Effect of Temperature on Geotechnical Properties ofRecycled Asphalt Shingle Mixtures

Ali Soleimanbeigi1; Tuncer B. Edil, Dist.M.ASCE2; and Craig H. Benson, F.ASCE3

Abstract: Shear strength, compressibility, and hydraulic conductivity of recycled asphalt shingles (RASs) mixed with bottom ash (BA) orstabilized with self-cementing fly ash (FA) were evaluated in a systematic manner at temperatures ranging from 5 to 35�C, representingseasonal field temperature variation. Increasing temperature reduced the shear strength and increased the compressibility and hydraulicconductivity of compacted RAS-BA and RAS-FAmixtures.When the temperature increased from 5 to 35�C, the effective friction angle (f9) ofthe compacted RAS-BAmixture containing 25%RAS decreased from 41 to 35�. The mixture containing 50%RASs decreased from 41 to 29�.Thef9 of the compacted RAS-FAmixture containing 20%FAdecreased from 46 to 26�; however, the effective cohesion (c9) increased from 45to 70 kPa, and the compressive strength remained higher than that of compacted sand. In contrast, the secondary compression ratio (Caɛ)increased with temperature. The Caɛ of the RAS-BA mixture is an exponential function of temperature. Thermal cycling induced thermalprecompression to the compacted RAS-BA and RAS-FA mixtures and significantly reduced the compressibility. Thermal precompressionreduced Caɛ for the compacted RAS-BA mixture containing 25% RASs from 0.0078 to 0.0004 and that of the RAS-FA mixture from 0.0016 to0.0002, which are comparable withCaɛ for compacted sand (Caɛ 5 0:0003). Therefore, to reduce long-term settlement of an embankmentmadewith materials containing RAS, construction is recommended during warm seasons. In this way, most settlement occurs during construction,and settlement during service life of the embankment becomes negligible. The hydraulic conductivity of the compacted RAS-BA and RAS-FAmixtures increased with increasing temperature, which is beneficial to drainage capacity of structural fills containing RASs. DOI: 10.1061/(ASCE)GT.1943-5606.0001216. © 2014 American Society of Civil Engineers.

Author keywords: Recycled asphalt shingle (RAS); Bottom ash (BA); Fly ash (FA); Temperature; Structural fill; Geotechnical properties.

Introduction

Approximately 11 million Mg of waste asphalt roofing shingles aregenerated each year in the United States. Of these, 10 million Mgconsist of tear-off roofing shingles, and 1 million Mg are factoryscraps [Vermont Agency of Natural Resources (VANR) 1999;Zickell 2003; Sengoz and Topal 2004; Townsend et al. 2007].Typical asphalt shingles are comprised of 16–35% asphalt binder,2–15% cellulose felt, 20–38% mineral granule/aggregate, and8–40% mineral filler/stabilizer (Krivit 2007; Townsend et al.2007). Asphalt shingle waste can be ground and used in hot mixasphalt (HMA), cold patch to repair pavements, supplemental fuelin the cement kiln dust industry, and temporary paving in ruralroads and trails. All of these applications, however, reuse only10–20%of the asphalt shinglewaste, and the remainder is landfilled(W. Tuley, personal communication, 2011). Identifying other potential

uses for recycled asphalt shingle (RAS) material is one means toincrease the fraction of reclaimed shingles that are reused.

Structural fill, including highway embankment fills or backfillbehind retaining walls, is a high-volume application that could uselarge quantities of RASs. Such use will contribute to a more sus-tainable roadway infrastructure by reducing energy and naturalresources consumption and greenhouse gas (GHG) emissions as-sociated with mining and production of conventional structural fillmaterials (Kilbert 2002; Gambatese and Rajendran 2005; Carpenteret al. 2007; U.S. EPA 2009; Lee et al. 2010). Soleimanbeigi et al.(2012, 2013) evaluated the engineering properties of RASs, RASsmixed with bottom ash (BA), and RASs stabilized with self-cementing fly ash (FA) at room temperature (T 5 22�C) andfound that the shear strength and drainage capacity of compactedRASs are comparable with those of compacted sand. [Friction angleand hydraulic conductivity of compacted RASs are, respectively,33� and 4:53 1024 cm=s, which are comparable with a frictionangle of 37� and hydraulic conductivity of 1:93 1023 cm=s forcompacted Wisconsin glacial outwash sand (GOS)]. However,RASs exhibit excessive compressibility, whereas compacted RAS-BA or RAS-FA mixtures with appropriate BA or FA contents havecomparable compressibility with that of compacted sand and aretherefore acceptable for structural fills. Therefore, the suitability ofRASs for inclusion in structuralfill was verified at room temperature.However, because RAS particles contain a viscous asphalt binder,which is sensitive to temperature (Roberts et al. 1996; West et al.2010; ASTM 2009), seasonal temperature changes may affect theengineering properties of the compacted RAS-BA and RAS-FAmixtures. Understanding the effect of typical field temperaturechange on geotechnical properties of RAS mixtures is important toensure stability, serviceability, and adequate drainage capacity ofstructural fills containing RASs. The effect of temperature on

1Research Scientist, Recycled Material Resource Center (RMRC-3G),Univ. of Wisconsin–Madison, Madison, WI 53706 (corresponding author).E-mail: soleimanbeig@wisc.edu

2Professor Emeritus and Director, Recycled Material Resource Center(RMRC-3G), Univ. of Wisconsin–Madison, Madison, WI 53706. E-mail:tbedil@wisc.edu

3Wisconsin Distinguished Professor, Director of Sustainability Re-search and Education, and Chair of Civil and Environmental Engineeringand Geological Engineering, Univ. of Wisconsin–Madison, Madison, WI53706. E-mail: chbenson@wisc.edu

Note. Thismanuscript was submitted onDecember 2, 2013; approved onSeptember 15, 2014; published online on October 14, 2014. Discussionperiod open until March 14, 2015; separate discussions must be submittedfor individual papers. This paper is part of the Journal of Geotechnical andGeoenvironmental Engineering, © ASCE, ISSN 1090-0241/04014097(14)/$25.00.

