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8/9/2019 Performance of Recycled Hot Mix Asphalt Mixtures

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PERFORMANCEOFRECYCLEDHOTMIXASPHALTMIXTURES

By

PrithviS.Kandhal

ShridharS.RaoDonaldE.WatsonBradYoung

PaperpublishedinTransportationResearchBoard,TransportationResearchRecord1507,1995

277TechnologyParkway Auburn,AL36830


2/19


By

PrithviS.Kandhal

AssistantDirectorNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

ShridharS.Rao

GraduateStudentAuburnUniversity,Alabama

DonaldE.Watson

AssistantStateMaterialsandResearchEngineerGeorgiaDepartmentofTransportation

BradYoung

AssociateResearchEngineerGeorgiaDepartmentofTransportation

PaperpublishedinTransportationResearchBoard,TransportationResearchRecord1507,1995


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DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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ABSTRACT

TheGeorgiaDepartmentofTransportation(GDOT)hasbeenconstructingrecycledasphalt

pavementsroutinelyforaboutfouryears.Thisresearchprojectwasundertakentoevaluatetheperformanceofrecycledpavementsincomparisontovirgin(control)asphaltpavements.

Fiveprojects,eachconsistingofarecycledsectionandacontrolsection,weresubjectedto

detailedevaluation.In-situmixproperties(suchaspercentairvoids,resilientmodulusandindirecttensilestrength),recoveredasphaltbinderproperties(suchaspenetration,viscosity,G*/sin*andG*sin*),andlaboratoryrecompactedmixproperties(suchasGyratoryStabilityIndexandconfined,dynamiccreepmodulus)weremeasured.Apairedt-teststatisticalanalysisindicatednosignificantdifferencesbetweenthesepropertiesofvirginandrecycledmixpavementswhichhavebeeninservicefrom1to2years.

Tenadditionalvirginmixpavementsand13additionalrecycledpavementswerealsoevaluated

astwoindependentgroups.Nostatisticallysignificantdifferenceswerefoundbetweenthe

recoveredasphaltproperties(penetrationandviscosity)ofthesevirginandrecycledpavementsinservice.

ThecurrentGDOTrecyclingspecificationsandmixdesignproceduresappeartobesatisfactory

basedontheresultsofthisstudy.

KeyWords:HotMixAsphalt,AsphaltConcrete,HotRecycling,Performance,Aging

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Kandhal,Rao,Watson,&Young


PrithviS.Kandhal,ShridharS.Rao,DonaldE.Watson,andBradYoung

INTRODUCTIONHotmixrecyclingofasphaltpavementsisincreasinglybeingusedasoneofthemajorrehabilitationmethodsbyvarioushighwayagenciesallovertheUnitedStates.Besidesgeneralsavingsincostsandenergyexpended,italsosavesournaturalresourcesandenvironment.TheGeorgiaDepartmentofTransportation(GDOT)hasbeenconstructingrecycledhotmixasphalt(HMA)pavementsroutinelyforaboutfouryears.MostoftherecycledpavementsinGeorgiahavebeenconstructedusingAC-20asphaltcementtypicallywith10to25%reclaimedasphaltpavementmaterial,whereasvirginHMApavementsaregenerallyconstructedusingAC-30asphaltcement.ItwasfeltnecessarytoevaluatetheperformanceoftheserecycledpavementsincomparisontovirginHMApavementsconstructedduringthesameperiod.ThiswoulddeterminewhethertherecycledHMApavementshaveperformedsimilartothevirginHMApavements.Itwouldalsoprovideinformationforadjustingthespecificationandmixdesignmethodforrecycledmixtures,ifneeded.

OBJECTIVES

Theprimaryobjectivesofthisprojectareasfollows:

1.Evaluatetheperformanceofin-servicerecycledandvirgin(control)HMApavementsfromthesameprojectbothvisuallyaswellasinthelaboratory,

2.ComparetheperformanceofrecycledHMAprojectswiththatofvirgin(control)HMAprojects,and

3.ReviewGDOT'spresentrecyclingspecificationsandrecommendchangeswherenecessary.

Thisstudywasdividedintotwotasks.Task1involvedidentifyingexistingfieldprojectswhichhaveusedbothrecycledandvirgin(control)mixesonthesameprojectandconductingadetailedcomparativeevaluation.Task2consistedofevaluatingatleast10recycledHMApavementsandatleast10virginmixpavementsconstructedindependentlythroughoutthestateduringthelast2-3years.Thepropertiesofthebindersrecoveredfromthemixturesoftheaboveprojectsformedadatabaseforcomparativepurposes.

Theworkforeachtaskinvolvedcollectingconstructiondataofalltheprojects,visualevaluation

ofthein-servicepavements,sampling,andextensivelaboratorytestingofthefieldcorestakenfromeachproject.

