Creep of compacted recycled asphalt pavement

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<ul><li><p>Creep of compacted recycled asphalt pavement</p><p>Chirayus Viyanant, Ellen M. Rathje, and Alan F. Rauch</p><p>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.</p><p>Key words: creep, recycled asphalt pavement, time-dependent behavior, rupture.</p><p>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.</p><p>Mots-cls : fluage, pavage dasphalte recycl, comportement en fonction du temps, rupture.</p><p>[Traduit par la Rdaction] Viyanant et al. 697</p><p>Introduction</p><p>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</p><p>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</p><p>Can. Geotech. J. 44: 687697 (2007) doi:10.1139/T07-022 2007 NRC Canada</p><p>687</p><p>Received 8 August 2006. Accepted 14 February 2007. Published on the NRC Research Press Web site at cgj.nrc.ca on 20 July2007.</p><p>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 &amp; May Engineers, Inc., Lexington, KY 40511, USA.</p><p>1Corresponding author (e-mail: e.rathje@mail.utexas.edu).</p></li><li><p>were recorded with time during the creep tests until creeprupture occurred or until 1 week had elapsed.</p><p>Material description</p><p>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.</p><p>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.</p><p>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.</p><p>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-</p><p>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.</p><p>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.</p><p>Creep behavior</p><p>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.</p><p>Generally, creep is a significant concern in clay soils dueto the viscous nature of clay minerals (Mitchell 1993).</p><p> 2007 NRC Canada</p><p>688 Can. Geotech. J. Vol. 44, 2007</p><p>Fig. 1. Gradation of RAP used in this study.</p></li><li><p>Nonetheless, creep has also been observed in sands, al-though the resulting creep deformations tend to be smallerthan those for clay (Augustesen et al. 2004). The mechanismof creep in RAP is related to the presence of the viscous as-phalt cement at the grain contacts. Because shear loads insoil are carried by friction at the grain contacts, the presenceof the viscous asphalt cement material found at the graincontacts will affect the shear stress and deformation re-sponse of RAP. Thus, although RAP is coarse grained, itexhibits creep deformations that are on the order of thoseobserved for clays.</p><p>Various models have been developed to describe the creepbehavior of soils. These models range from simple empiricalmodels to general elasto-visco-plastic models, as discussedby Liingaard et al. (2004). Empirical models are simple inthat they are fit to experimental data in an effort to mathe-matically describe the soil behavior, but they are limited tothe loading conditions applied in the laboratory experiments.Elasto-visco-plastic models are general stressstraintimemodels that can be applied to loading conditions beyond</p><p>those used to develop the experimental data, but they aresignificantly more complex than the empirical models. Thisstudy will focus on applying simplified empirical models tothe RAP creep data, in an effort to develop a general under-standing of the creep response of RAP as compared withother soils.</p><p>Singh and Mitchell (1968) developed a three parameterempirical model to describe the time-dependent deformationbehavior of soil during the primary and secondary stages ofcreep. The basic relationship developed by Singh and Mitch-ell (1968) to predict the time-dependent strain rate for agiven shear load is</p><p>[1] ( ) t A tt</p><p>Dm</p><p>= </p><p>e 1</p><p>where ( ) t is the strain rate as a function of time, D is thestress level (D = d df/ , where d is the deviator stress anddf is the deviator stress at failure), t is time, t1 is an arbi-trary reference time, and A, , and m are model parameters.Singh and Mitchell (1968) stated that eq. [1] is most suitablefor stress levels (D) from about 0.30 to 0.90, which arewithin the range of engineering interest. While eq. [1] hasmostly been used to describe the undrained creep-time re-sponse of clays, Singh and Mitchell (1968) indicated thatsimilar trends in creep behavior are observed in drained con-ditions and that the same functional model can be applied.</p><p>In eq. [1], the parameters A and predict the strain rate att = t1 as a function of the stress level (D), while the parame-ter m models the change in strain rate over time. Equa-tion [1] can be integrated with respect to time to deriveexpressions for strain as a function of time. Figure 2b showsschematically the variation of strain with log(time) for m 1.0. Distinctly different behaviors areobserved for these three ranges of m. For m &gt; 1.0 the straineventually stabilizes and approaches an asymptotic value.For m = 1.0 the strain increases linearly with log(time),while for m &lt; 1.0 the strain continuously increases with thesoil potentially reaching creep rupture. Thus, m is an impor-tant parameter for characterizing the creep potential of soils,with smaller values of m indicating high creep potential(Singh and Mitchell 1969). For clays, most m values reportedin the literature range from 0.7 to 1.2 (Augustesen et al.2004), although Singh and Mitchell (1969) indicate that val-ues as low as 0.4 are possible. For sands, less creep data isavailable, but most m values reported in the literature (e.g.,Murayama et al. 1984; Mejia et al. 1988) are close to 1.0. Iceand frozen soil are highly susceptible to creep, and m valuesof about 0.5 have been reported for these materials (Lacasseand Berre 2005). Although m is an important creep parameter,Lacasse and Berre (2005) emphasize that the actual value ofthe strain rate at a given stress level and time is also importantin understanding a materials creep susceptibility. Because Aand control strain rate values in the Singh and Mitchell(1968) model, these parameters should also be consideredwhen assessing the creep susceptibility of a material.</p><p>Creep rupture is defined as the failure of soil at the end oftertiary creep, where the axial strain becomes unbounded(Fig. 2a), and the time to rupture (tr) is the total elapsed timefrom the initiation of creep until final rupture. Traditionally,creep rupture has been treated separately from the Singh and</p><p> 2007 NRC Canada</p><p>Viyanant et al. 689</p><p>Fig. 2. (a) Time-dependent creep deformation under a constantstress level. (b) Effect of creep parameter m on predicted creepstrains (modified from Mitchell 1993).</p></li><li><p>Mitchell (1968) creep model. Saito and Uezawa (1961) de-veloped a relationship between log(tr) and log(min), wheremin is the minimum strain rate at the end of secondarycreep. This expression can be written using a power law</p><p>[2] log(tr) = log(b) a log(min)</p><p>where a and b are experimentally determined coefficients.Based on this relationship, the time to rupture (t...</p></li></ul>