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engineering properties of soil in applications where temperature isexpected to influence the soil response has been investigated bya number of investigators (Campanella and Mitchell 1968; Slegeland Davis 1977; Mitchell et al. 1982; Hueckel and Baldi 1990; DeBruyn and Thimus 1996; Graham et al. 2001; Cekerevac and Laloui2004; Abuel-Naga et al. 2007).

This paper describes the effect of temperature on shear strength,compressibility, and hydraulic conductivity of the compacted RAS-BA and RAS-FA mixtures using a thermomechanical system thatwas specifically developed for this study. The thermomechanicalsystem included temperature-controlled triaxial cells, one-dimensional(1D) compression cells, and flexible wall permeameters. Typicalranges of temperature, confining pressure, and vertical stress instructural fills were applied to test specimens. Recommendationswere provided regarding the design of structural fills with RAS-BAor RAS-FA mixtures.

Materials

A bulk sample of RASs was obtained from B. R. Amon & Sons inElkhorn,Wisconsin. Visual inspection indicated that the RASs werefree of impurities, such as wood chips, plastic, and nails. Warner(2007) concluded that the compacted RASs with a maximum par-ticle size (dmax) of 10mm have higher dry unit weight (gd), stiffness,and strength. Therefore, in this study, the RASs were scaled to10 mm. Fig. 1(a) shows a photograph of the RAS articles. The RASparticles are plate-like, irregular in shape, highly angular, and haverough surface texture. The grain-size analysis following ASTMD422-63 (ASTM 1963) indicated that most RAS particles are sandsized (between 0.075 and 4.75 mm) according to the Unified SoilClassification System (USCS). Table 1 provides information on thegrain-size indexes and material classification. The RAS particleswere classified as well-graded sand (SW) following the USCS. The

Fig. 1. (a) RAS particles; (b) grain-size distribution of RAS and BA particles; (c) RAS-BA mixture; (d) RAS-FA mixture

Table 1. Physical Properties of RAS, BA, and GOS

Materiala d10 (mm) d50 (mm) Cu Cc Fines (%) USCS classification wopt (%) gdmax (kN=m3) Gs

RAS 0.17 1.1 7.6 1.6 3.8 SW 8.0 11.3 1.74BA 0.19 0.9 6.3 0.8 1.9 SP 2.5 15.1 2.67GOS 0.21 0.5 3.1 0.8 0.0 SP 2.0 18.3 2.71RAS-BA (50–50) — — — — — — 7.4 12.8 —

RAS-BA (25–75) — — — — — — 6.3 13.5 —

RAS-FA (80–20) — — — — — — 11.0 13.8 —

Note:Cc 5 coefficient of curvature [d230=ðd10 3 d60Þ]; Cu 5 coefficient of uniformity (d60=d10); d10 5 effective particle size (particle size for which 10% of thesample is finer than d10); d50 5 average particle size (particle size for which 10% of the sample is finer than d50); Gs 5 specific gravity.aNumbers in parentheses indicate relative weight-based proportion in the mixtures.

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specific gravity (Gs) of the RASs, measured in accordance withASTMD854-00 (ASTM 2000b) (Method B), is 1.74. The lowGs ofthe RASs is attributed to the presence of asphalt binder and cellulosefibers, which byweight compose 18–40% of the RASs (Krivit 2007;Townsend et al. 2007).

The BA and self-cementing FA, both industrial byproducts, wereused to reduce compressibility of the RASs (Soleimanbeigi et al.2012, 2013, 2014) while enhancing sustainability of the mate-rials. Relatively incompressible granular BA achieves reduction incompressibility by reducing the relative amount of compressibleRASs in the mixture, whereas FA acts as a binder that stabilizes theRASs. The BA and FAwere obtained fromColumbia Power Stationin Portage, Wisconsin. The BA is a granular material with angularparticles that are internally porous and have rough surface texture.Figs. 1(c and d) show the RAS-BA and RAS-FA mixtures. Tocompare engineering properties of the RAS-BA and RAS-FAspecimens with those of a conventional granular material, a sam-ple ofWisconsin GOSwas used in this study. The grain-size indexesandGs of the BA andGOS are provided in Table 1. The BA particleshave a USCS classification for poorly graded sand (SP). The dif-ference in the grain-size distributions of the RASs and BA is small;therefore, the grain-size distribution of different RAS-BA mixtureswould be expected to fall within a narrow range [Fig. 1(b)]. TheGOSsample, classified as SP, was compris ed of subround to roundparticles with smooth surface texture (Soleimanbeigi et al. 2012).

Compaction tests for the RASs and BA and several RAS-BA andRAS-FAmixtures were conducted following standard Proctor effort(ASTM 2000a, Method B). The maximum dry unit weight (gdmax)

and optimum water content (wopt) of the materials are provided inTable 1. The RASs have a well-defined, albeit shallow, compactioncurvewith amaximumdry unitweight (gdmax) of 11:3 kN=m

3 and anoptimum water content (wopt) of 8%. Blending BA with the RASsresults in an increase in gd and reduces the sensitivity to water content.Blending the RASs with 20% FA increases gdmax to 13:8 kN=m

3 andwopt to 11%. The compacted RAS-FA mixture with 20% FA hassimilar compressibility to that of compacted sand, and therefore wasstudied herein for thermal effect.

Methods

Thermomechanical Testing System

The laboratory tests were conducted at temperatures similar to thoseexpected in the field. Variation of mean ground temperatures acrossthe United States and the seasonal temperature variation were usedto estimate the potential seasonal temperature range in embankmentfills in the contiguous United States (Soleimanbeigi 2012). Thetemperatures in embankment fills were estimated to range from25 to 35�C, depending on the season and locality. However, to avoidwater freezing in the testing apparatus and testing frozen materials,which are very stiff, the minimum temperature used was 5�C.