REVIEWOFLITERATURE

ResearchcarriedoutbyLittleandEpps(1),Littleetal.(2),Brown(3),Meyersetal.(4),andKandhaletal.(5)hasindicatedthatthestructuralperformanceofrecycledmixesisequalandinsomeinstancesbetterthanthatoftheconventionalmixes.

Thepropertiesoftherecycledmixturearebelievedtobemainlyinfluencedbytheaged

reclaimedasphaltpavement(RAP)binderpropertiesandtheamountofRAPinthemixture.Kiggunduetalwshowedthatmixturespreparedfromtherecycledbinderblendsgenerallyageataslowerratethanvirginmixtures.ThismaybeduetothefactthattheRAPbinderhasalreadyundergoneoxidationwhichtendstoretardtherateofhardening(4,6)KiggunduandNewman(7)haveindicatedthattherecycledmixtureswithstoodtheactionofwaterbetterthanthevirginmixtures.DunningandMendenhall(8)havealsoshownthatthedurabilityofrecycledasphaltconcretemixturesisgreaterthanthatoftheconventionalmixtures.

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TheamountoftheRAPusedinarecycledmixturedependsonthetypeofhotmixplantbeing

usedforpreparingthemixandalsoonenvironmentalconsiderations.However,thegradationofaggregateinRAP(especiallythematerialpassingNo.200sieve)canalsobeamajorlimitingfactor.ThespecifiedmaximumpermissibleamountsofRAPvaryfromstatetostate.The

GeorgiaDepartmentofTransportationlimitstheamountofRAPto40percentofthetotalrecycledmixtureforcontinuoustypeplantsandto25percentforbatchtypeplants(9).SomestatesallowhigherpercentagesofRAPforallplants.AccordingtotheGDOTspecificationtheRAPbinderwhenblendedwithvirginasphaltcementshouldgiveaviscositybetween6,000poisesto16,000poisesafterthethinfilmoventest.

SAMPLINGANDTESTINGPLAN(TASK1)

Onlyfiveexistingfieldprojectscouldbeidentifiedforthistask,whichhadbothrecycledandvirginHMAmixturesonthesameprojectinthewearingcourse.Theselectionoftheseprojectsassuredthattherecycledandthevirginsectionsusedthesamevirginaggregatesinthemixtures,wereproducedbythesameHMAplant,wereplacedandcompactedbythesameequipmentand

crew,andweresubjectedtothesametrafficandenvironmentduringservice.Theprojectdetailsofboththerecycledandthecontrol(virgin)wearingcoursemixturesforthe

fiveprojectsaregiveninTable1.Asshowninthetable,GeorgiaDOTalsousesAC-20Special(designatedasAC-20S)whichisrequiredtohaveapenetrationrangeof60to80.

Table1.ProjectConstructionDetails(Task1)

VirginAsphaltCement MixPropertiesProperties

Site County Section Ageyrs RAP Grade Viscosity Pen. Asphalt %AirNo. % (60C)Pa-s 25C Content Voids

(%) (mat)

18C Coffee Virgin 1.50 0 AC-30 299 *** 6.0 9.018R Coffee Recycled 1.50 15 AC-30 299 *** 5.7 9.3

22C Ware Virgin 1.75 0 AC-30 270 *** 6.0 6.6

22R Ware Recycled 1.75 10 AC-20S 191 *** 5.7 6.9

23C Chatham Virgin 1.50 0 AC-30 281 *** *** ***

23R Chatham Recycled 1.50 25 AC-20 199 *** 5.4 6.5

25C Emmanuel Virgin 2.25 0 AC-30 297 *** 5.8 7.9

25R Emmanuel Recycled 2.25 20 AC-20 206 *** 57 7.4

28C Columbia/ Virgin 1.50 0 AC-30 305 *** 6.0 8.3

Richmond

28R Columbia/ Recycled 1.50 20 AC-30 305 *** 5.8 7.8Richmond

***Datanotavailable

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Therecycledandthecontrolsectionsforallfiveprojectswerevisited.Arepresentative150m

(500ft)longtestareawasselectedineachsectionfordetailedevaluation.Thepavementwasvisuallyevaluatedforsurfacedistresssuchasrutting,raveningandweathering,alligator(fatigue)cracking,andtransversecracking.

Atotalofeight101-mm(4-inch)diametercoresandfour152-mm(6-inch)diametercoreswereobtainedfromeach150-m(500-ft)representativesectionfromtheoutsidewheeltrack.Coreswereobtainedatanintervalof30-m(100-ft).

LaboratorytestswereconductedonthefieldcoresaccordingtotheflowchartshowninFigure

1.Allthefieldcoresweresawedtorecoveronlytherecycledorthecontrol(virgin)wearingcourseportionofthepavement.

