loaded wheel testers in the united states: state of the practice

8/9/2019 Loaded Wheel Testers in the United States: State of the Practice

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LOADEDWHEELTESTERSINTHE UNITEDSTATES:STATEOFTHE PRACTICE

By

L.AllenCooleyJr.

PrithviS.KandhalM.ShaneBuchananFrankFeeAmyEpps

PublishedinTransportationResearchBoard,TransportationResearchCircularE-C016,July2000

277TechnologyParkway Auburn,AL36830


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LOADEDWHEELTESTERSINTHEUNITEDSTATES:STATEOFTHEPRACTICE

By

L.AllenCooleyJr.

ResearchEngineerNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

PrithviS.Kandhal

AssociateDirectorNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

M.ShaneBuchananSeniorResearchAssociate

NationalCenterforAsphaltTechnologyAuburnUniversity,Alabama

FrankFee

AsphaltEngineerCITGOAsphalt

Moylan,Pennsylvania

AmyEpps

AssistantProfessor

TexasA&MUniversityCollegeStation,Texas

PublishedinTransportationResearchBoard,TransportationResearchCircularE-C016,

July2000


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DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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CooleyJr.,Kandhal,Buchanan,Fee,&Epps

LOADEDWHEELTESTERSINTHEUNITEDSTATES:STATEOFTHEPRACTICE

L.AllenCooleyJr.,PrithviS.Kandhal,M.ShaneBuchanan,FrankFee,andAmyEpps

FOREWORDLoadedwheeltesters(LWT)arebecomingincreasinglypopularwithtransportationagenciesastheyseektoidentifyhotmixasphaltmixturesthatmaybepronetorutting.ThisE-CircularprovidesthestateofthepracticeontheuseofLWTswithintheUnitedStatesasobtainedfromareviewofliterature.TheintentistoprovidebackgroundinformationonLWTsusedintheUnitedStatesanddescribethekeytestparameters,limitations,materialsensitivities,andboundaryconditionsutilizedbyvariousLWTs.

INTRODUCTION

Permanentdeformation,orrutting,inhotmixasphalt(HMA)pavementshasbeenandcontinues

tobeamajorproblemintheUnitedStates.RuttingisdefinedastheaccumulationofsmallamountsofunrecoverablestrainresultingfromappliedwheelloadstoHMApavement.Thisdeformationiscausedbyconsolidationorlateralmovement,orboth,oftheHMAundertraffic.Shearfailure(lateralmovement)inaHMApavementgenerallyoccursinthetop100mm(4in)oftheHMAstructure(1).Ruttingnotonlydecreasestheusefullifeofapavementbutalsocreatesasafetyhazardforthetravelingpublic.Inrecentyears,thepotentialforruttingonthenation'shighwayshasincreasedduetohighertrafficvolumesandtheincreaseduseofradialtiresthattypicallyexhibithigherinflationpressures.

Astandardizedlaboratoryequipmentandtestprocedurethatpredictsfield-ruttingpotential

wouldbeofgreatbenefittotheHMAindustry.Currently,themostcommontypeoflaboratoryequipmentofthisnatureisaloadedwheeltester(LWT).

InanefforttoidentifyHMAmixturesthatmaybepronetorutting,manytransportationagencieshavebegunusingLWTsassupplementstotheirmixdesignprocedure.TheLWTsallowforanacceleratedevaluationofruttingpotentialinthedesignedmixes.However,inorderfortheseagenciestouseLWTswithconfidence,thereneedstobeanacceptablecorrelationbetweenruttinginthelaboratoryandactualfieldrutting.SomeagenciesusingLWTshaverecognizedthisfactandhaveconductedresearchtodeterminethedegreeofcorrelationbetweenfieldperformanceandresultsfromlaboratoryLWTs.

