Laboratory characterization of recycled crumb-rubber-modified asphalt mixture after extendedaging

Soon-Jae Lee, Hakseo Kim, Chandra K. Akisetty, and Serji N. Amirkhanian

Abstract: This paper presents a limited study that characterizes the recycling of artificially aged crumb-rubber modified(CRM) mixtures depending on their recycling percentage and aggregate type. Eight mixtures [six recycled mixtures con-taining rubberized reclaimed asphalt pavement (RAP) and two control virgin mixtures] were designed and tested. Twotypes of aggregates were used in this research project. The rubberized RAP used in the study was artificially aged in thelaboratory using an accelerated aging processes. The percentages of the RAP, by total weight of the mix, incorporated intothe recycled mixtures were 15%, 25%, and 35%. Evaluation of all mixtures included the following testing procedures: ten-sile strength ratio (TSR), asphalt pavement analyzer (APA), resilient modulus, and indirect tensile strength (ITS) afterlong-term oven aging. The results from this study showed that (i) the recycled aged CRM mixtures (with 15%, 25%, and35% rubberized RAP) can satisfy the current Superpave mixture requirements, including moisture susceptibility and ruttingresistance, and (ii) in general, there was no significant difference between the control and the recycled CRM mixtures forthe properties evaluated in this study.

Key words: recycling, crumb-rubber modified (CRM), moisture susceptibility, rutting resistance, resilient modulus.

Resume : Cet article presente une etude partielle caracterisant le recyclage de melanges de fragments de caoutchouc(« CRM ») vieillis artificiellement; divers pourcentages de recyclage et types d’agregats ont ete analyses. Huit melanges(six melanges recycles contenant du revetement asphaltique recupere (« RAP ») caoutchoute et deux melanges vierges dereference) ont ete concus et mis a l’epreuve. Deux types d’agregats ont ete utilises dans ce projet de recherche. Le« RAP » caoutchoute utilise dans cette etude a ete vieilli artificiellement en laboratoire par des procedes de vieillissementaccelere. Les pourcentages de « RAP », en poids total de melange incorpore dans les melanges recycles, etaient de 15, 25et 35 %. L’evaluation de tous les melanges comportait les procedures d’essai suivantes : coefficient de resistance en trac-tion, analyseur des revetements asphaltiques, module de resilience et resistance en traction indirecte apres un long vieillis-sement au four. Les resultats de cette etude montrent que (i) les melanges vieillis de fragments de caoutchouc recycles(avec 15, 25 et 35 % de « RAP » caoutchoute) peut rencontrer les exigences actuelles d’un melange Superpave, incluantla susceptibilite a l’humidite et la resistance a la creation d’ornieres, et (ii) regle generale, il y avait peu de differences en-tre les melanges de reference et de fragments de caoutchouc recycles quant aux proprietes evaluees dans la presente etude.

Mots-cles : recyclage, melange de fragments de caoutchouc (« CRM »), susceptibilite a l’humidite, resistance a la creationd’ornieres, module de resilience.

[Traduit par la Redaction]

Introduction

The use of conventional reclaimed asphalt pavement(RAP) has become routine in a number of states throughoutthe USA. Thus, satisfactory mix design procedures havebeen developed (McDaniel and Anderson 2001). The Fed-

eral Highway Administration reports that approximately 73million tons (1 ton = 907.2 kg) of RAP are used in highwayconstruction each year, lowering material costs for thenewly placed asphalt mixes and eliminating the disposalcost of the RAP (Shen et al. 2007a).

The asphalt pavement industry is recycling approximatelytwo million scrap tires out of 300 million scrap tires pro-duced each year in the USA. Rubberized asphalt binder hasbecome more popular because of its reported advantages in-cluding: thinner lift, increased pavement life, retarded reflec-tion cracking, decreased traffic noise, reduced maintenancecost, decreased pollution, and increased environmental qual-ity (Liang and Lee 1996; Huang et al. 2002).