Figure1.CoreTestingPlan(Task1)

Themixfromthe152-mm(6-inch)diametercoreswasreheatedto133C(300F)andrecompactedinthelaboratoryusingtheU.S.CorpsofEngineersGyratoryTestMachine(GTM).Thiswasdonetoevaluatetheruttingpotentialandshearpropertiesoftherecycledandthecontrolmixes.Propertiessuchasgyratorystabilityindex(GSI),gyratoryelasto-plasticindex(GEPI),androllerpressure,whicharediscussedlater,weredeterminedasthemixwas

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compacted.Airvoidcontentsofthecompactedspecimenswerealsoobtained.Confined,

dynamiccreeptestswereperformedontherecompactedspecimensat60C(140F),138kPa(20psi)confiningpressureand828kPa(120psi)verticalpressure.Dynamicloadingwasappliedforadurationofonehourandthenthespecimenwasallowedtorecoverfor30minutes.

TheextractionoftheagedasphaltcementfromtheHMAmixtureswasaccomplishedbytheASTMD2172(MethodA)procedureusingtrichloroethylene.TheasphaltcementfromthesolutionwasrecoveredusingtheRotovaporapparatusasrecommendedbytheSHRPprogram.Therecoveredasphaltcementwastestedforviscosityat60C(140F)andpenetrationat25C(77F).Also,thecomplexshearmodulus(G*)andthephaseangle(*)oftherecoveredasphaltbinderweredeterminedusingadynamicshearrheometer(DSR)accordingtotheAASHTOPerformanceGradedBinderSpecification(MP1)fortheStateofGeorgia.Testtemperaturesof64C(148F)and22C(72F)wereusedtodeterminethepotentialforruttingandfatiguecracking,respectively,ascontributedbythebinder.

Thetestsconductedonthe101-mm(4-inch)diameterfieldcoreswere(a)airvoidcontent,(b)

resilientmodulusat25C(77F),andGindirecttensiletestat25C(77F).TESTDATA(TASK1)

Asmentionedearlier,visualevaluationoftherecycledandthecontrolsectionswascarriedout.AsummaryoftheobservationsmadeduringthepavementevaluationispresentedinTable2.Theseresultswereanalyzedandquantifiedtocomparetherelativeperformanceofrecycledandcontrolsections.LaboratorytestswereconductedonthecoresobtainedfromtheprojectsitesaccordingtothetestingplanshowninFigure1.

In-situMixProperties

Figure2showsthetestdataobtainedonthefieldcoresfromthe5projects(bothrecycledand

controlsections).Thetestdataincludesairvoidcontent,resilientmodulusat25C(77F),andindirecttensilestrengthat25C(77F).

RecompactedMixProperties

ThemixobtainedfromthefieldcoreswasheatedandrecompactedintheGTMasmentionedearlier.BasedonNCAT'sexperience,averticalpressureof828kPa(120psi)and10initialgyrationanglewasused.Thecommongyratoryindicesandshearpropertiessuchasgyratorystabilityindex(GSI),gyratoryelasto-plasticindex(GEPI),androllerpressurewereobtained(10).

Figure3givestheaveragerecompactedmixpropertiessuchasairvoidcontent,GEPI,GSIand

rollerpressure.Figure4showstheaveragetestdataobtainedfromtheconfined,dynamiccreep

tests.

RecoveredAsphaltBinderProperties

Therecoveredasphaltcementwastestedforpenetrationat25C(77F)andabsoluteviscosityat60C(140F).TheresultsfromthesetestsareshowninFigure5.Twosamplesweretestedfromeachsectionand,therefore,thetestdatareportedistheaverageoftwotests.

Sincethemainconcernofthisprojectisperformance,itwasdecidedtodeterminethe

rheologicalpropertiesoftherecoveredbinderusingperformancebasedSHRPbindertestssuchasthedynamicshearrheometer(DSR).ThistestingwascarriedoutaspertheSHRPasphaltbinderspecificationproposedfortheStateofGeorgia.Theviscoelasticbehaviorofthe

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Table2.PavementSurfaceEvaluation(Task1)

SiteNo. County MixType AverageRut Raveling& Alligator Transverse Longitudinal OtherSurfaceDepth(inch) Weathering Cracking Cracking Cracking Distress

18C Coffee Virgin 0.069 None None Low None WBLhasmore

transversecracks

18R Coffee Recycled 0.069 None None None None None

22C Ware Virgin 0.069 None None None None None

22R Ware Recycled 0.063 None None None None None

23C Chatham Virgin 0.044 None None None None None

23R Chatham Recycled 0.066 None None None None None

25C Emmanuel Virgin 0.009 None None Low Low Map

(continuous) Cracking

25R Emmanuel Recycled 0.063 None None Low Low Long.Refl.

