道路工程外文文献翻译沥青混合料的应用理论和原则Word文件下载.docx
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AsphaltMixtures-Applications,TheoryandPrinciples
1.Applications
Asphaltmaterialsfindwideusageintheconstructionindustry.Theuseofasphaltasacementingagentinpavementsisthemostcommonofitsapplications,however,andtheonethatwillbeconsideredhere.
Asphaltproductsareusedtoproduceflexiblepavementsforhighwaysandairports.Theterm“flexible”isusedtodistinguishthesepavementsfromthosemadewithPortlandcement,whichareclassifiedasrigidpavements,thatis,havingbeamstrength.Thisdistinctionisimportantbecauseitprovidestheykeytothedesignapproachwhichmustbeusedforsuccessfulflexiblepavementstructures.
Theflexiblepavementclassificationmaybefurtherbrokendownintohighandlowtypes,thetypeusuallydependingonwhetherasolidorliquidasphaltproductisused.Thelowtypesofpavementaremadewiththecutback,oremulsion,liquidproductsandareverywidelyusedthroughoutthiscountry.Descriptiveterminologyhasbeendevelopedinvarioussectionsofthecountrytotheextentthatonepavementtypemayhaveseveralnames.However,thegeneralprocessfollowedinconstructionissimilarformostlow-typepavementsandcanbedescribedasoneinwhichtheaggregateandtheasphaltproductareusuallyappliedtotheroadbedseparatelyandtheremixedorallowedtomix,formingthepavement.
Thehightypeofasphaltpavementsismadewithasphaltcementsofsomeselectedpenetrationgrade.
Fig.·1Amodernasphaltconcretehighway.Shoulderstripingisusedasasafelyfeature.
Fig.·2AsphaltconcreteattheSanFranciscoInternationalAirport.
Theyareusedwhenhighwheelloadsandhighvolumesoftrafficoccurandare,therefore,oftendesignedforaparticularinstallation.
2.Theoryofasphaltconcretemixdesign
Hightypesofflexiblepavementareconstructedbycombininganasphaltcement,ofteninthepenetrationgradeof85to100,withaggregatesthatareusuallydividedintothreegroups,basedonsize.Thethreegroupsarecoarseaggregates,fineaggregates,andmineralfiller.Thesewillbediscussedindetailinlaterchapter.
Eachoftheconstituentpartsmentionedhasaparticularfunctionintheasphaltmixture,andmixproportioningordesignistheprocessofensuringthatnofunctionisneglected.Beforetheseindividualfunctionsareexamined,however,thecriteriaforpavementsuccessandfailureshouldbeconsideredsothatdesignobjectivescanbeestablished.
Asuccessfulflexiblepavementmusthaveseveralparticularproperties.First,itmustbestable,thatistoresistanttopermanentdisplacementunderload.Deformationofanasphaltpavementcanoccurinthreeways,twounsatisfactoryandonedesirable.Plasticdeformationofapavementfailureandwhichistobeavoidedifpossible.Compressivedeformationofthepavementresultsinadimensionalchangeinthepavement,andwiththischangecomealossofresiliencyandusuallyadegreeofroughness.Thisdeformationislessseriousthantheonejustdescribed,butit,too,leadstopavementfailure.Thedesirabletypeofdeformationisanelasticone,whichactuallyisbeneficialtoflexiblepavementsandisnecessarytotheirlonglife.
Thepavementshouldbedurableandshouldofferprotectiontothesubgrade.Asphaltcementisnotimpervioustotheeffectsofweathering,andsothedesignmustminimizeweathersusceptibility.Adurablepavementthatdoesnotcrackorravelwillprobablyalsoprotecttheroadbed.Itmustberememberedthatflexiblepavementstransmitloadstothesubgradewithoutsignificantbridgingaction,andsoadryfirmbaseisabsolutelyessential.
Rapidlymovingvehiclesdependonthetire-pavementfrictionfactorforcontrolandsafety.Thetextureofthepavementsurfacesmustbesuchthatanadequateskidresistanceisdevelopedorunsafeconditionsresult.Thedesignprocedureshouldbeusedtoselecttheasphaltmaterialandaggregatescombinationwhichprovidesaskidresistantroadway.
Designprocedureswhichyieldpavingmixturesembodyingallthesepropertiesarenotavailable.Soundpavementsareconstructedwherematerialsandmethodsareselectedbyusingtime-testedtestsandspecificationsandengineeringjudgmentsalongwithaso-calleddesignmethod.
Thefinalrequirementforanypavementisoneofeconomy.Economy,again,cannotbemeasureddirectly,sincetrueeconomyonlybeginswithconstructioncostandisnotfullydeterminableuntilthefullusefullifeofthepavementhasbeenrecorded.If,however,therequirementsforastable,durable,andsafepavementaremetwithareasonablesafetyfactor,thenthebestinterestsofeconomyhaveprobablybeenservedaswell.