Temperature-controlled triaxial compression, 1D compression,and flexible-wall hydraulic conductivity tests were conducted inconventional cells equipped with a heating and cooling system thatmaintained constant temperature between 5 and 35�C. A schematic

Fig. 2.Thermomechanical system: (a) temperature-controlled triaxial cell; (b) temperature-controlled 1D compression cell; (c) temperature-controlledpermeameter

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of the system is shown inFig. 2.Acoil of copper tubing (6-mmoutsidediameter) was wound around the specimen to circulate heated orcooled water. A 30-mm separation distance was maintained betweenthe copper coil and the specimen in the triaxial cell to avoid contactduring shearing. The temperature-controlled 1D compression cellconsisted of a stainless steel ring, which was placed inside a PVCcylinder filled with water to conduct heat from the copper coil to thespecimen.A10-mmdistance between the ring and the copper coilwasused in the cell. The same copper coil was wound at a distance of 10mm around the specimen in the flexible-wall permeameter.

Test temperatures above room temperature were obtained bycirculating heated water in the coil using a pump placed outside of aheating water bath. Tygon tubing (ThermoFisher Scientific, Fitch-burg, Wisconsin) was used to connect the pump to the coil tominimize temperature loss during water circulation. Temperaturesin thebath (Tb), in the cell chamber (Tc), and inside the specimen (Ts)were measured using three Type K thermocouples. A LabVIEW 8.5program was used to control heating in the bath; therefore, thetemperature of the test specimen could bemaintainedwithin60:5�Cof the target temperature.

Temperatures less than room temperature were obtained by cir-culating water from a cooling bath (PVC box filled with ice inequilibriumwithwater). The flow rate from the pumpwas controlledby a program written in LabVIEW so that the temperature of thespecimen was maintained within60:5�C of the target temperature.

Because insertion of the thermocouples inside the specimensdisturbs them, after the initial calibration tests, a correlation betweenthe Ts and Tc was obtained to estimate the specimen temperaturefrom the cell temperature. The required time to bring Ts to the targettemperature was approximately 100 min in the triaxial and 1Dcompression cells and 240 min in the permeameter.

Specimen Preparation

Details of the testing program are provided in Table 2. For each test,mixtures of RAS-BA or RAS-FAwere compacted atwopt and 95% ofgdmax. The specimens for triaxial compression tests were compactedinfive layers in a splitmoldwith74-mminsidediameter (i.d.) and148-mm height. Specimens for 1D compression tests were compacted inthree layers in stainless steel rings with 101-mm i.d. and 46-mmheight, whereas the specimens for hydraulic conductivity tests werecompacted in a split mold with 150-mm i.d. and 116-mm height

followingASTMD5084-03 (ASTM1995). The number of tamps perlayer using a standard Proctor hammer (ELE International, Loveland,Colorado) was determined by trials to achieve the target density.

The compacted RAS-FA specimens were carefully removedfrom the molds, wrapped using plastic sheeting to keep the watercontent constant according to ASTM C593-06 (ASTM 2006), andcured in a 100% humidity room at 22�C for 28 days.

Test Procedure

Triaxial CompressionAfter assembling the temperature-controlled triaxial system, eachspecimen was backpressure-saturated according to ASTMD4767-04(ASTM 2004) so that a B value greater than 95% was attained. Eachspecimen was then consolidated for 24 h at an applied effective con-fining pressure (i.e., s395 35, 70, or 140 kPa) at room temperature.Because of relatively high hydraulic conductivity of both RAS-BAand RAS-FA specimens (Soleimanbeigi 2012), pore-water pressurequickly dissipated after s39 was applied. The specimen volume changeduring isotropic consolidation was monitored in the backpressureburette until no further significant volume changewas observed.Afterconsolidation, the specimen temperature was brought to the targettemperature (i.e., 5 or 35�C) over 100 min. Axial loading was thencarried out at an axial strain rate of 3:0%=h, which was considered toprovide a drained condition during loading based on computationsmade using the pore-water expulsion rate during consolidation. Thevolume change of each specimen during shearing was recorded fromthe water elevation in the graduated backpressure burette.

1D CompressionThe RAS-BA and RAS-FA specimens were compressed under threetarget vertical effective stresses (i.e., sv95 50, 100, and 200 kPa),which represent typical road embankment overburden pressuresestimated based on the average highway embankment height of4.5m (Wright 1996) and the unitweight of the compacted sand. Eachspecimen was loaded at room temperature from 12.5 kPa up to thetarget sv9, with a load-increment duration (LID) of 24 h and load-increment ratio (LIR) of 1. At a given sv9, the target temperature wasinduced to the specimen for at least 10 days. The compressibility ofeach RAS-BA or RAS-FA specimens under an applied sv9 wasevaluated at 5, 22 (i.e., room temperature), and 35�C (Table 2). Tocompare the compressibility of the compacted RAS-BA mixtures

Table 2. Thermal Test Program for Mechanical Properties of RAS-BA Mixtures and FA-Stabilized RAS

Type of test Materiala s39 or sv9 (kPa)b T (�C) Number of tests

Triaxial compression RAS-BA (50–50) 35, 70, 140 5, 22, 35 9RAS-BA (25–75) 35, 70, 140 5, 22, 35 9BA 35, 70, 140 5, 22, 35 9FA-stabilized RASc 35, 70, 140 5, 22, 35 9

1D compression RAS-BA (50–50) 50, 100, 200 5, 22, 35 9RAS-BA (25–75) 50, 100, 200 5, 22, 35 9BA 50, 200 5, 35 4FA-stabilized RAS 50, 200 5, 22, 35 6RAS-BA (25–75)d 200 35–22 1FA-stabilized RASd 200 35–22 1RAS-GOS (50–50) 50, 100, 200 22, 35 6RAS-GOS (25–75) 50, 100, 200 22, 35 6

Hydraulic conductivity RAS-BA (50–50) 35, 70, 140 5, 22, 35 9FA-stabilized RAS 35, 70, 140 5, 22, 35 9

aNumbers in parentheses indicate relative weight-based proportion in the mixtures.bs39 in triaxial compression and hydraulic conductivity tests, sv9 in 1D compression tests.cContains 20% self-cementing FA.dThermal cycle tests.

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with that of the compacted RASs mixed with natural sand, 1Dcompression tests were also performed on compacted RAS-GOSmixtures at 22 and 35�C.