(continuous) Crack

28C Columbia/ Virgin 0.013 None None None None NoneRichmond

28R Columbia/ Recycled 0.078 None None None None OpenSurfaceRichmond Texture

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Figure2.In-SituMixProperties(Task1)

Figure3.RecompactedMixProperties(Task1)6


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Figure4.CreepModulusValues(Task1)

Figure5.RecoveredAsphaltBinderProperties(Task1)

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recoveredasphaltcementwascharacterizedbymeasuringtheComplexShearModulus(G*)and

thePhaseAngle(*)oftheasphaltcement.G*istheratioofthemaximumshearstiffness(Tmax)tomaximumshearstrain(Ymax).Thetimelagbetweentheappliedstressandtheresultingstrainisthephaseangle*.

AccordingtoSHRPspecifications,ruttingpotentialorthepermanentdeformationofthemixis

controlledbylimitingtheruttingfactorG*/Sin*athightesttemperaturestovaluesgreaterthan2.2kPaafterRTFOT(rollingthinfilmoventest)aging.Forthisstudy,thespecificationtesttemperatureforthestateofGeorgiaforruttingwasassumedtobe64C(148F)sincetherecommendedSHRPpavinggradewhichsatisfiesrequirementsformostofthegeographicalareaofGeorgiaisPG64-22.Higherruttingfactorsat64C(148F),indicatebetterruttingresistance.

Fatiguecrackingnormallyoccursatlowtomoderatetemperatures.AccordingtotheSHRP

specifications,itiscontrolledbylimitingthefatiguecrackingfactorG*Sin*oftherecoveredasphaltbindertovalueslessthan5000kPaatthetesttemperature.ThetesttemperatureforfatiguecrackinginGeorgiawasassumedtobe22C(72F).ThefivepavementsectionsinTask1havebeeninservicefrom1to3years,averagingabouttwoyears.TherecoveredasphaltbinderisthereforeexpectedtobesofterthanthecorrespondingPressureAgingVessel(PAV)residuewhichsimulatesfivetotenyearsinservice.However,thefatiguefactorcanbeusedinthisstudyforcomparingthepotentialfatiguebehaviorofthecontrolandrecycledsections.Lowerfatiguecrackingfactorsindicatebetterabilityfortheasphaltbindertodissipatestresswithoutcracking.

Figure5showsthevaluesofG*/Sin*andG*Sin*obtainedforthesefiveprojects.

ANALYSISOFTESTDATA(TASK1)

Apairedt-testisappropriateforanalyzingthetestdatainTask1todetermineifthereisa

significantdifferencebetweenthetestvaluesobtainedinrecycledandcontrolsection.Task1consistsoffivepairs(projects),eachpairconsistingofonerecycledandonecontrolsection.Averagetestvalueswereusedintheanalysis.Table3givesthepairedt-testresults.

VisualEvaluation

TheextentofdistresswasquantifiedandisreportedinReference10.Nosignificantrutting,raveningandalligatorcrackinghasoccurredinanyofthesites(Table2).RuttingoccurswhentheHMAmixistoosoft.Alligator(fatigue)crackingcanoccuriftheHMAmixistoostifforbrittle.Theoverallpavementsurfaceevaluationandratingindicatesthatbothrecycledandcontrolsectionsareperformingequallywellaftertheaverageservicelifeofabouttwoyears.Itwouldbeinterestingtorevisitthesetestsectionsinthefuturetoascertaintheirrelativeperformance.

In-situAirVoids

Thepairedt-testanalysisasreportedinTable3indicatesnosignificantdifferencebetweentheairvoidsinrecycledandcontrolsections.Averageairvoidsinthefiveprojectsare7.0and6.5percent,respectively,incontrolandrecycledsections.AirvoidssignificantlyaffecttherateofagingofasphaltbindersinHMApavements.Highairvoidsacceleratetheagingprocess.Itisencouragingtoknowthattherecycledsectionsdonothaveairvoidshigherthanthoseinthecontrolsections.

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Table3.Pairedt-testResults(Task1)

Averageof5Projects

Property Control Recycled tcalca AreDifferencesSignificantat5%Level?

A.In-SituMix

AirVoids(%) 7.9 6.5 0.818 No

ResilientModulus@25C 6,530 6,150 0.469 No(MPa)

IndirectTensileStrength 1,393 1,289 3.994 Yes@25C(kPa)

B.RecompactedMix

AirVoids(%) 4.1 3.1 2.022 No

GSI 1.1 1.1 2.181 No

GEPI 1.1 1.3 7.467 YesRollerPressure(kPa) 57.9 40.7 4.192 Yes

CreepModulus(MPa) 46.3 65.8 1.378 No

C.RecoveredAsphalt

Penetration@25C(0.1 20 20 0.047 Nomm)

Viscosity@60C(Pa-s) 5,466 4,688 0.850 No

G*/Sin(*)(kPa)@64C 17.9 15.4 1.012 No

G*Sin(*)(kPa)@22C 1,356 1,288 0.371 Noa tcritical=2.776for5numberofobservations(samplesize)at5%levelofsignificance.