LOADEDWHEELTESTERSUSEDINTHEUNITEDSTATES

SeveralLWTscurrentlyarebeingusedintheUnitedStates.TheyincludetheGeorgiaLoadedWheelTester(GLWT),AsphaltPavementAnalyzer(APA),HamburgWheelTrackingDevice(HWTD),LCPC(French)WheelTracker,PurdueUniversityLaboratoryWheelTrackingDevice

(PURWheel),andone-thirdscaleModelMobileLoadSimulator(MMLS3).FollowingaredescriptionsforeachoftheseLWTs.

GeorgiaLoadedWheelTester

TheGLWT,showninFigure1,wasdevelopedduringthemid-1980sthroughacooperativeresearchstudybetweentheGeorgiaDepartmentofTransportationandtheGeorgiaInstituteofTechnology(2).DevelopmentoftheGLWTconsistedofmodifyingawheel-trackingdeviceoriginallydesignedbyC.R.BenedictofBenedictSlurrySeals,Inc.,totestslurryseals(3).TheprimarypurposefordevelopingtheGLWTwastoperformefficient,effective,androutinelaboratoryrutprooftestingandfieldproductionqualitycontrolofHMA(4).

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Figure1.GeorgiaLoadedWheelTester(GLWT)

TheGLWTiscapableoftestingHMAbeamorcylindricalspecimens.Beamdimensionsare

generally125mmwide,300mmlong,and75mmhigh(5inx12inx3in).CompactionofbeamspecimensfortestingintheGLWThasvariedgreatlyaccordingtotheliterature.TheoriginalworkbyLai(2)utilizeda"loadedfoot"kneadingcompactor.HeatedHMAwas"spooned"intoamoldasaloadedfootassemblycompactedthemixture.Aslidingrack,ontowhichthemoldwasplaced,wasemployedasthekneadingcompactorwasstationary.Westetal.(5)utilizedastaticcompressiveloadtocompactspecimens.HeatedHMAwasplacedintoamoldandacompressiveforceof267kN(60,000lbs)wasloadedacrossthetopofthesampleandthenreleased.Thisloadsequencewasperformedatotaloffourtimes.In1995,LaiandShami(6)describedanewmethodofcompactingbeamsamples.Thismethodutilizedarollingwheeltocompactbeamspecimens.

Laboratorypreparedcylindricalspecimensaregenerally150mmindiameterand75mmhigh.

Compactionmethodsforcylindricalspecimenshaveincludedthe"loadedfoot"kneading

compactor(2)andaSuperpavegyratorycompactor(7).

Bothspecimentypesaremostcommonlycompactedtoeither4or7percentairvoidcontent.

However,someworkhasbeenaccomplishedintheGLWTatairvoidcontentsaslowas2percent(8).

TestingofsampleswithintheGLWTgenerallyconsistsofapplyinga445-N(100-lb)loadontoa

pneumaticlinearhosepressurizedto690kPa(100psi).Theloadisappliedthroughanaluminumwheelontothelinearhose,whichresidesonthesample.Testspecimensaretrackedbackandforthundertheappliedstationaryloading.Testingistypicallyaccomplishedforatotalof8,000loadingcycles(onecycleisdefinedasthebackwardandforwardmovementoversamplesbythewheel).However,someresearchershavesuggestedfewerloadingcyclesmaysuffice(5).

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TesttemperaturesfortheGLWThaverangedfrom35Cto60C(95Fto140F).Initialwork

byLai(2)wasconductedat35C(95F).ThistemperaturewasselectedbecauseitwasGeorgia'smeansummerairtemperature( 3).Testtemperatureswithintheliteraturesubsequentlytendedtoincreaseto40.6C(105F)(3,5,9,10,11),46.1C(115F)(11),50C(122F)(3,8),

and60C(140F)(8).Attheconclusionofthe8,000cycleloadings,permanentdeformation(rutting)ismeasured.Rutdepthsareobtainedbydeterminingtheaveragedifferenceinspecimensurfaceprofilebeforeandaftertesting.Atemplatewithsevenslotsthatfitsoverthesamplemoldandamicrometeraretypicallyusedtomeasurerutdepth( 2).