To evaluate the effect of thermal cycle on compressibility, aRAS-BA specimen with 25% RASs was compacted at 35�C insidethe 1D compression ring and incrementally loaded (LID5 24 h,LIR5 1) from sv95 12:5 to 200 kPa at 35�C. The specimen tem-peraturewas then reduced to room temperature, and the compressiontest was continued for 3 weeks. A thermal cycle test was alsoconducted on a RAS-FA specimen. Temperature of the RAS-FAspecimen compressed for 10 days at 35�C and sv95 200 kPa wasreduced to room temperature, and the compression test was con-tinued for 5 weeks.

Hydraulic ConductivityThe procedures for saturation and consolidation of the specimens inthe temperature-controlled flexible wall permeameter were identicalto those in triaxial compression tests. The volumetric change ofthe each specimen was measured during heating and cooling. Thefalling head–rising tail hydraulic conductivity tests were conducted

on the RAS-BA andRAS-FA specimens at 5, 22, and 35�C (Table 2).The hydraulic gradients of 0.5 and 2.0 were, respectively, applied toRAS-BA and RAS-FA specimens per ASTM D5084-03 (ASTM1995) recommendation.

Results

Shear Strength

RAS Mixed with BAFigs. 3(a and b) show the stress-strain and volumetric changebehavior of the compacted BA. The change of temperature from 5 to35�Chad negligible effect on the stress-strain and volumetric changebehavior. However, as the RAS content in the compacted RAS-BAmixture increased to 25%, increasing the temperature from roomtemperature (22�C) to 35�C, the peak deviator stress (sdf9 ) was re-duced and the axial strain corresponding to sdf9 ðɛf Þ of the RAS-BAspecimens was increased [Fig. 3(c)]. As illustrated in Fig. 3(d),rising temperature also made the volumetric change behavior more

Fig. 3. (a and b) Effect of temperature on stress-strain and volumetric change behavior of compactedBA; (c and d) effect of temperature on stress-strainand volumetric change behavior of the compacted RAS-BAmixture with 25%RASs; (e and f) modes of failure of the compacted RAS-BAmixtures atdifferent temperatures

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contractive. During shearing, the specimen bulged without de-veloping a clear failure plane [Fig. 3(e)]. On the other hand, whenthe specimen temperature was reduced to 5�C, the sdf9 increased andthe volumetric behavior became less contractive. Shearing of thespecimen occurred along a clear failure plane as shown in Fig. 3(f).The observed stress-strain and volumetric change behavior of theRAS-BA specimens at different temperatures are similar to thoseobserved for clay (Burghignoli et al. 2000; Cekerevac and Laloui2004; Abuel-Naga et al. 2007) but with different mechanisms thataffect the interactions between the particles.

The sensitivity of the stress-strain and volumetric change be-havior of the RAS-BA specimens with respect to temperaturechange is attributed to the presence of the asphalt binder in RASparticles. The viscosity of asphalt binder in RAS is reduced withincreasing temperature (Roberts et al. 1996; ASTM 2009). Becausethe applied shear stress is sustained by friction between the particles,presence of viscous asphalt binders at the contact surfaces betweenRAS particles and also RAS-BA particles is postulated to makethe shear strength and compressibility of the RAS-BA specimenssensitive to temperature change. At reduced viscosity, three com-pression mechanisms are expected to accelerate under an applieddeviator stress (sd9), considering the particle composition of asphaltshingles: (1) shear strain at the contact surfaces between the RASparticles andRAS-BAparticles, (2) deformability of individualRASparticles in the mixture, and (3) penetration of BA particles orgranules (separated from RAS particles) into the asphalt bindermaterial in RASs. Expedition of these mechanisms can explain theincrease in compressibility of the RAS-BA specimen; therefore, ata given applied sd9, the axial strain of the specimen increases. On theother hand, when the specimen temperature is reduced, the viscosityof the asphalt binder in the RAS particles is expected to increase andslow down these compression mechanisms in the RAS-BA speci-mens. Therefore, at a given applied sd9, the axial strain of thespecimen is reduced as illustrated in Fig. 3(c). Increased viscosity ofasphalt binders in RAS increases the stiffness and rolling tendency ofRAS particles over each other; therefore, the RAS-BA specimenstend to exhibit dilative behavior during shearing.

The effective friction angle (f9) and cohesion intercept (c9) ofcompactedBAandRAS-BAmixtures at different temperatureswereobtained from Mohr-Coulomb failure envelopes. Fig. 4(a) showsvariation of f9 with temperature. Although the f9 of BA is es-sentially insensitive to temperature as expected from the stress-strainresponse, the f9 of the compacted RAS-BA mixture containing 25%RASs is reduced from 41 to 36� (reflecting 12% reduction) when thetemperature rises from 5 to 35�C. Increasing RAS content in themixture makes thef9 evenmore sensitive to temperature. Thef9 forthe mixture containing 50% RASs is reduced by 27% from 41 to 29�.Fig. 4(b) shows the variation of c9 with temperature. The slightdevelopment of c9 in the RAS mixture after compaction is possiblycaused by bonding between the RAS and RAS-BA particles by theasphalt binder constituent in the RASs. The existence of bondingwas evident by stability of the RAS-BA specimens after dismantlingof the split mold. Each compacted RAS-BA specimen had enoughstrength to put the membrane on without deploying suction. In-creasing temperature also reduces c9 of the mixtures as shown inFig. 4(b). The reduction rate beyond 22�C is higher for the mixturewith higher RAS content. When the temperature increases from 5 to35�C, the c9 for the mixture with 25% RASs is reduced by ap-proximately 20% (from approximately 25 to 20 kPa), while for themixture with 50%RASs, c9 is reduced by approximately 50% (fromapproximately 30 to 15 kPa). Increasing temperature generallycorresponds to a reduction of shear strength (both f9 and c9) forbituminous mixtures primarily because of the reduction of asphaltbinder viscosity (Chen et al. 2006;Wang et al. 2008; Jun et al. 2009).