RecoveredAsphaltBinderPropertiesThepairedt-test(Table3)indicatesnosignificantdifferencebetweenthepenetrationofcontrolandrecycledsections.Amongthefiverecycledsections,AC-20orAC-20SasphaltcementshavegenerallyresultedinrelativelyhigherpenetrationvaluescomparedtoAC-30asphaltcements(Figure5).Site25and28usedthesameamountofRAP(20percent)butdifferentgradesofasphaltcements.Site28withAC-30gavealowerpenetrationvaluethanSite25withAC-20asphaltcement.Site18hasthelowestpenetrationofallthesitesbecausethispavementhadrelativelyhigherairvoidcontentatthetimeofconstructioncomparedtotheremainingpavements(Table1).

TheviscosityhistogramshowninFigure5indicatescomparableviscosityvaluesforcontrolandrecycledsectionsinallsitesexceptSite25.Generally,theviscosityvaluesareconsistentwiththepenetrationvalues.ExceptSite25,theuseofAC-30asphaltcementgenerallyresultedinrelativelyhigherviscosityvaluescomparedtoAC-20orAC-20Sasphaltcements.Thepairedt-test(Table3)indicatesnosignificantdifferencebetweentheviscosityofthecontrolandtherecycledsections.

TheG*/Sin*(ruttingfactor)histogramshowninFigure5showscomparablevaluesforcontrolandrecycledsectionsinallsitesexceptSite25.Thepairedt-test(Table3)indicatesnosignificantdifferencebetweentheG*/Sin*valuesofthecontrolandtherecycledsections.Thismeansthebindersareequallyresistanttorutting.ItisinterestingtonotethatG*/sin *histogramandtheviscosityhistogramhavesimilartrends.Bothtestswereconductedathightemperatures:

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DSRat64C(148F)andviscosityat60C(140F).Theviscouscomponentofthecomplex

shearmodulusG*,therefore,appearstobedominantintherecoveredasphaltcements.

TheG*Sin*(fatiguecrackingfactor)histogramshowninFigure5indicatesthatallvaluesare

wellbelow5000kPaasexpected.Thepairedt-test(Table3)indicatesnosignificantdifferencebetweentheG*Sin*valuesofcontrolandrecycledsections.Thismeanstheyareequallyresistanttofatiguecracking.TheG*Sin*histogramisconsistentwiththepenetrationhistogram(Figure5)becausebothtestsareconductedatveryclosetemperatures.Again,theuseofAC-30asphaltcement(Sites18and28)generallygavehighG*Sin*values(morepronetofatiguecracking)comparedtoAC-20orAC-20Sasphaltcements.Site18hasthehighestG*Sin*valuebecausethispavementhadrelativelyhighairvoidcontentatthetimeofconstructionasmentionedearlier.


TheresilientmodulushistogramshowninFigure2indicatescomparablevaluesforcontrolandrecycledsectionsinallsitesexcept25and28.However,thepairedt-testindicatesnosignificantdifferencebetweenthecontrolandrecycledsectionswhenallfiveprojectsareconsideredinstatisticalanalysis(Table3).Sincetheresilientmodulusisanindicatorofthemixstrengthunderdynamicloading,bothcontrolandrecycledsectionsappeartohavecomparablestructuralstrengths.

Theindirecttensilestrengthhistogram(Figure2)showsthatthetensilestrengthvaluesofthe

controlmixesareslightlyhigherthanthoseoftherecycledmixesinallthefivesites.Thepairedt-test(Table3)indicatesthereisasignificantdifferencebetweentheindirecttensilestrengthvaluesofthecontrolandtherecycledsections.Sincetheseprojectsareonly1to2yearsold,theimplicationsofthistest,ifany,arenotevidentvisuallyintheformofsomedistress.

RecompactedMixProperties

TheairvoidshistogramshowninFigure3indicateslowerairvoidsinrecycledsections(exceptSite23)comparedtocorrespondingcontrolsections.However,ifall5projectsareconsidered,thepairedt-test(Table3)indicatesnostatisticallysignificantdifferencebetweenthecontrolandrecycledsections.BothcontrolandrecycledmixesinSite23wererecompactedinthelaboratorytoverylowairvoidscontentsof1.7and2.2percent,respectively.Thisindicatesapotentialforruttinginthefutureifthesiteissubjectedtoheavytrafficloads.Site23hasthelowestin-situairvoids(Figure2)atthepresenttimebutithasnotrutted(Table2)becausethein-situairvoidsaremorethan3.5percentatthepresenttime.

