路桥工程专业英语第二课

第二课沥青及沥青混合料Asphalt$MixAsphaltText:Asphalt&AsphaltPavingMaterials1AsphaltDefinedTheblackcementingagentknownasasphalthasbeenusedforroadconstructionforcenturies.Althoughtherearenaturaldepositsofasphalt,orrockasphalt,mostusedtodayisproducedduringtherefiningofcrudeoil.Asphaltisaconstituentofmostpetroleumsandisisolatedthroughtherefiningprocess.Asphaltiscalledabituminousmaterialbecauseitcontainsbitumen,ahydrocarbonmaterialsolubleincarbondisulfate.Thetarobtainedfromthedestructivedistillationofsoftcoalalsocontainsbitumen.Bothpetroleumasphaltandcoaltararereferredtoasbituminousmaterials.Becausetheirpropertiesdiffergreatly,petroleumasphaltshouldnotbeconfusedwithcoaltar.Petroleumasphaltiscomposedalmostentirelyofbitumenwhilethebitumencontentincoaltarisrelativelylow.Thetwomaterialsshouldbetreatedasseparateentities.Oneofthecharacteristicsandadvantagesofasphaltasanengineeringconstructionandmaintenancematerialsisitsversatility.Althoughasemi-solidatordinarytemperratures,asphaltmaybeliquefiedbyapplyingheat,dissolvingitinsolvents,oremulsifyingit.Asphaltisastrongcementthatisreadilyadhesiveandhighlywaterproofanddurable,makingitparticularlyusefulinroadbuilding.Itisalsohighlyresistanttotheactionsofmostacids,alkalis,andsalts.1.1AsphaltBinderAsphaltisproducedinavarietyoftypesandgradesrangingfromhard-brittlesolidstonearwater-thinliquids.Thesemi-solidformknownasasphaltbinderisthebasicmaterialusedinHot-MixAsphalt(HMA)pavements.Liquidasphaltisproducedwhenasphaltbinderisblendedor“cutback”withpetroleumdistillatesoremulsifiedwithwaterandanemulsifyingagent.Atambientairtemperatures,asphalitbinderisablack,sticky,highlyviscousmaterial.Itisastronganddurablebinderwithexcellentadhesiveandwaterproofingcharacteristics.Applyingheat,whichfacilitatesmixingwithmineralaggregatestoproduceHMA,canreadilyliquefyasphaltbinders.ThelargestuseofasphaltbinderisforHMA.Aftercompactingandcoolingtoairtemperature,HMAisaverystrongmaterialwiththeabilitytosustainheavytrafficloadswhileremainingflexibleenoughtowithstandambientenvironmentalconditionsandstress.Over96percentofthehard-surfacedroadsintheUnitedStatesarepavedusingHMA.1.2EmulsifiedAsphaltsEmulsifiedasphalts(alsoknownasemulsions)arelow-viscositymixturesoftinyasphaltbinderdroplets,waterandemulsifyingagents.Theemulsifyingagentcoatsthesurfacesoftheasphaltdropletsandkeepsthemsuspendedinthewaterpriortoapplication.Afterapplication,theasphaltemulsionbreaksandthewaterseparatesandevaporates.Emulsionsarebrownishincolorduringapplication,butafterbreaking,theasphaltbinderreturnstoitsoriginalblackcolor.EmulsionsareusedforaTackCoatbetweensubsequentlayersofHMAtoaidinbindingthelayerstogether.Cut-backasphaltsarelow-viscosityliquidasphaltmixturesmanufacturedbydiluting(cuttingback)AsphaltBinderswithpetroleumsolvents(cutterstockordiluent).Afterapplication,thepetroleumsolventevaporates,leavingtheasphaltbinderresidue.Cut-backasphaltsmaybeusedasatackcoatbetweensubsequentlayersofHMA,particularlywhenambientairtemperaturesarecool.2.AsphaltBinderGradingAsphaltbindersappropriateforpavementconstructionwerepreviouslygradedbasedonresistancetopenetrationand/orviscositymeasures.Currently,asphaltbindersaregradedbasedonthetemperaturerangeoverwhichthebinderretainscertaindesirablecharacteristics.Thesedesirablecharacteristicsincludeadequateflexibilitytoresistcoldtemperaturecrackingandsufficientrigiditytoresistwarmtemperaturerutting.ThecurrentgradingsystemisknownasthePerformanceGrading(PG)system.PerformancegradingspecificationsweredevelopedaspartoftheStrategicHighwayResearchProgram(SHRP)andareamajorcomponentofSUPERPAVE.Bindersarespecifiedonthebasisoftheclimateandpavementtemperaturesinwhichthebinderisexpectedtoserve,pavementAgePerformancegraded(PG)bindersusedinMinnesotavaryfromnorthtosouthandwithintendeduse;however,PG58-28isthemostcommomlyusedgrade.Thefirstnumber(58)representstheaverage7-daymaximumpavementdesigntemperatureindegreesCelsius.Thismaximumtemperatureestablishestheuppertemperaturelimitforthebindertiretainadequaterigiditytoresistrutting.Thesecondnumber(-28)representstheminimumpavementdesigntemperatureindegreesCelsius.Theminimumtemperatureestablishedthelowerlimitforthebindertoretainsufficientflexibilitytoresistthermalcracking.Physicalpropertiesofthebindersaremeasuredatvarioustemperaturesbothbeforeandafterlaboratoryaging.ThelaboratoryagingisconductedtosimulatefieldcinditionsimposedduringtheHMAproductionprosessaswellasfromlong-termenvironmentalexposure.3.Hot-MixAsphalt(HMA)Hot-MixAsphaltisknownbymanydifferentnames:HMA,asphaltconcrete,plantmix,bituminousconcrete,andmanyothers.Itisacombinationoftwoprimaryingredients—aggregatesandasphaltbinder.Theaggregatestotalapproximately95percentofthetotalmixturebyweight.Theyaremixedwithapproximately5percentasphaltbindertoproduceHMA.Theaggregatesandasphaltarecombinedinamanufacturingplantcapableofproducingspecifiedmaterials.Plantequipmentincludes:coldbinsforstorageandcontrolledproportioningofgradedaggregate;adryerfordryingandheatingaggregatestotherequiredmixingtemperature;apugmillordrumforcombiningthegraded,heatedag