Overall, the friction angle of the RAS-BA specimensmostly remainsabove 30�, which is within the range off9 for compacted sandy soils(U.S. Navy 1986; Holtz et al. 2011; Andersen and Schjetne 2013).Therefore, it is considered sufficient to provide stability for the sideslopes of highway embankments, which are typically flatter than3H:1V (Wright 1996). The slight developed c9 is neglected for amore conservative design. In general, seasonal temperature changein the United States does not appear to endanger stability of typicalhighway embankments containing RASs up to 50%.

RAS Stabilized with FAThe engineering properties of FA-stabilized RAS (or RAS-FAspecimens) at room temperature are adequate for use as structuralfill (Soleimanbeigi et al. 2013). The effect of temperature change onthe stress-strain and volumetric change behavior of the RAS-FAspecimens is similar to those of the RAS-BA specimens. Increasingtemperature reduced sdf9 and increased ɛf while making the volu-metric change behavior more contractive. Fig. 5(a) shows thevariation of f9 and c9 of the RAS-FA specimens with temperature.When the temperature increases from 5 to 35�C, f9 noticeablyreduces by 43% from 46 to 26�; however, c9 increases from 44 to 71kPa. The reduction in f9 is attributed to the reduction of viscosity ofasphalt binders in RASs, which increase the shear strain and reducesthe friction resistance at the particle contact surfaces. The increase ofc9 is attributed to the acceleration of hydration of self-cementing

Fig. 4. (a) Variation of effective friction angle and (b) cohesion in-tercept of compacted RAS-BA mixtures with temperature

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FA when the temperature increases (Rao and Shivananda 2005;Veisi et al. 2010), which creates stronger bonds between the RASparticles in the RAS-FA specimens. Fig. 5(b) compares sdf9 of theRAS-FA specimens with that of the compacted GOS specimens thatwere isotropically compressed at s395 70 kPa and sheared at dif-ferent temperatures. Increasing the temperature from 5 to 35�C hada negligible effect onsdf9 of the compacted GOS (2056 6 kPa), but itreducedsdf9 of theRAS-FAspecimens by40%(from580 to 340 kPa).However, sdf9 of the RAS-FA specimens remained consistentlyhigher than that of the compacted GOS. The shear strength of theRAS-FA mixture in a typical field temperature range thereforeremains adequate to provide stability for use as structural fill inhighway embankments or backfill.

1D Compression

RAS Mixed with BAFig. 6 presents the variation of vertical strain (ɛv) with time (t) ofthree replicate RAS-BA specimens containing 25% RASs, com-pressed one-dimensionally under sv95 200 kPa and at three dif-ferent temperatures. After the initial compression for 24 h, thespecimen heated to 35�C and exhibited higher ɛv compared with areplicate specimen compressed at room temperature. On the otherhand, ɛv was significantly decreased when the specimen was cooledto 5�C. Sensitivity of the viscosity of asphalt binders in RAS par-ticles with temperature is postulated to change the compressibility ofthe RAS-BA specimens at different temperatures. The mechanismsresponsible for this behavior at the particle level are considered to be

the same as those described for shear behavior. Increasing tem-perature generally corresponds to a significant reduction of stiffnessand an increase of permanent deformation for bituminous mixtures(Tayebali et al. 1994; Sondag et al. 2002; Palit et al. 2004; Fu andHarvey 2007; Cao et al. 2009) caused by reducing the asphalt binderviscosity. The increased compressibility with increasing tempera-ture was also observed in peat and clay (Campanella and Mitchell1968; Plum and Esrig 1969; Fox and Edil 1996; Hanson 1996;Delage et al. 2000) but with different mechanisms than those in thecompacted RAS-BA mixtures or other bituminous material.

The data points in the ɛv-t curves (Fig. 6) can be differentiatedwith respect to time (t) (after the initial 24-h compression), to obtainthe vertical strain rates ( _ɛv 5Dɛv=Dt). The vertical strain rates werecalculated at three vertical stresses (i.e., sv95 50, 100, and 200 kPa).At a given sv9, the strain rates were calculated at three target tem-peratures and different elapsed t. Fig. 7(a) shows the variation of _ɛvin the log scale with temperature at different elapsed t; _ɛv decreaseswith t after inducing the target temperature. The _ɛv log-linearlyincreases with temperature, with identical slopes at different t. Ata given t, the slopes of the log _ɛv-T curves are also identical atdifferent sv9 for the RAS-BA specimens as shown in Fig. 7(b) fort5 1,000 min. The slope of the log _ɛv-T curves is defined asa thermal coefficient of compression and is denoted as CTɛ, with theunit of per degree Celsius (1/�C). Based on Figs. 7(a and b), CTɛ isobserved to be independent of stress and elapsed time and thereforeis regarded as an inherent property of a compactedRAS-BAmixture.The CTɛ, however, changes with different mixture compositions.Fig. 7(c) shows that CTɛ increases with increasing RASs content inthe mixture. Increasing the temperature-sensitive asphalt binder ofRASs in the compacted RAS-BAmixture promotes the compressionmechanisms described in the previous section. The equation of thebest fitting line over the data points in Fig. 7 is given by

d ln _ɛvdT

¼ CTɛ (1)

Compression of the compacted RAS-BA mixtures in an em-bankment of a large lateral extent can be considered as 1D. Settle-ments caused by lateral deformation are generally small comparedwith the settlements caused by consolidation and secondary com-pression of foundation soil, elastic settlement, and long-term creep

Fig. 5. (a) Variation of effective friction angle and cohesion withtemperature; (b) deviator stress at failure of the RAS-FA specimen withtemperature

Fig. 6.Variation of ɛv with time for the RAS-BA specimens at differenttemperatures

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of the fill mass. If the factor of safety against external instabilityduring construction remains greater than 1.4, settlement caused bylateral deformation is likely to be less than 10% of the end-of-primary settlement (Terzaghi et al. 1996). The long-term settlementof a wide embankment can be estimated using the compressibilityparameters obtain from 1D compression tests and the followingrelationship:

s ¼ Pni51

si ¼ Pni51

hiCaɛ,i log

�tto

�(2)

where s 5 total embankment settlement; si 5 settlement of a layerwith thickness hi; n5 number of layers (H5 nhi,H5 embankmentheight); Caɛ,i 5 secondary compression ratio of the ith layer; t5 time; and to 5 an arbitrary reference time.