TheGyratoryStabilityIndex(GSI)isameasureofthestabilityofthemix.GSIinexcessof1.00

indicatesanincreaseinplasticity.TheGSIhistogramshowninFigure3indicatesthattheGSIvaluesofthecontrolandrecycledmixesarecomparable.ItalsoindicatesthatSite23maybe

subjecttoruttingorplasticdeformation.Thepairedt-test(Table3)alsoindicatesnostatisticallysignificantdifferencebetweenthecontrolandrecycledmixes.

TheGyratoryElasto-PlasticIndex(GEPI)isameasureofinternalfrictionpresentinthemix.

GEPIvaluesnearonearefoundinfreshstableHMAmixwithlowshearstrain.TheGEPIhistogramshowninFigure3indicatesconsistentlyhighervaluesforrecycledmixescomparedtocontrolmixes.Thismeanstherecycledmixeshavelessinternalfrictioncomparedtocontrolmixes.Thiscouldbeattributedtothefactthat100percentvirginaggregateparticleshavemoreinterlockingeffectthantherecycledmixwhichcontainsamixtureofvirginaggregateparticlesandRAPparticles.TheRAPparticlesareusuallynotasangularasvirginaggregateparticlesduetothemillingoperation.Also,theRAPparticlesarealreadycoatedwithasphaltbinderandtherefore,maynothavearoughsurfacetexture.Thepairedt-test(Table3)indicatesthatthe

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GEPIvaluesofcontrolandrecycledmixesarestatisticallysignificantlydifferent.

TherollerpressurevaluesmeasuredduringtheGTMcompactionprocedureareshowninthe

histograminFigure3.Itisevidentfromthehistogramthattherecycledmixeshavelowerroller

pressurecomparedtocontrolmixesinallcases.Thepairedt-testalsoindicatesthatthedifferenceisstatisticallysignificant.Therollerpressureisameasureofresistancetodeformationofthemix.Ahigherrollerpressureindicatesgreaterresistanceofthemixagainstdeformation.

ThecreephistogramshowninFigure4indicateshighercreepmodulus(higherresistanceto

permanentdeformation)forrecycledmixescomparedtocontrolmixesinallsitesexceptSite18.However,thepairedt-test(Table3)showsthatthedifferencesarenotstatisticallysignificant.ThecreepmodulusdataareunlikeGEPIandGSIdatawhichshowedthattherecycledmixeshavelowerresistancetopermanentdeformationcomparedtocontrolmixes.Nosignificantruttinghasbeenobservedinthefieldinrecycledandcontrolsectionsasyet(Table2).

SAMPLINGANDTESTINGPLAN(TASK2)

Thistaskconsistedofevaluating13projectsinvolvingonlytherecycledwearingcoursesand10projectsinvolvingonlyvirginmixwearingcoursesconstructedgenerallyduringthesameperiodthroughoutthestateofGeorgia.TheresultsobtainedfromtheseprojectscombinedwiththosefromTask1formedadatabasefordetermininggeneraltrendsinthecharacteristicsandperformanceofrecycledmixesascomparedtovirginmixesusedinthewearingcourses.VisualevaluationofallprojectsinTask2wasperformedasinTask1.Surfacedistresseslikeruttingandcrackingweremeasuredandquantified.Four152-mm(6-inch)diametercoreswereobtainedatanintervalofabout30m(100ft)fromtherepresentativetestareaforfurtherlaboratorytesting.Thecoreswereobtainedfromtheoutsidewheeltrackarea.Thefieldcoresweresawedtoseparatethetoprecycledorthevirginmixlayerforthetesting.Afterdeterminingthedensity(andthereforeairvoidcontent)ofthecores,asphaltcementwasextractedandrecoveredfromthecoresfollowingtheproceduresmentionedinTask1.

Penetrationat25C(77F)andviscosityat60C(140F)oftherecoveredasphaltcements'werethendetermined.

TESTDATA(TASK2)

TheprojectsselectedinTask2consistedofpavementsthroughoutGeorgiawhichwereconstructedusingeithertherecycledmixorthevirginmix.SpecificrecycledprojectdetailsaregiveninTable4.Visualpavementsurfaceevaluationofthesepavementswasconductedinthebeginningoftheproject.Theobservationsaregivenelsewhere(10).Mostofthevirginandrecycledpavementsdonotexhibitanydistressatthistimeandhavecomparableperformance.