AsphaltPavementAnalyzer

TheAPA,showninFigure2,isamodificationoftheGLWTandwasfirstmanufacturedin1996byPavementTechnology,Inc.TheAPAhasbeenusedtoevaluatetherutting,fatigue,andmoistureresistanceofHMAmixtures.SincetheAPAisthesecondgenerationoftheGLWT,it

followsthesameruttestingprocedure.Awheelisloadedontoapressurizedlinearhoseandtrackedbackandforthoveratestingsampletoinducerutting.SimilartotheGLWT,mosttestingiscarriedoutto8,000cycles.UnliketheGLWT,samplesalsocanbetestedwhilesubmergedinwater.

Figure2.AsphaltPavementAnalyzer(APA)

TestingspecimensfortheAPAcanbeeitherbeamorcylindrical.Currently,themostcommonmethodofcompactingbeamspecimensisbytheAsphaltVibratoryCompactor(12).However,somehaveusedalinearkneadingcompactorforbeams(13).ThemostcommoncompactorforcylindricalspecimensistheSuperpavegyratorycompactor(14).Beamsaremostoftencompactedto7percentairvoids,whilecylindricalsampleshavebeenfabricatedtoboth4and7

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percentairvoids(13).Testscanalsobeperformedoncoresorslabstakenfromanactual

pavement.

TesttemperaturesfortheAPAhaverangedfrom40.6Cto64C(105Fto147F).Themost

recentworkhasbeenconductedatorslightlyaboveexpectedhighpavementtemperatures(14,15).

WheelloadandhosepressurehavebasicallystayedthesameasfortheGLWT,445Nand690

kPa(100lband100psi),respectively.Onerecentresearchstudy(15)diduseawheelloadof533N(120lb)andhosepressureof830kPa(120psi)withgoodsuccess.

HamburgWheel-TrackingDevice

TheHWTD,showninFigure3,wasdevelopedbyHelmut-WindIncorporatedofHamburg,Germany(16).ItisusedasaspecificationrequirementforsomeofthemosttraveledroadwaysinGermanytoevaluateruttingandstripping.TestswithintheHWTDareconductedonaslab

thatis260mmwide,320mmlong,andtypically40mmhigh(10.2inx12.6inx1.6in).Theseslabsarenormallycompactedto71percentairvoidsusingalinearkneadingcompactor.

Figure3.HamburgWheelTrackingDevice(HWTD)

TestingintheHWTDisconductedunderwaterattemperaturesrangingfrom25Cto70C(77Fto158F),with50C(122F)beingthemostcommontemperature(17).LoadingofsamplesintheHWTDisaccomplishedbyapplyinga705-N(158-lb)forceontoa47-mm-widesteelwheel.Thesteelwheelisthentrackedbackandforthovertheslabsample.Testsamplesareloadedfor20,000passesoruntil20mmofdeformationoccurs.Thetravelspeedofthewheelisapproximately340mmpersecond(16).

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AsshowninFigure4,resultsobtainedfromtheHWTDconsistofrutdepth,creepslope,

strippinginflectionpoint,andstrippingslope.Thecreepslopeistheinverseofthedeformationratewithinthelinearregionofthedeformationcurveafterpostcompactionandpriortostripping(ifstrippingoccurs).Thestrippingslopeistheinverseofthedeformationratewithinthelinear

regionofthedeformationcurve,aftertheonsetofstripping.Thestrippinginflectionpointisthenumberofwheelpassescorrespondingtotheintersectionofthecreepslopeandthestrippingslope.ThisvalueisusedtoestimatetherelativeresistanceoftheHMAsampletomoisture-induceddamage(17).