The recycling method for reclaimed asphalt pavementcontaining crumb-rubber modified (CRM) binders needs tobe investigated as many CRM pavements were paved10*20 years ago and it is time for some of them to be re-cycled. There have been limited reports to address the CRMpavement recycling issue, mainly conducted by some state

Received 4 February 2008. Revision accepted 1 August 2008.Published on the NRC Research Press Web site at cjce.nrc.ca on6 November 2008.

S.-J. Lee.1 Department of Technology, Texas State University –San Marcos, San Marcos, TX 78666, USA.H. Kim, C.K. Akisetty, and S.N. Amirkhanian. Department ofCivil Engineering, Clemson University, Clemson, SC 29634,USA.

Written discussion of this article is welcomed and will bereceived by the Editor until 31 March 2009.

1Corresponding author (e-mail: soonjae93@gmail.com).

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departments of transportation (DOTs) (Gunkel 1994; Crock-ford et al. 1995; Albritton et al. 1999; Bischoff and Toepel2004). A previous laboratory study showed that it is possibleto incorporate 15% rubberized RAP into hot-mix asphalt(HMA) pavement (Shen et al. 2006).

This study investigated the properties of recycled aged

CRM mixtures prepared using typical recycling practices(McDaniel and Anderson 2001). The two CRM mixtureswere produced in the laboratory using two aggregate sourcesand artificially aged in an accelerated aging process. Theaged CRM mixtures were recycled using the same virgin ag-gregate and two base binders (PG 64–22 for 15% recyclingand PG 58–22 for 25% and 35% recycling). Several proper-ties of all mixtures, including moisture susceptibility, ruttingresistance, resilient modulus, and indirect tensile strengthafter long-term aging, were evaluated. Figure 1 shows aflow chart of the experimental design used in this study.

Materials and test program

Materials

Crumb rubber modifierThe crumb rubber modifier produced by mechanical

Fig. 1. Flow chart of experimental design procedures. CRM, crumb-rubber modified; TSR, tensile strength ratio; APA, asphalt pavementanalyzer; ITS, indirect tensile strength.

Table 1. Gradation of crumb rubber used in this study.

Sieve No.*Amountretained (%)

Cumulative amountretained (%)

30 (600) 0 040 (425) 9.0 9.050 (300) 31.9 40.980 (180) 32.9 73.8100 (150) 7.6 81.4200 (75) 18.6 100.0*mm size of sieve shown in parentheses.

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shredding at ambient temperature was obtained from onesource: –40 mesh (0.425 mm) and used with a gradation asshown in Table 1, which is widely used to produce the CRMmixtures in South Carolina. To ensure that the consistencyof the crumb rubber modifier was maintained throughoutthe study, only one batch of crumb rubber was used in thisstudy.

Asphalt bindersTwo CRM binders were used to produce the eight CRM

mixtures by mixing the base binders (PG 64–22 or PG 58–22) with 10% (by weight of the binder) ambient crumb rub-ber modifier. Two binder types selected in this study arecommonly used for asphalt paving in both conventional andrecycling mixes in South Carolina and several other statesthroughout the USA. The properties of all these binders arelisted in Table 2. The CRM binder was manufactured in thelaboratory by mixing the crumb rubber modifier with thebinder at 177 8C using an open blade mixer at a blendingspeed of 700 rpm for 30 min (Shen et al. 2005; Lee et al.2006). This mixing condition matches the field practicesused in South Carolina to produce field CRM mixtures.

AggregatesThe mineral aggregates used for this study were obtained

from two different locations in South Carolina and desig-nated as aggregate sources B and L. Table 3 shows the prop-erties of two aggregates. Hydrated lime, used as an anti-stripadditive, was added at a rate of 1% by dry mass of aggre-gate.

Crumb-rubber modified mixture production and agingThe two CRM mixtures were produced through the Super-

pave mix design procedures, and these mixtures were artifi-cially aged using an accelerated aging process (an ovenaging for 2 d at 100 8C) in the laboratory. The long-termoven aging is considered to simulate the pavement agingafter 10*20 years of service in the field (Kliewer et al.1995). The aggregate gradations for the mixtures are repre-sented in Fig. 2.