Table4.SpecificRecycledProjectsDetails(Task2)

VirginAsphalt No.of Specified Supplied %RAP AgeofGrade Projects Penetration Penetration Used Pavement

(Years)

AC-10 2 80+ 98-123 40 3.50

AC-20 10 60+ 82-93 10-30 1.5-3.0

AC-20S 4 60-80 67-84 10-25 1.25-5.0

AC-30 2 60+ --- 15-20 2.0-3.5

Bulkspecificgravityofthecoreswasdeterminedtocalculatetheairvoidcontent.AsinTask1,asphaltcementwasrecoveredfromthefieldcores.Penetrationandviscosityoftherecoveredasphaltbinderweredetermined.

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Figure6showstherangeandaverageofthetestdataonin-situairvoidcontent,penetration,and

viscosityforbothvirginandrecycledpavements.

Figure6.Histogramsof%AirVoids,PenetrationandViscosity(Task2)

ANALYSISOFDATA(TASK2)TheTask2partofthestudyinvolvedtheevaluationof15independentvirginmixpavementsand18independentrecycledmixpavementstoobtainatestdatabaseforcomparison.TheseprojectsarelocatedthroughouttheStateofGeorgia.Itshouldberealizedthatmanyvariables(suchasthepercentageandpropertiesofRAPused,gradeofthevirginasphaltcement,andageofthepavement)areinvolvedinthe18recycledmixprojectsasshowninTable4.

Becauseofthesevariables,therecoveredasphaltcementpropertiesareexpectedtovary

significantly.Pavementsurfaceevaluationratingsofvirginmixpavementsandrecycledmixpavementsaregivenelsewhere(10).Therewasnosignificantoveralldifferenceintheperformanceofvirginandrecycledpavementsatthetimeofinspection(January/February1993).

Thetestdata(airvoids,penetrationorviscosity)obtainedinTask2canbetreatedastwo

independentgroups(virginmixesandrecycledmixes)ofunequalsizes(15virginand18recycledmixes).Oneoftheobjectivesofthisstudyistodetermineifthecharacteristicsoftherecycledmixesaresignificantlydifferentfromthoseofthevirginmixes.ThiscanbeaccomplishedbyperformingstatisticalanalysisusingtheIndependentSamplest-test,fortestingtheequalityofmeansofpercentairvoids,penetrationandviscositydataat5percentlevelofsignificance.Itisassumedthatthesamplemeansarereasonableestimatesoftheirrespectivepopulationmeans.Table5givestheresultsofthestatisticalanalysis.

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Table5.TestingEqualityofMeansbyt-test(Task2 )

SampleSize Average StandardDeviation tcalc tcritical AreDifferencesProperty Significantat

Virgin Recycled Virgin Recycled Virgin Recycled 5%Level?

In-SituAirVoids 14 18 5.8 6.3 2.27 2.34 0.62 2.040 No

(%)

Penetrationat25C 15 18 18.3 19.1 3.7 5.0 0.51 2.037 No

(0.1mm)

Viscosityat60C 15 18 7,310 5,820 7,150 3,330 0.79 2.037 No

(Pa-s)

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TheairvoidshistogramgiveninFigure6showstheminimum,averageandmaximumvaluesofairvoidsinvirginandrecycledpavements.Theaveragein-situairvoidsinvirginandrecycled

pavementsare5.8and6.3percent,respectively.Thet-testanalysis(Table5)indicatesnostatisticallysignificantdifferencebetweentheairvoidsofvirginandrecycledpavementsatthetimeofcoresampling.

RecoveredAsphaltBinderProperties

Thepenetrationhistogramshowingtheminimum,averageandmaximumvaluesofpenetrationinvirginandrecycledpavementsisalsogiveninFigure6.Theaveragepenetrationvaluesofvirginandrecycledpavementsare18and19,respectively.Thet-testanalysis(Table5)indicatesnostatisticallysignificantdifferencebetweenthepenetrationvalueofthetwopavementtypes.

Theaverageviscosityvaluesofvirginandrecycledpavementsare73,098poisesand58,196

poises,respectively,asshownintheviscosityhistogram(Figure6).Thet-testanalysisindicatesthereisnosignificantdifferencebetweentheviscosityvaluesofthetwopavementtypes.

CONCLUSIONSANDRECOMMENDATIONS

Thefollowingconclusionscanbedrawnfromthisstudy.

1.Bothvirginandrecycledsectionsofthe5projectsinTask1areperformingsatisfactorilyafter1to2yearsinservicewithnosignificantrutting,raveningandweathering,andfatiguecracking.

