortlandcementareavailabletheworldover.（Itshouldbenoted,however,thatthenecessaryreinforcingbarsforconcretemaynotbereadilyavailableinlessdevelopedcountries.）

Concreteisnaturallyfireproofandneedsnoseparateprotectionsystem.Becauseofitsmass,itcanalsoserveasaneffectivebarriertosoundtransmission.

Inviewingthenegativeaspects,concreteisunfortunatelyquiteheavyanditisoftennotedthataconcretestructureexpendsalargeportionofitscapacitymerelycarryingitself.Attemptstomakeconcretelessdense,whilemaintaininghighqualitylevels,havegenerallyresultedinincreasedcosts.Nevertheless,useoflightweightconcretecansometimesresultinoveralleconomies.

Concreterequiresmorequalitycontrolthanmostotherbuildingmaterials.Moderntransit-mixedconcretesuppliersateavailabletoallU.S.urbanareasandthemixisusuallyofauniformlyhighquality.Field-or-job-mixedconcreterequiresknowledgeablesupervision,however.Inanytypeofconcretework,missingormislocatedreinforcingbarscanresultinelementswithreducedloadcapacities.Poorhandlingand/orcuringconditionscanseriouslyweakenanyconcrete.Fortheseandotherreasons,mostbuildingcodesrequireindependentfieldinspectionsatvariousstagesofconstruction.

Properconcreteplacementisalsosomewhatdependentupontheambientweatherconditions.Extremelyhightemperaturesand,moreimportant,thosebelow（ornear）freezingcanmakeconcreteworkverydifficult.

Concretederivesitsstrengthfromthefactthatpulverizedportlandcement，whenmixedwithwater，hardensbyaprocesscalledhydration．Inanidealmixture，concreteconsistsofaboutthreefourthssandandgravel（aggregate）byvolumeandonefourthcementpaste．Thephysicalpropertiesofconcretearehighlysensitivetovariationsinamixtureofthecomponents，soaparticularcombinationoftheseingredientsmustbecustom-designedtoachievespecifiedresultsintermsofstrengthorshrinkage．Whenconcreteispouredintoamoldorform，itcontainsfreewater，notrequiredforhydration，whichevaporates．Astheconcretehardens，itreleasesthisexcesswateroveraperiodoftimeandshrinks．Asaresultofthisshrinkage，finecracksoftendevelop．Inordertominimizetheseshrinkagecracks，concretemustbehardenedbykeepingitmoistforatleast5days．Thestrengthofconcreteincreasesintimebecausethehydrationprocesscontinuesforyears；

asapracticalmatter，thestrengthat28daysisconsideredstandard．

Concretedeformsunderloadinanelasticmanner．Althoughitselasticmodulusisonetenththatofsteel，similardeformationswillresultsinceitsstrengthisalsoisonetenththatofsteel．Concreteisbasicallyacompressivematerialandhasnegligibletensilestrength．

c、AdvantagesandDisadvantagesofConcreteandItsWater-CementRatio

ConcreteisamixtureofPortlandcement,water,sand,andcrushedgravelorstone.Thewaterandcementformacementpasteinwhichthesandandstoneorgravelaremixed.Thesandandstoneorgraveltogethermakeuptheaggregateofaconcretemixture.Theaggregateservesnostructuralfunction.Itismerelyafillerthataddslow-costbulktothecementpaste;

itusuallymakesupabout75percentofagivenmassofconcrete,byvolume,althoughapooraggregatecanreducethe4strengthofabatchofconcreteconsiderably,goodaggregateaddsonlyslightlytothestrengthofthecement.

Thetwoprincipaladvantagesofconcreteasaconstructionmaterialareitsrelativecheapnessandtheeasewithwhichitcanbehandledandplacedwhileitisintheplasticstate.Theprincipalstructuraladvantagesofconcreteareitsgreatcompressivestrengthanditsdurability.Concretecanwithstandveryhighcompressiveloads.Thisiswhatmakesconcretesosuitableforthefoundations,walls,andcolumnsofbuildings,andfordrivewaysandwalksaswell.

Theprincipalstructuraldisadvantageofconcreteisitspoortensilestrength.Thatis,itcannotwithstandpullingorbendingloadswithoutcrackingorbreaking.Forthisreason,steelrods,orreinforcementsteel,areoftenembeddedinconcrete,thereinforcementsteelprovidingthetensilestrengththeconcretelacks.Concretewithreinforcementsteelembeddedinitisreinforcedconcrete.

Inadditiontoitspoortensilestrength,concrete,likemostconstructionmaterials,expandsinhotweatherandwhenwetandcontractsincoldweatherandasitdriesout.Unlessthesemovementsareallowedforduringconstruction,theconcretewillcrack.

And,contrarytocommon