Figure4.TypicalHamburgWheelTrackerTestResults

AslightmodificationoftheHWTDwasmadebytheSuperfosConstruction,U.S.(previouslyCouch,Inc.).Thisdevice,showninFigure5,wasreferredtoastheSuperfosConstructionRutTester(SCRT).TheSCRTusedslabspecimenswithsimilardimensionsastheHWTD.Theprimarydifferencebetweenthetwowastheloadingmechanism.TheSCRTappliedan82.6-kg(180-lb)verticalloadontoasolidrubberwheelwithadiameterof194mmandwidthof46mm.Thisloadingconfigurationresultedinacontactpressureofapproximately940kPa(140psi)andcontactareaof8.26cm2(1.28in2)whichwasappliedataspeedofapproximately556mmpersecond(18).

Testtemperaturesrangingfrom45Cto60C(113Fto140F)havebeenusedwiththeSCRT.

RecentresearchwiththeSCRThasused60Casthetesttemperature(18,19).Anairvoidcontentof6percentwasgenerallyusedfordense-gradedHMAsamples(18).

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Figure5.SuperfosConstructionRutTester(SCRT)

ResultsfromtheSCRTareidenticaltothosefromtheHWTDandincluderutdepth,creepslope,

strippingslope,andstrippinginflectionpoint.

AnotherslightmodificationoftheHWTDistheEvaluatorofRuttingandStripping(ERSA)

equipment.ThisdevicewasbuiltbytheDepartmentofCivilEngineeringattheUniversityofArkansas(20).

TestingofcylindrialorbeamsamplesintheERSAcanbeconductedineitherwetordry

conditions.A47-mmwidesteelwheelisusedtoloadspecimenswith705N(160lb)for20,000cyclesora20-mmrutdepth,whicheveroccursfirst.

LCPC(French)WheelTracker

TheLaboratoireCentraldesPontsetChauses(LCPC)wheeltracker[alsoknownastheFrench

RuttingTester(FRT)],showninFigure6,hasbeenusedinFranceforover15yearstosuccessfullypreventruttinginHMApavements(21).Inrecentyears,theFRThasbeenusedintheUnitedStates,mostnotablyinthestateofColoradoandFHWA'sTurnerFairbankHighwayResearchCenter.

TheFRTiscapableofsimultaneouslytestingtwoHMAslabs.Slabdimensionsaretypically180

mmwide,50mmlong,and20to100mmthick(7.1inx19.7inx0.8to3.9in)(22).SamplesaregenerallycompactedwithaLCPClaboratory-tiredcompactor(23).

Loadingofsamplesisaccomplishedbyapplyinga5000-N(1124-lb)loadontoa400x8Treb

Smoothpneumatictireinflatedto600kPa(87psi).Duringtesting,thepneumatictirepassesoverthecenterofthesampletwicepersecond( 23).

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Figure6.LCPC(French)WheelTracker(FRT)

WithinFrance,testtemperaturesforFRTtestingaregenerally60C(140F)forsurfacecourses

and50C(122F)forbasecourses.However,ithasbeensuggestedthattemperatureslowerthan60C(140F)canbeusedforcolderregionswithintheUnitedStates(22).

RutdepthswithintheFRTaredefinedbydeformationexpressedasapercentageoftheoriginal

slabthickness.Deformationisdefinedastheaveragerutdepthfromaseriesof15measurements.Thesemeasurementsconsistofthreemeasurementstakenacrossthewidthofaspecimenatfivelocationsalongthelengthoftheslab.A"zero"rutdepthisgenerallydefinedbyloadingasampleatambienttemperaturefor1,000cycles(23).

PurdueUniversityLaboratoryWheelTrackingDevice

Asthenamestates,thePURWheel,showninFigure7,wasdevelopedatPurdueUniversity(24).PURWheeltestsslabspecimensthatcaneitherbecutfromtheroadwayorcompactedinthe

laboratory.Slabspecimensare290mmwideby310mmlong(11.4inx12.2in)(25).Thicknessesofslabsamplesdependuponthetypemixturebeingtested.Forsurfacecoursemixes,asamplethicknessof38mm(1.5in)isusedwhilebinderandbasecoursemixesaretestedatthicknessesof51mmand76mm(2inand3in),respectively(25).