Withtheserequirementsinmind,thefunctionsoftheconstituentpartscanbeexaminedwithconsiderationgivetohoweachpartcontributestonow-establishedobjectivesorrequirements.Thefunctionsoftheaggregatesistocarrytheloadimposedonthepavement,andthisisaccomplishedbyfrictionalresistanceandinterlockingbetweentheindividualpiecesofaggregates.Thecarryingcapacityoftheasphaltpavementis,then,relatedtothesurfacetexture(particularlythatofthefineaggregate)andthedensity,or“compactness,”,oftheaggregates.Surfacetexturevarieswithdifferentaggregates,andwhilearoughsurfacetextureisdesired,thismaynotbeavailableinsomelocalities.Densemixturesareobtainedbyusingaggregatesthatareeithernaturallyorartificially“wellgraded”.Thismeansthatthefineaggregateservestofillthevoidsinthecoarseraggregates.Inadditiontoaffectingdensityandthereforestrengthcharacteristics,thegradingalsoinfluencesworkability.Whenanexcessofcoarseaggregateisused,themixbecomesharshandhardtowork.Whenanexcessofmineralfillerisused,themixesbecomegummyanddifficulttomanage.
Theasphaltcementintheflexiblepavementisusedtobindtheaggregateparticlestogetherandtowaterproofthepavements.Obtainingtheproperasphaltcontentisextremelyimportantandbearsasignificantinfluenceonalltheitemsmarkingasuccessfulpavement.Achiefobjectiveofallthedesignmethodswhichhavebeendevelopedistoarriveatthebestasphaltcontentforaparticularcombinationofaggregates.
3.Mixdesignprinciples
Certainfundamentalprinciplesunderliethedesignproceduresthathavebeendeveloped.Beforetheseprocedurescanbeproperlystudiedorapplied,someconsiderationoftheseprinciplesisnecessary.
Asphaltpavementsarecomposedofaggregates,asphaltcement,andvoids.Consideringtheaggregatealone,allthespacebetweenparticlesisvoidspace.Thevolumeofaggregatevoidsdependsongradingandcanvarywidely.Whentheasphaltcementisadded,aportionoftheseaggregatevoidsisfilledandafinalair-voidvolumeisretained.Theretentionofthisair-voidvolumeisveryimportanttothecharacteristicsofthemixture.Thetermair-voidvolumeisused,sincethesevoidsareweightlessandareusuallyexpressedasapercentageofthetotalvolumeofthecompactedmixture.
Anasphaltpavementcarriestheappliedloadbyparticlefrictionandinterlock.Iftheparticlesarepushedapartforanyreason,thenthepavementstabilityisdestroyed.Thisfactorindicatesthatcertainlynomoreasphaltshouldbeaddedthantheaggregatevoidscanreadilyhold.However,asphaltcementissusceptibletovolumechangeandthepavementissubjecttofurthercompactionunderuse.Ifthepavementhasnoairvoidswhenplaced,orifitlosesthemundertraffic,thentheexpandingasphaltwilloverflowinaconditionknownasbleeding.Thelossofasphaltcementthroughbleedingweakensthepavementandalsoreducessurfacefriction,makingtheroadwayhazardous.
Fig.·3Crosssectionofanasphaltconcretepavementshowingtheaggregateframeworkboundtogetherbyasphaltcement.
Theneedforaminimumair-voidvolume(usually2or3percent)hasbeenestablished.Inaddition,amaximumair-voidvolumeof5to7percentshouldnotbeexceed.Anexcessofairvoidspromotesravelingofthepavementandalsopermitswatertoenterandspeedupthedeterioratingprocesses.Also,inthepresenceofexcessairtheasphaltcementhardensandageswithanaccompanyinglossofdurabilityandresiliency.
Theair-voidvolumeofthemixisdeterminedbythedegreeofcompactionaswellasbytheasphaltcontent.Foragivenasphaltcontent,alightlycompactedmixwillhavealargevoidsvolumeandalowerdensityandagreaterstrengthwillresult.Inthelaboratory,thecompactioniscontrolledbyusingaspecifiedhammerandregulatingthenumberofblowsandtheenergyperblow.Inthefield,thecompactionandtheairvoidsaremoredifficulttocontrolandtestsmustbemadenospecimenstakenfromthecompactedpavementtocheekonthedegreeofcompactionbeingobtained.Trafficfurthercompactthepavement,andallowancemustbemadeforthisinthedesign.Asystematiccheckingofthepavementoveranextendedperiodisneededtogivenfactualinformationforaparticularmix.Achangeindensityofseveralpercentisnotunusual,however.
Asphaltcontenthasbeendiscussedinconnectionwithvariousfacetsoftheixdesignproblem.Itisaveryimportantfactorinthemixdesignandhasabearinganallthecharacteristicsldasuccessfulpavement:
stability,skidresistance,durability,andeconomy.Ashasbeenmentioned,thevariousdesignproceduresareintendedtoprovideameansforselectingtheasphaltcontent.Thesetestswillbeconsideredindetailinafuturechapter,buttherelationshipbetweenasphaltcontentandthemeasurablepropertiesofstability,unitweight,andairvoidswillbediscussedhere.
Fig.4Variationsinstability,unitwe