Recycling of aged crumb-rubber modified mixturesThe CRM binder produced using a base binder of PG 64–

22 was used for 0% and 15% mixture recycling. The bindergrade was selected based on the previous research indicatingthat, for 15% recycling, the PG grade of the asphalt binderadded for the recycling can be the same as that used in 0%recycling (Kandhal and Mallick 1997; Shen et al. 2006). Inthe cases of 25% and 35% recycling, the base binder of PG58–22 was selected to produce the virgin CRM binder forthe recycling (Fig. 1). National Cooperative Research High-way Program (NCHRP) 452 states that higher percentageRAP (>20%) can be used with a softer RAP binder; that is,if the RAP is more than 20%, then the base binder gradeneeds to be reduced by one PG grade as the higher percent-age RAP increases the stiffness of the mixture, thereby caus-ing cracking problems in asphalt pavement.

The eight CRM mixtures (two aggregates � four recy-cling percentages of 0%, 15%, 25%, and 35%) were de-signed according to AASHTO M 323–04 (AASHTO 2007).Optimum asphalt contents (OAC) were determined fromthese designs and were used to produce the recycled agedCRM mixtures.

Indirect tensile strengthThe indirect tensile strength (ITS) properties were meas-

Table 2. Properties of the binders used in this study.

Test propertiesBase 1(PG 58–22)

Base 2(PG 64–22)

CRM 1(PG 58–22 + 10% rubber)

CRM 2(PG 64–22 + 10% rubber)

Unaged binderViscosity @ 135 8C (Pa�s) 0.287 0.409 0.771 1.302G*/sin d @ 64 8C (kPa) 0.590 1.194 — —G*/sin d @ 70 8C (kPa) — — 0.883 1.633

RTFO aged residueG*/sin d @ 64 8C (kPa) 1.054 2.430 — —G*/sin d @ 70 8C (kPa) — — 2.295 2.774

RTFO + PAV aged residueG*sin d @ 25 8C (kPa) 1208.7 2335.0 1102.7 1963.5Stiffness @ –12 8C (MPa) 114 187 86 149m-value @ –12 8C 0.378 0.337 0.350 0.322

Note: G*/sin d, rutting resistance factor; RTFO, rolling thin film oven; PAV, pressure aging vessle; m-value, low temperaturecracking property.

Table 3. Properties of aggregates B and L.

Properties Standard method Aggregate B Aggregate LApparent specific gravity AASHTO T 85 (AASHTO 2008) 2.860 2.680Bulk specific gravity AASHTO T 85 (AASHTO 2008) 2.820 2.650Absorption AASHTO T 85 (AASHTO 2008) 0.5 0.6Los Angeles abrasion AASHTO T 96 (AASHTO 2003) 26 52Soundness AASHTO T 104 (AASHTO 1998) 0.6 0.2

Note: Aggregate B, marble-schist; aggregate L, granite.

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ured to evaluate the moisture susceptibility of the mixturesaccording to the South Carolina (SC) Department of Trans-portation (DOT) specification: SC-T-70. Two sets of threesamples each were tested at 25 8C in dry and wet states.The samples were 150 mm diameter having a height of95 mm and an air-void content of 7 ± 1%. The ITS and ten-sile strength ratio (TSR) values were calculated and the re-sults were reported as the average value.

Asphalt pavement analyzerThe asphalt pavement analyzer (APA) test in this study

was conducted on cylindrical samples, manufactured by aSuperpave gyratory compactor (SGC), with an air-void con-tent of 4 ± 0.5%, a height of 75 mm, and a diameter of150 mm. The test temperature was 64 8C, the hose pressurewas 690 kPa, and the wheel load was 445 N. The rut depthwas recorded and measured manually after 8000 cycles.