LaboratorysamplesarecompactedusingalinearcompactoralsodevelopedbyPurdue

University(25).ThiscompactorwasbaseduponasimilarcompactorownedbyKochMaterialsinpreparingsamplesfortheHWTD(26).TheprimarydifferencebeingthatthePurdueversioncancompactlargerspecimens.Samplesarecompactedtoanairvoidcontentrangeof6to8percent.

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Figure7.PurdueUniversityLaboratoryWheelTrackingDevice(PURWheel)

PURWheelwasdesignedtoevaluateruttingpotentialand/ormoisturesensitivity(25).Testsamplescanbetestedineitherdryorwetconditions.Moisturesensitivityisdefinedastheratioofthenumberofcyclesto12.7mmofruttinginawetconditiontothenumberofcyclesto12.7mmofruttinginthedrycondition.The12.7-mmrutdepthisusedtodifferentiatebetweengoodandbadperformingmixeswithrespecttorutting(25).

LoadingoftestsamplesinPURWheelisconductedutilizingapneumatictire.Agrosscontact

pressureof620kPa(90psi)isappliedtothesample.Thisisaccomplishedbyapplyinga175-kg(385-lb)loadontothewheelthatispressurizedto793kPa(115psi).Aloadingrateof332mm/secisapplied.Testingisconductedto20,000wheelpassesoruntil20mmofruttingisdeveloped(24).

PURWheelisverysimilartotheHWTD.However,oneinterestingfeatureaboutPURWheelisthatitcanincorporatewheelwanderintotesting(25).ThisfeatureisuniqueamongtheLWTscommonintheUnitedStates.

ModelMobileLoadSimulator(MMLS3)

Theone-thirdscaleMMLS3wasdevelopedrecentlyinSouthAfricafortestingHMAineitherthelaboratoryorfield.Thisprototypedevice,showninFigure8,issimilartothefull-scaleTexasMobileLoadSimulator(TxMLS)butscaledinsizeandload.Thescaledloadof2.1-kN(472-lb)isapproximatelyone-ninth(thescalingfactorsquared)oftheloadonasingletireofanequivalentsingleaxleloadcarriedondualtires(27).

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Figure8.ModelMobileLoadSimulator(MMLS3)

TheMMLS3canbeusedfortestingsamplesindryorwetconditions.Anenvironmentalchambersurroundingthemachineisrecommendedtocontroltemperature.Temperaturesof50Cand60Chavebeenusedfordrytests,andwettestshavebeenconductedat30C.MMLS3samplesare1.2m(47in)inlengthand240mm(9.5in)inwidth,withthedeviceapplyingapproximately7200single-wheelloadsperhourbymeansofa300-mm(12-in)diameter,80-mm(3-in)widetireatinflationpressuresupto800kPa(116psi)withatypicalvalueof690kPa(100psi).Wandercanbeincorporateduptothefullsamplewidthof240mm.PerformancemonitoringduringMMLS3testingincludesmeasuringrutdepthfromtransverseprofilesanddeterminingSeismicAnalysisofSurfaceWavesmodulitoevaluateruttingpotentialanddamageduetocrackingormoisture,respectively.Rutdepthcriteriaforacceptableperformancearecurrentlybeingdeveloped( 28).

Currentlythereisnostandardforlaboratoryspecimenfabrication,althoughresearchisbeing

proposedtotheTexasDepartmentofTransportation.

EFFECTOFTESTPARAMETERSANDMIXTUREPROPERTIESONLWTRESULTS

AsshowninthepreviousdescriptionsonLWTs,allhavesimilaroperatingprinciples.Essentially,aloadistrackedbackandforthoveraHMAtestsample.Therefore,theeffectofvarioustestparametersandmaterialconstituentsshouldbesimilarforeach.FollowingaredescriptionsofhowdifferenttestparametersandconstituentscanaffectLWTresults.