The secondary compression ratio is most commonly defined as(Ladd et al. 1977)

Caɛ ¼ dɛvd log t

(3)

The Caɛ value of each RAS-BA specimen, compressed at a targetsv9 and temperature, was determined from the ɛv-log t curve overa 1,000-min period after inducing the temperature. This Caɛ wasnearly constant over 1,000 min and chosen as an index of thetemperature effects. The data points in Fig. 8(a) show the variationofCaɛ with temperature at different sv9 for the compacted RAS-BAmixture containing 25% RASs. At a given sv9, the measured Caɛ

increases with increasing temperature. Similar behavior was alsoobserved for RAS-BA specimens containing 50% RASs. FromEq. (3), the strain rate is obtained as

_ɛv ¼ Caɛ ln 10t

(4)

Substitution of Eq. (4) into Eq. (1) obtains

D lnCaɛ ln 10

tDT

¼ CTɛ → lnCaɛ ln 10

t

����T2 ln

Caɛ ln 10t

����To

¼ CTɛDT→ ln

�Caɛ,T

Caɛ,To

�¼ CTɛDT

Therefore

Caɛ,T ¼ Caɛ,ToeCTɛðT2ToÞ (5)

where To 5 reference temperature; and Caɛ,To 5 secondary com-pression ratio measured at To. Eq. (5) indicates that Caɛ for theRAS-BA specimens is an exponential function of temperature change;therefore, the rate of settlement of an embankment fill constructedwith the compacted RAS-BA mixture varies at different seasons.Parameter CTɛ is an indicator of the degree of temperature de-pendency of Caɛ . The variation of CTɛ with the RAS content wascalculated from the slopes of log _ɛv-T curves in Fig. 7(c) and isshown in Fig. 8(c). Temperature was also demonstrated to affect thedeviator creep strain rate of the compacted RAS-BA mixture (be-cause of a constant deviator stress present at the embankment sideslopes) as an exponential function (Soleimanbeigi et al. 2013).

Fig. 8(a) also shows that at a given temperature, Caɛ increaseswith increasing sv9. An increase of Caɛ with sv9 has also been ob-served in clay (Mesri and Castro 1987), peat (Fox and Edil 1996),and sand (Mesri and Vardhanabhuti 2009). Increasing the appliedvertical stress appears to increase the microshear stresses at thecontact surfaces between the RAS and RAS-BA particles, whichexpedites the shear strain along the particle contact surfaces.Soleimanbeigi (2012) showed that in the 1D compression tests onthe RAS-BA specimens, _ɛv log-linearly increases with sv9 [Fig. 8(b)].The slope of the log _ɛv-logsv9 curves, denoted as m, was defined asthe stress coefficient of compression and is independent of elapsedtime after loading. The log _ɛv-logsv9 curves have identical slopes atdifferent t as shown in Fig. 8(c) and can be expressed as

d ln _ɛvd lnsv9

¼ m (6)

Substitution of Eq. (4) into Eq. (6) and a simple manipulationobtains

Fig. 7.Variation of the strain rate of compactedRAS-BAmixtureswithtemperature

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Caɛ ¼ Caɛo

�sv9svo9

�m

(7)

where svo9 5 a reference vertical stress; and Caɛ,vo 5 secondarycompression ratio measured at svo9 . Eq. (7) is also valid for RASsmixed with other types of granular material, including foundryslag (FS) and GOS (Soleimanbeigi 2012). Parameter m is an in-dicator of the degree of stress dependency of Caɛ and depends onthe RAS content in the mixture. Fig. 8(c) shows the variation ofm with the RAS content in the compacted RAS-BA mixture.Practical implication of Eq. (7) is reflected in Eq. (2), indicatingthat a granular embankment fill containing RASs settles at dif-ferent rates along the embankment elevation. Combining Eqs. (5)and (7) obtains

Caɛ ¼ CaɛoeCTɛðT2ToÞ

�sv9svo9

�m

(8)

Eq. (8) is an empirical relationship that estimates Caɛ of the RAS-BA specimens at T and sv9 from the measured Caɛo at To and svo9 .The predicted Caɛ values as a function of temperature and sv9using Eq. (8) are plotted in Fig. 8(a) from a single data point ofCaɛo 5 0:0029, measured at To 5 22�C and svo9 5 100 kPa for theRAS-BA specimen containing 25% RASs. The predicted Caɛ-Tcurves capture the variation of measured Caɛ with temperature andsv9. The same CTɛ and m obtained from 1D compression tests on the

RAS-BA mixtures were used for prediction of Caɛ of the com-pacted RAS-GOS mixtures. Fig. 9 shows the predicted versusmeasured Caɛ for the compacted RAS-BA and RAS-GOS mix-tures. Because Caɛ exponentially increased with temperature, thescale of the graph was changed to a logarithm to clearly show the

Fig. 8. (a) Variation of Caɛ of the RAS-BA specimens with temperature at different sv9; (b) variation of strain rate with sv9; (c) variation of CTɛ and mwith RASs content in the RAS-BA specimen

Fig. 9. Predicted versus measured Caɛ for compacted RAS-BA andRAS-GOS mixtures

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range of the Caɛ values. The coefficient of determination, R2, is0.96, which shows that Eq. (8) provides a significant represen-tation of experimental Caɛ for the RAS-BA and RAS-GOSmixtures independent of the type of granular material used inthe mixture and can be recommended for the prediction of Caɛ ofgranular structural fills containing RASs at different temperaturesand stress levels.

The Caɛ values at intermediate temperatures were interpolatedfrom themeasuredCaɛ-T data points and plotted versusRAS contentin Fig. 10. Fig. 10 allows for estimation of the required RAS contentin the mixture based on a desired Caɛ (i.e., long-term settlementlimit), field temperature, and stress level. Previous research resultsshowed that compressibility of the compactedBA is greater than thatof the compacted natural sand (Soleimanbeigi 2012). The acceptableBA content (.50%) that makes the compressibility of the com-pacted RAS-BA mixture appropriate for structural fill can be safelyreplaced by conventional granular material. The results obtainedherein can be used conservatively for the compacted RASs mixedwith sands and gravels.

