Mix Type Selection for Perpetual Pavements-Newcomb

MixTypeSelectionforPerpetualPavementsDavidE.NewcombKentR.HansenVice-President,Research&TechnologyDirectorofEngineeringNationalAsphaltPavementAssociationNationalAsphaltPavementAssociation5100ForbesBlvd.5100ForbesBlvd.Lanham,MD20706Lanham,MD20706Phone:[phone]Phone:[phone]email:[e-mail]:[e-mail]ng-lifeasphaltpavementsorPerpetualPavementscameaboutinthe1960s,althoughtheywerenotrecognizedassuchatthattime.UsingthicklayersofHMAwasseenasawayofreducingthetotalsectionthicknessofthepavement.Sincethen,ithasbecomeapparentthatthisapproachtodesigncanleadtoasphaltpavementsthatonlyneedperiodicresurfacing.Recently,strategieshavebeendevisedtoensuretheproperstructuraldesignofthesepavements.OneofthekeystosuccessinPerpetualPavementsistodesignthedifferentlayersaccordingtothefunctionstheyserve.Thispaperaddressestheneedtoselectthepropermixturesfordifferentapplicationswithinthepavementstructure.Densegraded,opengraded,andstonematrixasphalt(SMA)mixturesarediscussedandrecommendationsaremadefortheiruses.INTRODUCTIONBackgroundPerpetualpavements,alsocalledextendedlifeorlong-lastingasphaltpavements,arenotnewtoU.S.roads.Thedesignandconstructionoffull-depthanddeep-strengthasphaltpavementstructureshavebeencommonplacesincethe1960s,whentheanalysisoftheAASHOandotherroadtestsintheU.S.andBritainshowedthatlayersofasphaltmixturescouldbesuccessfullysubstitutedforthickerlayersofgranularmaterials.(1)Whiletheplanningandplacementofsuchasphaltpavementsdonotguaranteetheirultimateperformance,itiswellknownthatwhentheyareproperlyconstructedthesepavementscanlastfordecadeswithonlyminimaltrafficdisruptiontooccasionallyrenewthesurfacematerial.Oneofthechiefadvantagesofthesepavementsisthattheoverallsectionofthepavementisthinnerthanthoseemployingthickgranularbasecourses.Becausethetotalthicknessofasphaltboundlayersisgreater,thepotentialfortraditionalbottom-upfatiguecrackingandstructuralruttingarereduced,andpavementdistressmaybeconfinedtotheupperlayerofthestructure.Avoidingfatiguecrackingthatpropagatesfromthebottomofthepavementandavoidingstructuralruttingisimportantbecausethelong-termremedyforthesedistressestypicallyinvolvesreconstructionofthepavementsection.WhensurfacedistressreachesacriticallevelinaPerpetualPavement,aneconomicalsolutionistoremovetheverytoplayerandreplaceittothesamelevelwithahigh-qualityHMAsurface.Theobservationsconcerninglong-lifeasphaltpavementperformanceintheU.S.havebeenconfirmedbyEuropeanexperience(2,3).Infact,thecurrentEuropeaneffortsindevelopinglong-lifeasphaltpavementstandards(4,5)arebeingmirroredintheU.S.TheFederalHighwayAdministration(6)andstatessuchasIllinois(7)haveembracedtheideaoflong-lifeasphaltpavementshavebeguninitiativestomorethoroughlydefinethedesign,constructionandmaintenancestandards.AnumberofU.S.laboratoryandfieldstudiesonPerpetualPavementsarecontinuinginanefforttorefinethetechnology.Carpenteretal.(8)reportedonalaboratorystudytoidentifyafatigueendurancelimitforhotmixasphalt,aneffortthathasbeenexpandedbytheAsphaltInstitute(9)andthroughNationalCooperativeHighwayResearchProgram(NCHRP)Project1-38(10).ThePerpetualPavementconcepthasbeenusedintherehabilitationandreconstructionofaportionofInterstateHighway710insouthernCalifornia.Duringthedesignandconstruction,innovativemethodswereemployedtoensurelong-lifepavementandrapidplacementoftheroadway(11).Thisprojectincorporatedtheideaofusinga“rich”bottomlayerinthehopesofimprovingdurabilityandpossiblyfatiguebehavior.Arutresistantsurfacelayerusingahighlypolymerizedbinderwasalsoemployed.HMAmixturesweresubjecttoextensivemechanistictestinginthehopesofproducingalong-lastingstructure.Itisnowundertrafficandperformancemonitoringisbeingdone.Beyondtherealmofnewconstruction,othereffortssuchasthosebyRoweetal.(12)haveexaminedmeansforevaluatingandmaintainingPerpetualPavementsonaportionofInterstateHighway287inNewJersey.Theyfoundthatwhenthepavementisstructurallysound,theplacementofathinoverlayusingapolymermodifiedbinderintheHMAcouldkeepthepavementinanexcellentservicecondition.Recenteffortsinmaterialsselection,mixturedesign,performancetesting,andstructuraldesignofferthemeanstoobtainlong-lastingperformancefromasphaltpavementstructures(greaterthan50years)whileperiodicallyreplacingthepavementsurface.Thethrustistocombinearutresistant,impermeable,andwearresistanttopstructurallayerwitharutresistantanddurableintermediatelayerandafatigueresistantanddurablebaselayerasshowninFigure1.MaxTensileStrainPavementFoundationRutResistantIntermediateCourse75–100mm(3–4”)DurableBase100–250mm(4–10”)38–75mm(1.5-3”)SMA,OGFCorSuperpave}ZoneofHighCompressionFigure1.PerpetualPavementDesignConcept.ScopeThispaperwillexaminetheroleofhotmixasphalt(HMA)typeselectionrelativetothedesiredperformanceofPerpetualPavements.Thecharacteristicsofthebasecourse,intermediatecourse,andwearingsurfacewillbediscussedintermsofthetypesofmixessatisfyingtheserequirements.PERPETUALPAVEMENTDESIGNCurrently,thepracticeofpavementdesignisundergoingatransitionfrompurelyempiricalmethodstomechanistic-empiricaldesign.Empir