2.ThedifferencesbetweenthefollowingpropertiesofvirginandrecycledsectionsofTask1projectswerefoundtobenotstatisticallysignificantata5percentlevelofsignificancebasedonpairedt-test.(a)In-situmixpropertiessuchaspercentairvoidsandresilientmodulusat25C(77F)

(b)Agedasphaltbinderpropertiessuchaspenetrationat25C(77F),viscosityat60C(140F),SHRPruttingfactorG*/Sin*at64C(148F),andSHRPfatiguecrackingfactorG*Sin*at22C(72F).GRecompactedmixpropertiessuchaspercentairvoids,GyratoryStabilityIndex(GSI),andconfineddynamiccreepmodulusat60C(140F).

3.Thedifferencesbetweentheindirecttensilestrengthat25C(77F),theGyratoryElasto-PlasticIndex(GEPI),andtherollerpressurevaluesofvirginandrecycledsectionsofTask1projectswerefoundtobestatisticallysignificantata5percentlevelofsignificance.

4.Task2pavementsweretreatedastwoindependentgroups(virginmixesandrecycledmixes)ofunequalsizes(15virginand18recycledmixes)forstatisticalanalysis.IndependentSamplest-testwasusedfortestingtheequalityofmeansofpercentairvoids,penetrationandviscosityofthetwogroups.Nostatisticallysignificantdifference

wasfoundinthesethreeproper-tiesofvirginandrecycledpavementsata5percentlevelofsignificance.

5.Therewasnosignificantoveralldifferenceintheperformanceofvirginandrecycledpavementsbasedonvisualinspection.

6.BasedontheevaluationofTask1andTask2pavements,itcanbeconcludedthattherecycledpavementsaregenerallyperformingaswellasthevirginpavementsatthepresenttimeinGeorgia.Therefore,itcanbeimpliedthattheexistingGeorgiaDOTrecyclingspecifications,recycledmixdesignproceduresandqualitycontrolaresatisfactory.Thespecificationtoachieveaviscosityof6,000to16,000poisesfortheblend(RAPbinder+virginbinder)appearsreasonablebasedonthepresentdata.

7.Someselectedvirginandrecycledpavements(especiallythoseincludedinTask1)shouldbereevaluatedafteranother2-3yearsservice.Thisshouldincludepavement

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surfaceevaluationanddeterminationofagedasphaltpropertiesofthesame

representativesectionusedinthisstudy.

ACKNOWLEDGMENTS

ThisresearchprojectwassponsoredbytheGeorgiaDOTincooperationwiththeFederalHighwayAdministration.TheassistanceofRobertJohnsonofNCATwhoconductedtheperformanceevaluationofallprojectsandobtainedfieldcores,isappreciated.TheauthorsacknowledgethecooperationandvaluableadviceoftheGeorgiaDOTpersonnel:Messrs.RonaldCollins,GeneSlaughterandHarryMcGaugheyincarryingoutthisresearchproject.REFERENCES1.D.H.LittleandJ.A.Epps.EvaluationofCertainStructuralCharacteristicsofRecycled

PavementMaterials.AAPT,Vol.49,1980,pp219-251.2.D.H.Little,R.J.Holmgreen,Jr.,andJ.A.Epps.EffectofRecyclingAgentsontheStructural

PerformanceofRecycledAsphaltConcreteMaterials.AAPT,Vol.50,Feb.1981,pp32-63.3.E.R.Brown.EvaluationofPropertiesofRecycledAsphaltConcreteHotMix.U.S.Army

EngineerWaterwaysExperimentStation,FinalReport#CL-84-2,Feb1984.4.F.Meyers,G.R.Tessier,R.Haas,andT.W.Kennedy.StudyofHotMixRecyclingof

AsphaltPavements.RoadsandTransportationAssociationofCanada,Report#TP2964E,1983,Ottawa,Ontario.

5.P.S.Kandhal,E.R,Brown,andS.Cross.GuidelinesforHotMixRecyclinginGeorgia.GeorgiaDOTProjectNo.8807,FinalReport,Sept.1989.

6.B.M.Kiggundu,B.N.Humphrey,andD.M.Zallen.RecyclingAgentSelectionandTentativeSpecification.NewMexicoEngineeringResearchInstitute,FinalReport#ESL-TR-84-47,March1985.

7.B.M.KiggunduandJ.K.Newman.Asphalt-AggregateInteractionsinHotRecycling.Final

Report#ESL-TR-87-07,NewMexicoEngineeringResearchInstitute,July1987.8.R.L.DunningandR.L.Mendenhall.DesignofRecycledAsphaltPavementsandSelectionofModifiers.RecyclingofBituminousPavements,ASTMSTP662,1978,pp35-46.

9.GeorgiaDepartmentofTransportationSpecifications.Section402:HotMixRecycledAsphaltConcrete.1993.

10.P.S.Kandhal,S.S.Rao,andB.Young.PerformanceofRecycledMixturesinStateofGeorgia.FHWAReportNo.FHWA-GA-94-9209,January1994.

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