Effect of the Thermal Cycle on CompressibilityTemperature rise increased compressive strain of RAS-BA speci-mens in 1D compression tests (Fig. 6). The higher compressivestrain results in lower void space in the specimen. Therefore, whenthe temperature is reduced back to room temperature, the specimenthat was compressed at a temperature higher than room temperatureis expected to have higher stiffness and lower compressibilitycompared with the replicate specimen compressed just at roomtemperature throughout the test. Fig. 11 shows vertical strain versustime of twoRAS-BA specimens containing 25%RASs, which werecompacted and incrementally loaded to sv95 200 kPa. Specimen 1was loaded at room temperature, and Specimen 2 was loaded at35�C. The temperature of Specimen 2 was then reduced back toroom temperature 24 h after the target sv95 200 kPa was applied,whereas the compression test was continued. As shown in Fig. 11,under each incremental sv9, Specimen 2 exhibited higher verticalstrain than Specimen 1. Therefore, at the target sv9, Specimen 2 hadlower void space compared with Specimen 1. The measuredCaɛ forSpecimen 2 over 3 weeks at sv95 200 kPa was 0.0004, which isonly 5% of the corresponding Caɛ 5 0:0078 of Specimen 1. Thisimplies that temperature rise induced the thermal precompressionto Specimen 2 and reduced Caɛ to as low as that for WisconsinGOS (Caɛ 5 0:0003) and other low compressible granular material(Table 3). Thermal precompression also reduced deviator strain andstrain rate of the compacted RAS-BA and RAS-FA mixtures underconstant applied deviator stress (Soleimanbeigi et al. 2013). Asa result, construction of embankments containing RASs is rec-ommended during warm seasons. At higher temperatures, viscosityof asphalt binders in RAS particles is reduced, and the void space ofthe fill containing RASs reduces at a higher rate. Therefore, mostcompression occurs during construction, and the potential forsettlement during the following seasons is significantly reduced.Compaction temperature also affected density, strength, and de-formational behavior of bituminous mixtures. The increase ofcompaction temperature reduces the air void in the asphalt mixture(Azari et al. 2003; Lee et al. 2007;Akisetty et al. 2009) and increasesthe shear modulus (Azari et al. 2003; Sanchez-Alonso et al. 2012).Thermal precompression has also been shown effective in reduc-ing compressibility of other materials, such as seafloor sediments(Houston et al. 1985) and peat (Edil and Fox 1994; Fox and Edil1996; Hanson 1996). However, these materials involved differentcompression mechanisms at elevated temperatures than those forthe compacted RAS-BA mixtures.

RAS Stabilized with FAFig. 12 shows compression curves of three replicate RAS-FAspecimens compressed at sv95 200 kPa. After 24 h of compres-sion, the temperature of each specimen was brought to the targettemperature. At 5 and 22�C, the vertical strain increases with the

Fig. 10. Variation of Caɛ with RASs content at different temperaturesand sv9

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logarithm of time at a constant rate. However, when the temperaturerises to 35�C, the vertical strain rapidly increases and exhibitsa nonlinear response with respect to the elapsed time. Unlike theRAS-BA specimens, the log _ɛv-T relationship for the RAS-FAspecimens is linear only immediately after the temperature changeand exhibits nonlinear behavior at subsequent elapsed times. Thecoefficient of thermal compression, CTɛ, immediately after the tem-perature change is 0.168 1/�C. The predictedCaɛ using Eq. (6) agreedwell with the measured Caɛ.

The temperature of the specimen heated to 35�C for 10 days wasreduced to 22�C, and the compression was continued for an addi-tional 5 weeks. The measured Caɛ at a reduced temperature was0.0002, which is 12% ofCaɛ for the RAS-FA specimen isothermallycompressed at 22�C (Fig. 12). The thermal cycle induced thermalprecompression to the RAS-FA specimen and reduced Caɛ to aslow a value as that for compacted Wisconsin outwash sand(Caɛ 5 0:0003) (Table 3). As a result, construction of RASs sta-bilized with FA is recommended during warm seasons. This way,most of the compression occurs during construction; therefore,negligible settlement is expected during the following seasons.

Hydraulic Conductivity

The hydraulic conductivities of the RAS-BA and RAS-FA speci-mens show a tendency to increase with increasing temperature asshown in Fig. 13(a). When the temperature was increased from 5 to35�C, the hydraulic conductivity of the RAS-BA specimens, whichwere isotropically compressed at s395 70 kPa, increased by 40%from 93 1024 to 1:33 1023 cm=s, whereas the hydraulic con-ductivity of the RAS-FA specimens increased by 85% from2:63 1024 to 4:83 1024 cm=s. Two mechanisms are hypothe-sized for changing hydraulic conductivities of the RAS-BA andRAS-FA specimens with respect to temperature: (1) change in thevoid ratio of the specimens because of compressibility of the RASmixtures; and (2) change in viscosity of permeating water. Thehydraulic conductivity of a porous medium is separated into theproduct of two multiples: one reflecting property of the porousmedium and the other reflecting fluid properties (Lambe andWhitman 1969; Holtz et al. 2011)

K ¼ krgm

(9)

where K 5 hydraulic conductivity of the porous medium; k 5 in-trinsic permeability of the porous medium; r 5 density of water; m5 viscosity of water; and g 5 gravitational acceleration. The in-trinsic permeability is generally related to the void ratio and averagepore or particle diameter (Carman 1937; Taylor 1948; Lambe andWhitman 1969). Fig. 13(b) shows that the volumetric compressivestrain of the RAS-BA and RAS-FA specimens increases with in-creasing temperature, which indicates reduction of void space inthe specimens. The reduction in void space has a reducing effecton hydraulic conductivity, whereas the measured hydraulic con-ductivity of the specimens increased with increasing temperature[Fig. 13(a)]. However, when the temperature is increased from 5 to35�C, the water density is only slightly reduced (0.8%), whereasthe water viscosity is reduced by half from 1:523 1023 to 0:723 1023 Ns=m2 (Korson et al. 1969). Reduction of viscosity of per-meating water appears to be the dominating factor in increasing thehydraulic conductivity of the RAS-BA and RAS-FA specimens[Eq. (9)] with increasing temperature.

