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PERFORMANCEOFRECYCLEDHOTMIXASPHALTMIXTURES
By
PrithviS.Kandhal
ShridharS.RaoDonaldE.WatsonBradYoung
PaperpublishedinTransportationResearchBoard,TransportationResearchRecord1507,1995
277TechnologyParkway Auburn,AL36830
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PERFORMANCEOFRECYCLEDHOTMIXASPHALTMIXTURES
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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PERFORMANCEOFRECYCLEDHOTMIXASPHALTMIXTURES
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.
In-situMixProperties
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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In-situMixProperties
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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