WithintheoperatingspecificationsforeachoftheLWTs,twotestparametersarealways

specified:airvoidsandtesttemperature.Thisisprimarilyduetothefactthatthesetwo

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parametershavethemosteffectontestresults;especiallyrutdepths(29).Asairvoidsincrease,

rutdepthsalsoincrease.Thishasbeenshownbyseveralresearchstudies(8,29).Likewise,astesttemperatureincreases,rutdepthsalsoincrease(8,30,31,32).Unfortunately,nothingcouldbefoundintheliteratureabouttheeffectofairvoidsandtesttemperatureonmoisture

susceptibilityresults.AirvoidcontentsforeachoftheLWTsaregenerallyspecifiedbasedupontwoconcepts(12).First,somebelievethatspecimenairvoidcontentsshouldbeapproximately7percent,sincethisairvoidcontentrepresentstypicalas-constructeddensity.Othersbelievethattestspecimensshouldbecompactedto4percentairvoids,asactualshearfailureofmixesusuallytakesplacebelowapproximately3percent.

Anothertestparameterthatcansignificantlyaffecttestresultsisthetypeandcompaction

methodoftestsamples(29).Thetwopredominant"types"oftestspecimensarecylindersandbeams/slabs.Forruttingandmoisturesusceptibility,theliteratureindicatesthatthetwosampletypesdoprovidedifferentrutdepthsandstrippinginflectionpoints;however,bothtypes

generallyrankmixessimilarly(20,33,34).Theprimaryreasonthesetwotypesofspecimensdonotproducethesamerutdepthsisthattheyaregenerallycompactedbydifferentmethods.Forinstance,cylindricalspecimensaretypicallycompactedusingtheSuperpavegyratorycompactorwhilebeamsamplesaregenerallycompactedwithavibratoryorkneadingcompactor.Themethodofcompactioninfluencesthedensity(airvoid)gradientsandaggregateorientationwithinsamples(35,36).

Formoisturesusceptibility,researchhasshownthatdifferentsampletypesalsoyielddifferent

strippinginflectionpoints,evenonsamplescompactedsimilarly(20).Severalresearchershaveevensawncylindricalsamplessothatthey"butt"upagainsteachotherandcomparedtobeamspecimens(20,32).Thissampleconfigurationhasalsoshowndifferencesinstrippinginflectionpointsbetweenbeamsandcylinders.However,similartorutting,thecylindricalandbeamsspecimenstendtorankmixessimilarlywithrespecttomoisturedamage.

Anothertestparameterthatsignificantlyaffectstestresultsisthemagnitudeofloading.Awide

rangeofloadingsareusedinthedifferentdevices.AlthougharecentstudyindicatedthatsmallchangesinthemagnitudeofloadingmaynotaffectLWTrutresults(29),previousresearchhasshownthatsignificantdifferencesinloadingscanaffecttestresults(2).

Dependinguponwhetherruttingormoisturetestingistobeconducted,sampleconditioning

priortoLWTtestingisdifferent.Forrutting,ithasbeenshownthatsixhoursatthetesttemperatureissufficient(29).Ifsamplesarenotpreheatedsufficiently,lowrutdepthscanbeexpected.Conditioningofsamplesformoisturetestingpurposesgenerallytakesplaceunderwater(12).Nospecifictimeintervalhasbeenrecommended.Someusershaveutilizedfreeze-thawcyclestoconditionspecimenspriortomoisturetestinginLWTs(31).Duringactualmixingandcompactingoftestsamples,ithasbeensuggestedthatsamplesbeshort-termagedusingthe

Superpaveprotocols(14,32).Thisshort-termagingprocedureisbelievedtoagethemixturesimilartoagingthatoccursthroughfieldproductionandplacement.

SeveralresearchstudieshaveshownthatLWTscandifferentiatebetweenasphaltbindertypes

(7,8,14,32).ResearchershavecomparedidenticalaggregatesandgradationsbutusingdifferentbindergradesinLWTs.Whentestedatsimilartemperatures,mixescontainingstiffergradesofasphaltbinderwillprovidelowerrutdepths.RuttingtendstofollowtheG*/sin*ofthebinderwhentestedusingtheDynamicShearRheometer( 14,32).

