Creep of compacted recycled asphalt pavement

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  • Creep of compacted recycled asphalt pavement

    Chirayus Viyanant, Ellen M. Rathje, and Alan F. Rauch

    Abstract: Recycled asphalt pavement (RAP) can be beneficially used as fill in the construction of earth structures suchas embankments and retaining structures. Experiments were conducted to evaluate the creep response of compactedRAP under sustained deviatoric stresses. Constant stress, consolidated-drained triaxial tests were performed on 100 mmdiameter, compacted RAP specimens at multiple confining stresses and deviator stress levels. The test data displayedclassic creep behavior, with clearly identifiable primary and secondary creep observed in all specimens. Tertiary creepand creep rupture were observed in specimens tested at larger deviator stress levels. The creep response of RAP wassignificant at confining pressures less than about 272 kPa, while the creep response was less severe at larger confiningpressures. Upper yield stress levels, representing the deviator stress levels below which creep rupture does not occur,were identified and shown to be confining stress dependent. In general, the creep potential of RAP is significant andshould be considered in design. The developed creep models can be used to predict the time-dependent deformation ofearth structures utilizing RAP backfill.

    Key words: creep, recycled asphalt pavement, time-dependent behavior, rupture.

    Rsum : Les pavages dasphalte recycl (PAR) peuvent tre utiliss avec bnfice comme remblai dans la construc-tion de structures en terre, telles les remblais et les structures de soutnement. On a conduit des expriences pour va-luer la raction en fluage du PAR sous des contraintes dviatoriques soutenues. On a ralis des essais triaxiauxconsolids drains contrainte connstante sur des spcimens de PAR consolids de 100 mm de diamtre sous plusieurscontraintes de confinement et niveaux de contraintes dviatoriques. Les rsultats des essais ont montr un comporte-ment classique de fluage, avec fluage primaire et secondaire clairement identifiable dans tous les spcimens. Le fluagetertiaire et la rupture en fluage ont t observs dans des spcimens tests de plus hauts niveaux de contrainte dvia-torique. La raction en fluage du PAR tait significative des pressions de confinement infrieures environ 272 kPaalors que la raction en fluage tait moins svre des pressions de confinement plus leves. Les niveaux suprieursde contrainte de limite lastique, reprsentant les niveaux de contrainte dviatorique en dec desquels la rupture enfluage ne se produit pas, ont t identifis et se sont rvls dpendants de la contrainte de confinement. En gnral, lepotentiel de fluage du PAR est significatif et devrait tre pris en compte dans la conception. Les modles de fluage d-velopps peuvent tre utiliss pour prdire la dformation en fonction du temps des structures en terre utilisant un rem-blai de PAR.

    Mots-cls : fluage, pavage dasphalte recycl, comportement en fonction du temps, rupture.

    [Traduit par la Rdaction] Viyanant et al. 697

    Introduction

    Recycled asphalt pavement (RAP) is removed and (or) re-processed pavement material containing bituminous asphaltcement and aggregate. More than 73 million tons of RAP areprocessed each year in the United States (Kelly 1998) withmuch of it re-used in pavement construction. RAP is an at-tractive alternative for backfill because pavement demolitionmaterials can be re-used on site with minimal disposal costs.Additionally, in areas where select backfill is scarce, usingrecycled materials such as RAP can eliminate the need totransport select fill from a significant distance. Thus, there

    are economic and environmental incentives for using RAPas backfill in new construction. However, asphalt pavementis susceptible to creep under the large cyclic loads impartedon roadways, and RAP may also display creep under thesustained shear stresses found in embankments and retainedfill. This experimental study was undertaken to quantify thecreep response of RAP under deviatoric loading and to de-velop empirical models to predict creep deformations andcreep rupture for RAP. Constant axial stress, consolidated-drainedtriaxial tests were performed on 100 mm diameter, compactedRAP specimens. Tests were performed at multiple confiningstresses and at different deviator stress levels. Axial strains

    Can. Geotech. J. 44: 687697 (2007) doi:10.1139/T07-022 2007 NRC Canada

    687

    Received 8 August 2006. Accepted 14 February 2007. Published on the NRC Research Press Web site at cgj.nrc.ca on 20 July2007.

    C. Viyanant. Bechtel Corporation, 3000 Post Oak Boulevard, P.O. Box 2166, Houston, TX 77056-6503, USA.E.M. Rathje.1 University of Texas, Austin, TX 78712-1076, USA.A.F. Rauch. Fuller, Mossbarger, Scott & May Engineers, Inc., Lexington, KY 40511, USA.