The effect of specimen compression or dilation because oftemperature change on hydraulic conductivity of the RAS-BAspecimens can be quantified using Eq. (9). If there was no change

Fig. 11. Effect of construction at elevated temperature on compress-ibility of the compacted RAS-BA mixture containing 25% RASs

Table 3. Secondary Compression Ratio of Different Materials

Materiala Caɛ

Berthierville clay (Mesri and Castro 1987) 0.0185California tar sand (Mesri and Castro 1987) 0.0014Micaceous Antelope Valley sand (Lade and Liu 1998) 0.0011Lake Michigan Beach sand (Mesri et al. 1990) 0.0004Wisconsin outwash sand 0.0003RAS-BA (25–75) 0.0078RAS-BA (25–75): thermally precompressed 0.0004RAS-FA (80–20) 0.0030RAS-FA (80–20): thermally precompressed 0.0002aNumbers in parentheses indicate relative weight-based proportion in themixtures.

Fig. 12. Variation of vertical strain with time of the RAS-FA speci-mens at different temperatures

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in intrinsic permeability with the temperature, the ratio of hydraulicconductivity at an elevated temperature (K) to the hydraulic con-ductivity at room temperature (Ko) would be obtained from

KKo

¼ r

ro×mo

m(10)

where ro 5 water density; and mo 5 water viscosity at roomtemperature. Values of r and m as a function of temperature wereobtained fromKorson et al. (1969). ThemeasuredK=Ko of the RAS-BA specimens along with the calculated K=Ko from Eq. (10) areplotted in Fig. 13(c). Deviation between themeasured and calculatedK=Ko at different temperatures reflects the effect of specimenvolumetric change on hydraulic conductivity. At 35�C, the reductionin void space (compression) has an approximately 15% reducingeffect on hydraulic conductivity, whereas at 5�C the increase invoid space (dilation) has an approximately 10% increasing effect.Overall, the increase in hydraulic conductivity with temperaturefavorably increases drainage capacity of structural fills containingRASs and bears no concerns.

Summary and Conclusions

The premise that RASs are too compressible for use as structural fillmaterial led to an investigation of mixing RASs with a less com-pressible granular industrial byproduct, such as BA, or stabilizingwith a self-cementing FA. Such improvement resulted in acceptablestructural fill behavior at moderate temperatures. The effect of

seasonal temperature change, typically observed in the field, ongeotechnical properties of compacted RAS-BA and RAS-FA mix-tures was evaluated. The range of temperature considered hereinencompasses the typical seasonal temperature change observable inthe contiguous United States. A thermomechanical testing system,including temperature-controlled triaxial compression, 1D com-pression, and permeameter cells, was developed. Test procedureswere devised to closely simulate the field conditions in the labora-tory. Based on the test results, the following conclusions are made.• Shear strength of compacted RAS-BA and RAS-FA mixtures

consistently decreases with increasing temperature but remainswithin a range sufficient to provide stability for typical roadembankment fills in the climatic ranges of the United States(i.e., up to 35�C fill temperatures). The maximum RASs contentin the RAS-BA mixture is limited to 50% to provide adequateshear strength.

• Increasing temperature increased the vertical strain, strain rate,and secondary compression ratio of the compacted RAS-BA andRAS-FAmixtures. The log-strain rate of the compactedRAS-BAmixtures linearly increasedwith temperature with a slope definedas the coefficient of thermal compression. The coefficient ofthermal compression is independent of time and stress level and isregarded as an inherent property of the mixture. The secondarycompression ratio of the compacted RAS-BA mixtures is anexponential function of temperature and power function of thestress level; therefore, the rate of settlement of an embankmentfillcontaining RASs varies at different field temperatures and ele-vation points in the embankment.

Fig. 13. (a) Variation of hydraulic conductivity of the RAS-BA and RAS-FA specimens with temperature; (b) variation of volumetric strain withtemperature; (c) normalized hydraulic conductivity of the RAS-BA specimens with temperature

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• Thermal cycling induced thermal precompression to the com-pacted RAS mixtures and significantly reduced the compress-ibility. Thermal precompression reduced secondary compressionratios of the compactedRAS-BA andRAS-FAmixtures to as lowas that of compacted sand. Based on the results, to minimize thelong-term settlement, construction of embankments made withmaterials containing RASs is recommended during warm sea-sons to induce thermal precompression and to have most of theembankment settlement occur during the construction.

• Hydraulic conductivity of the compacted RAS-BA and RAS-FAmixtures increased with increasing temperature. The increase ofhydraulic conductivity is caused by a reduction ofwater viscositywith temperature. There is no concern regarding drainage capacityof the compacted RAS-BA or RAS-FA mixtures at elevatedtemperatures.The results obtained in this research are also specific to the type

andmaximum particle size of the RASs used. Thematerials used areprocessed industrial materials or byproducts. Therefore, their be-havior is expected to vary in a narrow range, and only one sourcewas used for the test materials. Although the overall behavior is notlikely to vary significantly, the quantitative values of the variousparameters may be different if materials from other sources are used;therefore, they should be evaluated for design. The RASs can beused in construction of highway embankment fills taking into ac-count their intrinsic characteristics, such as compressibility andtemperature sensitivity, during design and construction. The re-duction of the asphalt value is acknowledgedwhenRASs are used asbackfill. However, excess RASs being landfilled justifies its use asa fill material. Such high-volume use not only resolves the disposalproblem of the material but also helps provide a more sustainableroadway construction.

Acknowledgments

Funding for this research was provided by the Recycled MaterialResource Center (RMRC) and Solid Waste Research Program(SWRP) at the University of Wisconsin–Madison. The authorsalso greatly appreciate B. R. Amon & Sons for providing recycledasphalt shingles for this research.

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