AnothermixturecharacteristicthataffectsLWTresultsisnominalmaximumaggregatesize(2).

Foragivenaggregateandbindertype,mixeswithlargernominalmaximumaggregatesizegradationstendtoprovidelowerrutdepths.

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LWTRESULTSVERSUSFIELDPERFORMANCE

NumerousstudieshavebeenconductedtocompareresultsofLWTtestingtoactualfieldperformance.MostofthesestudieshavebeentorelateLWTrutdepthstoactualfieldrutting.

InthedevelopmentoftheGLWT,theresearchersusedfourmixesofknownfieldrutperformancefromGeorgia(2).Threeofthefourmixeshadshownatendencytorutinthefield.ResultsofthisworkshowedthattheGLWTwascapableofrankingmixturessimilartoactualfieldperformance.AsimilarstudyconductedinFlorida(5)usedthreemixesofknownfieldperformance.Oneofthesemixeshadverygoodruttingperformance,onewaspoor,andthethirdhadamoderatefieldhistory.Again,resultsfromtheGLWTwereabletorankthemixturessimilartotheactualfieldruttingperformance.

TheUniversityofWyomingandWyomingDepartmentofTransportationparticipatedinastudy

(11)toevaluatetheabilityoftheGLWTtopredictrutting.Forthisstudy,150-mmcoreswereobtainedfrom13pavementsthatprovidedarangeofruttingperformance.Resultsshowedthat

theGLWTcorrelatedwellwithactualfieldruttingwhenprojectelevationandpavementsurfacetypewereconsidered.Theeffectofelevationonrutdepthswasmostlikelyduetodifferentclimatesatrespectiveelevationintervals.

AftertheAPAcameonthemarket,theFloridaDepartmentofTransportationconductedastudy

(34)similartotheGLWTstudydescribedpreviously(5).Again,threemixesofknownfieldperformanceweretestedintheAPA.Withinthisstudy,however,beamsandcylinderswerebothtested.Resultsshowedthatbothsampletypesrankedthemixessimilartothefieldperformancedata.Therefore,theauthorsconcludedthattheAPAhadthecapabilitytorankmixesaccordingtotheirruttingpotential.

TheColoradoDepartmentofTransportationandtheFHWA'sTurnerFairbankHighway

ResearchCenterparticipatedinaresearchstudytoevaluatetheFRTandactualfield

performance(22).Atotalof33pavementsfromthroughoutColoradothatshowedarangeofruttingperformancewereused.TheresearchindicatedthattheFrenchruttingspecification(rutdepthoflessthan10percentofslabthicknessafter30,000passes)wastoosevereformanyofthepavementsinColorado.Byreducingthenumberofpassesforlow-volumeroadsanddecreasingthetesttemperatureforpavementslocatedinmoderatetohighelevations(i.e.,colderclimates),thecorrelationbetweentheFRTresultsandactualfieldruttingwasgreatlyincreased.

AnotherresearchstudybytheLCPCcomparedrutdepthsfromtheFRTandfieldrutting(37).

FourmixturesweretestedintheFRTandplacedonafull-scalecirculartesttrackinNantes,France.ResultsshowedthattheFRTcanbeusedasamethodofdeterminingwhetheramixturewillhavegoodruttingperformance.

TheFHWAconductedafieldpavementstudyatTurner-FairbankHighwayResearchCenter(38)

usinganacceleratedloadingfacility(ALF).HMAmixtureswereproducedandplacedoveranaggregatebaseonalineartestsection.ThreeLWTswereusedtotestmixesplacedontheALFinordertocompareLWTresultswithruttingaccumulatedundertheALF.ThethreeLWTsweretheFRT,GLWT,andHWTD.Baseduponthisstudy,theresultsfromtheLWTsdidnotalwaysrankthemixturessimilartotheALF.

