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—1—PavementHighwaypavementsaredividedintotwomaincategories:rigitandflexible.ThewearingsurfaceofarigidpavementisusuallyconstructedofPortlandcementconcretesuchthatitactslikeabeamoveranyirregularitiesintheunderlyingsupportingmaterial.Thewearingsurfaceofflexiblepavements,ontheotherhand,isusuallyconstructedofbituminousmaterialsuchthattheyremainincontactwiththeunderlyingmaterialevenwhenminorirregularitiesoccur.Flexiblepavementsusuallyconsistofabituminoussurfaceunderlaidwithalayerofgranularmaterialandalayerofasuitablemixtureofcoarseandfinematerials.CoarseaggregatesFineaggregatesTrafficloadsaretransferredbythewearingsurfacetotheunderlyingsupportingmaterialsthroughtheinterlockingofaggregates,thefrictionaleffectofthegranularmaterials,andthecohesionofthefinematerials.Flexiblepavementsarefurtherdividedintothreesubgroups:hightype,intermediatetype,andlowtype.High-typepavementshavewearingsurfacesthatadequatelysupporttheexpectedtrafficloadwithoutvisibledistressduetofatigueandarenotsusceptibletoweatherconditions.Intermediate-typepavementshavewearingsurfacesthatrangefromsurfacetreatedtothosewithqualitiesjustbelowthatofhigh-typepavements.Low-typepavementsareusedmainlyforlow-costroadsandhavewearingsurfacesthatrangefromuntreatedtoloosenaturalmaterialstosurface-treatedearth.Thecomponentsofaflexiblepavementincludethesubgradeorpreparedroadbed,thesubbase,basecourse,andthesurfacecourse(Fig.11.1).UppersurfacecourseMiddlesurfacecourseLowersurfacecourseTheperformanceofthepavementdependsonthesatisfactoryperformanceofeachcomponent,whichrequiresproperevaluationofthepropertiesofeachcomponentseparately.Thesubgradeisusuallythenaturalmateriallocatedalongthehorizontalalignmentofthepavementandservesasthefoundationofthepavementstructure.Thesubgrademayalsoconsistofalayerofselectedborrowmaterials,wellcompactedtoprescribedspecifications.CompactingplantCompactiondeviceCompactnessItmaybenecessarytotreatthesubgradematerialtoachievecertainstrengthpropertiesrequiredforthetypeofpavementbeingconstructed.—2—Locatedimmediatelyabovethesubgrade,thesubbasecomponentconsistsofasuperiorqualitytothatwhichgenerallyisusedforsubgradeconstruction.Therequirementsforsubbasematerialsareusuallygivenintermsofthegradation,plasticcharacteristics,andstrength.Whenthequalityofthesubgradematerialmeetstherequirementsofthesubbasematerial,thesubbasecomponentmaybeomitted.Incaseswheresuitablesubbasematerialisnotreadilyavailable,theavailablematerialcanbetreatedwithothermaterialstoachievethenecessaryproperties.Thisprocessoftreatingsoilstoimprovetheirengineeringpropertiesisknowasstabilization.Thebasecourseliesimmediatelyabovethesubbase.Itisplacedimmediatelyabovethesubgradeifasubbasecourseisnotused.Thiscourseusuallyconsistsofgranularmaterialssuchascrushedstone,crushedoruncrushed.Thespecificationsforbasecoursematerialsusuallyincludestricterrequirementsthanthoseforsubbasematerials,particularlywithrespecttotheirplasticity,gradation,andstrength.MaterialsthatdonothavetherequiredpropertiescanbeusedasbasematerialsiftheyareproperlystabilizedwithPortlandcement,asphalt,orlime.Insomecases,high-qualitybasecoursematerialsmayalsobetreatedwithasphaltorPortlandcementtoimprovethestiffnesscharacteristicsofheavy-dutypavementsThesurfacecourseistheuppercourseoftheroadpavementandisconstructedimmediatelyabovethebasecourse.Thesurfacecourseinflexiblepavementusuallyconsistsofamixtureofmineralaggregatesandasphalticmaterials.Itshouldbecapableofwithstandinghightirepressures,resistingtheabrasiveforcesduetotraffic,providingaskid-resistantdrivingsurface,andpreventingthepenetrationofsurfacewaterintotheunderlyinglayers.Thethicknessofthewearingsurfacecanvaryfrom3in.tomorethan6in.（inch，英寸，2.54cm）,dependingontheexpectedtrafficonthepavement.Itwasshownthatthequalityofthesurfacecourseofaflexiblepavementdependsonthemixdesignoftheasphaltconcreteused.Rigidhighwaypavementsusuallyareconstructedtocarryheavytrafficloads,althoughtheyhavebeenusedforresidentialandlocalroads.Properlydesignedandconstructedrigidpavementshavelongservicelivesandusuallyarelessexpensivetomaintainthantheflexiblepavements.ThePortlandcementconcretecommonlyusedforrigidpavementsconsistsofPortlandcement,coarseaggregate,fineaggregate,andwater.Steelreinforcingrodsmayormaynotbeused,dependingonthetypeofpavementbeingconstructed.Rigidhighwaypavementsbedividedintothreegeneraltype:plainconcretepavements,simplyreinforcedconcretepavements,andcontinuouslyreinforcedconcretepavement.Thedefinitionofeachpavementtypeisrelatedtotheamountofreinforcementused.Plainconcretepavementhasnotemperaturesteelordowelsforloadtransfer.—3—However,steeltiebarsareoftenusedtoprovideahingeeffectatlongitudinaljointsandtopreventtheopeningofthesejoints.Plainconcretepavementsareusedmainlyonlow-volumehighwaysorwhencement-stabilizedsoilsareusedassubbase.Jointsareplacedatrelativelyshorterdistances(10to20ft)thanwiththeothertypesofconcretepavementstoreducetheamountofcracking.Insomecase,thetransversejointsofplainconcretepavementsareskewedabout4to5ftinplan,suchthatonlyonewheelofavehiclepassesthroughth