Resilient modulus (MR)The resilient modulus (MR) test was carried out at temper-

atures of 5, 25, and 40 8C according to ASTM D4123(ASTM 2006). Four duplicate samples with a 150 mm diam-eter and 95 mm thickness were compacted using a SGC toan air-void content of 4 ± 0.5%. One of the four sampleswas used to measure the ITS value by which the repeatedload is determined, specifically 30%, 15%, and 5% of theITS was used as the repeated load for the tests at 5, 25, and40 8C, respectively. The resilient modulus value was re-ported as an average of three samples.

Indirect tensile strength after long-term oven agingThe ITS tests were conducted to evaluate the properties of

recycled aged CRM mixtures after artificial long-term aging.One set of three samples for each mixture was tested at25 8C in dry state. The samples were 150 mm diameter hav-ing a height of 95 mm and an air-void content of 4 ± 0.5%.

Analysis methodThe Statistical Analysis Software (SAS) program, version

8.0 (SAS Institute Inc., Cary, N.C. ), was used to conductanalysis of variance (ANOVA) and Fisher’s least significantdifference (LSD) comparisons with an a = 0.05. The pri-mary variables included the RAP percentages (0%, 15%,25%, and 35%) and the aggregate sources (B and L).

Results and discussions

Superpave mix designThe mix design results are shown in Table 4. In general,

the optimum asphalt contents (OAC) were found to be sim-ilar for the mixes of 0% (control) and 15% recycling, andfor those of 25% and 35% recycling. The difference ofOAC between 15% and 25% recycling can be attributed todifferent virgin binder grades used for each mix (PG 64–22for 15% recycling and PG 58–22 for 25% recycling). In ad-dition, the mixes made with aggregate L had higher OAC,compared with those made with aggregate B. This may beexplained by the fact that aggregate L tends to absorb moreasphalt binder than aggregate B.

Fig. 2. Aggregate gradations for two crumb-rubber modified mixtures.

Table 4. Superpave mix design results.

Mix properties

Mixtures made with aggregate L (% by weight) Mixtures made with aggregate B (% by weight)

0* 15 25 35 0* 15 25 35OAC (%) 6.2 6.0 5.7 5.7 5.0 5.1 4.9 4.7MSG 2.419 2.413 2.464 2.465 2.592 2.588 2.601 2.610

Note: OAC, optimum asphalt content; MSG, maximum specific gravity.*Control mix: 0% aged crumb-rubber modified (CRM) mix + 100% virgin CRM mix.

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Tensile strength ratioIn general, the ITS test provides two mixture properties

that are useful in characterizing HMA. The first property isa tensile strength that evaluates cracking potential and sec-ond is a moisture susceptibility that evaluates moisture dam-age potential in HMA. For moisture susceptibility, a highnumber typically relates to good performance. Figure 3 il-

lustrates the ITS test results of the recycled CRM mixturesin dry and wet conditions. The ITS values of mixtures con-taining 15% RAP were highest for all mixes within each ag-gregate source. The mixtures containing 25% and 35% RAPand made with a softer grade of PG 58–22 as a virgin bindershowed relatively lower ITS values than corresponding mix-tures (0% and 15%). The ITS values of all mixtures satisfied

Fig. 3. Indirect tensile strength (ITS) test results of recycled aged crumb-rubber modified mixtures made with aggregate sources L and B.TSR, tensile strength ratio.

Table 5. Statistical analysis results of the indirect tensile strength (ITS) values of recycledaged crumb-rubber modifed mixtures as a function of reclaimed asphalt pavement (RAP)percentage and aggregate source (a = 0.05): (a) dry ITS; (b) wet ITS.

Aggregate L Aggregate B

(a) Dry ITS1 2 3 4 1 2 3 4

Aggregate L 1 — N N N N S N N2 — S S N N N N3 — N S S N N4 — S S N N

Aggregate B 1 — N N N2 — S S3 — N4 —

(b) Wet ITS1 2 3 4 1 2 3 4

Aggregate L 1 — N N N N N N N2 — N N N N N N3 — N N S N N4 — N N N N

Aggregate B 1 — N N N2 — N N3 — N4 —

Note: RAP percentage: 1, 0% (control); 2, 15%; 3, 25%; 4, 35%; N, nonsignificant; S, significant.