    1Corresponding author (e-mail: e.rathje@mail.utexas.edu).

  • were recorded with time during the creep tests until creeprupture occurred or until 1 week had elapsed.

    Material description

    RAP is derived from demolished asphalt pavement and isgenerated by milling or full-depth removal of asphalt pave-ment. Milling involves the mechanical removal of up to50 mm of pavement in a single pass. Full-depth removal isusually achieved with a pneumatic pavement breaker or arhino horn on a bulldozer. The broken materials are trans-ferred to a central facility for a series of recycling processes,which include crushing, screening, conveying, and stacking.Asphalt pavement can also be pulverized in place and in-corporated into granular or stabilized base courses using aself-propelled pulverizing machine (FHWA 2000), whicheliminates the cost of transporting material to and from theprocessing facility. The processing practice generally yieldsRAP with a consistent gradation.

    The parent material for RAP is obviously the original as-phalt pavement. Asphalt pavement is a blend of aggregateand bituminous asphalt cement binder, with typical mixproportions ranging from 3% to 7% asphalt cement (Rob-erts et al. 1996). The processed RAP material contains ag-gregate particles that are coated with asphalt cement, suchthat asphalt cement is found at most of the grain contacts.Thus, while the properties of RAP are affected by tradi-tional geotechnical parameters such as in-place density,material gradation, and particle shape, the properties arealso affected by the presence and character of the asphaltcement binder. The asphalt cement binder is a hydrocarbonderived from the distillation of crude oil, and its properties(e.g., viscosity, ductility) are controlled by the type of vir-gin crude oil and the distillation process (Roberts et al.1996). AASHTO MP1a-04 (AASHTO 2004) provides stan-dard specifications for asphalt cement binders that arebased on achieving specific properties at specific tempera-tures, such that the appropriate asphalt cement performancegrade can be selected for the range of temperatures ex-pected in a region.

    For this study, a bulk sample of RAP was obtained from aTexas Department of Transportation stockpile within theCorpus Christi District. The asphalt cement content of theRAP was estimated using a nuclear gauge, which measuresthe hydrogen content of a material (ASTM method 4125-05)(ASTM 2005a). After correcting for the water content of theRAP, the asphalt cement content was estimated as 3.5%. Itwas not possible to determine the type of asphalt cementused in the parent hot-mix asphalt for the RAP used in thisstudy, but performance grades 7022 and 7622 are usedmost often in Texas.

    Figure 1 displays the grain size distribution of RAP sam-ples taken from four different locations in the RAP stock-pile. The grain size distribution of the RAP was veryconsistent in the stockpile. Less than 5% of the materialwas larger than 40 mm, and no particles larger than 75 mmwere observed. Only 2% of the material passes the No. 40sieve (0.425 mm), and there were no fines passing theNo. 200 sieve (0.075 mm). The Unified Soil ClassificationSystem (USCS) classification of this material is well-gradedgravel (GW). The gradation of the RAP across the stock-

    pile was consistent with gradations generated by commer-cial producers of RAP (Rathje et al. 2006) and generallymeets gradation specifications for earth structures such asretaining walls.

    Atterberg limit testing indicated that the RAP wasnonplastic, as the plastic limit could not be determined. Thespecific gravity (Gs) of RAP was determined by a weightedaverage of the measured values for particles larger than theNo. 4 sieve (ASTM method C127) (ASTM 2005b) and forparticles smaller than the No. 4 sieve (ASTM method D854)(ASTM 2005c). The weighted specific gravity was equal to2.33, based on measured values of 2.36 (ASTM 2005b) and2.28 (ASTM 2005c). This specific gravity is smaller thanthat for typical soil because it represents an effective specificgravity for the aggregate, asphalt cement coating, and voidsencapsulated by the asphalt cement.

    Creep behavior

    Deviatoric creep represents time-dependent shear defor-mations that occur under sustained shear stress (Mitchell1993). Figure 2a is a plot of axial strain versus time under aconstant deviator stress (d = l 3) for a soil experienc-ing creep. The curve displays three distinct regions of creepbehavior, primary, secondary, and tertiary creep, followed bycreep rupture (Mitchell 1993). Primary creep occurs imme-diately after application of the shear stress, and during thisstage the strain rate (slope of the straintime curve) de-creases with time. During secondary creep, the strain ratereaches a minimum value (min) and remains essentially con-stant over an extended period of time before the strain ratestarts to accelerate. This point of accelerating deformationrepresents the initiation of tertiary creep and leads to com-plete creep rupture at the end of the tertiary creep stage.

    Generally, creep is a significant c