AjointstudybytheFHWAandVirginiaTransportationResearchCouncil(15)evaluatedthe

abilityofthreeLWTstopredictruttingperformanceonmixturesplacedatthefull-scalepavementstudyWesTrack.ThethreeLWTsweretheAPA,FRT,andHWTD.Forthisresearch,10testsectionsfromWesTrackwereused.TherelationshipbetweenLWTandfieldruttingfor

allthree2LWTswasstrong.TheHWTDhadthehighestcorrelation(R2=0.91),followedbythe

APA(R=0.90)andFRT(R 2=0.83).

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Theonlystudyfoundintheliteraturedealingwithmoisturesusceptibilitywasconductedbythe

ColoradoDepartmentofTransportation(16).ThisstudycomparedresultsfromtheHWTDwithknownfieldperformanceintermsofstripping.TwentypavementsfromthroughoutthestateofColoradowereevaluated.TestresultsfromtheHWTDindicatedthatthestrippinginflection

pointandstrippingslopegenerallydistinguishedbetweengoodandpoorperformance.ThreestudiesbytheTexasDepartmentofTransportation(28,39,40)utilizedtheprototypeMMLS3todeterminetherelativeperformanceoftworehabilitationprocessesandestablishthepredictivecapabilityofthislaboratory-scaledevice.Forthefirsttwostudies,theMMLS3testedeightfull-scalepavementsectionsinthefieldadjacenttosectionstraffickedwiththeTxMLS.Fieldtestingcombinedwithadditionallaboratorytestingindicatedthatoneoftherehabilitationprocesseswasmoresusceptibletomoisturedamageandlessresistanttopermanentdeformationcomparedtothesecondprocess.Thissecondprocesswaslessresistanttofatiguecracking.Inaddition,acomparisonofpavementresponseunderfull-scale(TxMLS)andscaled(MMLS3)acceleratedloadingshowedgoodcorrelationwhenactualloadingandenvironmentalconditionswereconsidered.

AnongoingthirdstudyaimstotieMMLS3resultswithactualmeasuredperformanceoffoursectionsatWesTrack(28).Ahightestingtemperature(60C)wasselectedbasedonthecriticaltemperatureforpermanentdeformationduringa5-daytraffickingperiodduringwhichfailureoccurredforthreeofthefoursections(41,42).LimitedlaboratorytestingusingtheHWTDandtheAPAisalsoincludedinthisstudy,butonlytherankingsfromHWTDresultsshowgoodcorrelationwithactualperformance.ResultsindicatethattheMMLS3iscapableofcorrectlyrankingperformanceofthefourWesTracksections.

SUMMARY

Baseduponreviewofthelaboratorywheeltrackingdevicesandtherelatedliteraturedetailingthelaboratoryandfieldresearchprojects,thefollowingobservationsareprovided.

Bothcylindricalandbeamspecimens,dependinguponthetypeofwheeltrackingdevice,canbeusedtorankmixtureswithrespecttorutting.Resultsobtainedfromthewheeltrackingdevicesseemtocorrelatereasonablywellto

actualfieldperformancewhenthein-serviceloadingandenvironmentalconditionsofthatlocationareconsidered.

Thewheeltrackingdevicesseemtoreasonablydifferentiatebetweenperformancegradesofbinders.

Wheeltrackingdevices,whenproperlycorrelatedtoaspecificsite'strafficandenvironmentalconditions,havethepotentialtoallowtheuseragencytheoptionofapass/failor"go/nogo"criteria.Theabilityofthewheeltrackingdevicestoadequatelypredictthemagnitudeoftheruttingforaparticularpavementhasnotbeendeterminedatthistime.

Adevicewiththecapabilityofconductingwheel-trackingtestsinbothairandinasubmergedstatewilloffertheuseragencythemostoptionsofevaluatingtheirmaterials.

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loaded wheel testers in the united states: state of the practice

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