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the requirement set forth by the SC DOT (455 kPa or65 psi). With respect to the TSR values, most of the mix-tures showed higher TSR values than 85%, the criterionspecified by the SC DOT. Only the mixtures with 15%RAP (aggregate source B) showed a lower TSR value than85%. The higher dry ITS value of the mixture may be ex-plained as a possible reason of lower TSR value.

Using one-way analysis of variance, the statistical signifi-cance of the change in the ITS values as a function of RAPpercentage and aggregate source was examined and the re-sults are shown in Table 5. The data indicates that therewas no significant difference, at the a = 0.05 level, amongdry ITS values between the control CRM mixture (0% recy-cling) and the recycled CRM mixtures (15%, 25%, and 35%recycling) within each aggregate source. With respect to wetITS values, the difference between the control mixture andthe recycled mixtures was statistically insignificant in mostcases, regardless of RAP percentage and aggregate source.This is thought to be associated with the higher air-voidcontents (i.e., 6%*8%) of specimens for ITS tests; there-fore, increasing the variance of test results, especially in thewet ITS tests.

Asphalt pavement analyzerThe APA test is a general laboratory method for predict-

ing rutting potential (permanent deformation) in HMA. Alow rut depth typically indicates that good performance isexpected. Figure 4 shows the final rut depth values for themixtures that were compacted at an air-void content of 4 ±0.5%. From the APA test results, the deformation valuesafter 8000 cycles were found to be significantly below8 mm, the recommended value (Kandhal and Cooley 2003).The 15% recycled CRM mixtures had the lowest rut depthwithin each aggregate source, followed by the 0% recycledmixtures. As expected, the addition of RAP resulted instiffer mixes compared with the control mixes using the

same virgin binder grade of PG 64–22. However, the use ofa softer binder grade of PG 58–22 for 25% and 35% recy-cling was found to make the recycled mixes softer than thecontrol mixes, indicating more susceptibility to rutting athigh temperatures but more resistance to cracking at lowtemperatures.

The rut depth values of mixtures with aggregate source Bwere relatively lower than those with aggregate source L atall recycling percentages. The difference of the ruttingperformance is thought to be attributed to different binderproperties of the mixtures at the test temperature, providedthat all mixtures used the same gradation within each aggre-gate source.

Resilient modulusThe resilient modulus test is used to measure stiffness

change for HMA. A lower slope throughout test tempera-tures typically represents better performance. In particular, ahigher slope value at a low temperature tends to indicate acrack in the HMA earlier than the lower value. Figure 5shows the resilient modulus test results at temperatures of5, 25, and 40 8C. A general trend was found that the 25%and 35% recycled CRM mixtures had similar results withineach aggregate source and test temperature. The mixturesmanufactured with aggregate source B resulted in muchhigher resilient modulus values than those with aggregatesource L, especially at the test temperature of 5 8C. For themixtures made with aggregate source L, there was no signif-icant difference between 0% and 15% recycling at all testtemperatures at the 5% significance level. Similar to theITS and APA results, the resilient modulus tends to highlydepend on the virgin binder grade (PG 64–22 or PG 58–22)and the aggregate sources (L or B).

Indirect tensile strength after long-term oven agingThe effectiveness of the accelerated aging process used in

Fig. 4. Asphalt pavement analyzer test results of recycled aged crumb-rubber modified mixtures made with aggregate sources L and B.

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this study was evaluated in a previous study using the gelpermeation chromatography (GPC) technique (Shen et al.2006). The study investigated the large molecular size(LMS) of the binders extracted from the mixtures, then thecomparison of before- and after-aging results showed thatthe long-term aging of the mixtures in the laboratoryresulted in a significant increase of the LMS ratios (LMSratio = LMS value after aging / LMS value before aging).In general, the aging is considered to have a good relationwith an increase in the LMS of the asphalt binder (Kim andBurati 1993; Kim et al. 2006; Shen et al. 2007b).

In general, the low strength values after long-term agingare considered desirable attributes from the standpoint of re-sistance to cracking after 10*20 years of service. Figure 6

depicts the ITS test results of the recycled CRM mixturesthat were long-term aged for 2 d at 100 8C. Similar to theresults of ITS tests before aging, the 15% recycled CRMmixtures showed the highest ITS values among all mixeswithin each aggregate source. The mixtures containing 35%RAP, the highest recycling percentage used in this study, re-sulted in relatively lower ITS values than the control mix-tures (0% recycling) in both aggregates. Again, this resultcan be explained by the use of a softer virgin binder gradeof PG 58–22 for 35% recycling. In terms of the effect of ag-gregate source, the mixtures with aggregate source Bshowed higher ITS values than those with aggregate sourceL at all recycling percentages.

The statistical significance of the change in ITS values

Fig. 5. Resilient modulus test results for the recycled aged crumb-rubber modified mixtures made with aggregate sources (a) L and (b) B.

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depending on the recycling percentage and the aggregatesource was examined (Table 6). The statistical resultsshowed that the difference in the control (0% recycling) andthe 15% recycled mixtures was statistically insignificantwithin each aggregate source at the 5% significance level.The result is consistent with the previous research (Shen etal. 2006). Also, it was observed that there was no significantdifference among the ITS values between CRM mixturescontaining 25% RAP and 35% RAP, within each aggregatesource. When a comparison is made between aggregatesources, the aggregate sources were found to have a signifi-cant effect on the ITS values of recycled CRM mixturesafter long-term aging.

Summary and conclusions

This project was conducted to evaluate the properties oflaboratory-prepared recycled aged CRM mixtures madewith aggregate from two different sources. The CRM mix-

tures were artificially aged through accelerated aging proc-esses. The aged CRM mixtures were recycled at 0%, 15%,25%, and 35% of RAP. A series of mixture tests were con-ducted to obtain the TSR, the APA, the resilient modulus,and the ITS after long-term oven aging. Based on the exper-imental results, the following conclusions were drawn forthe limited materials used in this study:

(1) The optimum asphalt contents of the mixes producedusing the same virgin binder and aggregate source werefound to be similar regardless of recycling percentage.

(2) The ITS values of all recycled CRM mixtures werehigher than the specification set forth by the SC DOT.In most cases, there was no significant difference of theITS values between the control (0% recycling) and therecycled CRM mixtures.

(3) The final rut depths of all the mixtures were lower thanthe requirements set forth by the SC DOT. The aggre-gate source and virgin binder grade used for recycling

Fig. 6. Indirect tensile strength (ITS) test results of recycled aged crumb-rubber modified mixtures after long-term oven aging.

Table 6. Statistical analysis results of the indirect tensile strength (ITS) values of recycledaged crumb-rubber modified mixtures after long-term oven aging as a function of re-claimed asphalt pavement (RAP) percentage and aggregate source (a = 0.05).

ITS after long-term ovenaging

Aggregate L Aggregate B

1 2 3 4 1 2 3 4Aggregate L 1 — N S N S S N N

2 — S S S S N N3 — N S S S S4 — S S S S

Aggregate B 1 — N S S2 — S S3 — N4 —

Note: RAP percentage: 1, 0% (control); 2, 5%; 3, 25%; 4, 35%; N, nonsignificant; S, significant.

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were found to have a significant effect on the rutting re-sistance and resilient modulus properties.

(4) After long-term oven aging, the 15% recycled mixturesshowed the highest ITS values in both aggregate sources,but it was found that the ITS values of the control andthe 15% recycled mixtures were statistically insignificantwithin each aggregate source.

(5) The laboratory-prepared long-term aged CRM mixtureswere recycled up to 35% in this study, and in mostcases, the engineering properties of recycled aged CRMmixtures showed satisfactory results meeting current theSC DOT mixture requirements.

(6) The properties of the control and the recycled CRMmixtures were investigated using several mixture tests.However, the point stressed in this paper is not to spe-cify the observed number of each property, but to showa general trend of the engineering properties of themixtures. Of importance, it should be mentioned that adifferent virgin binder grade was used depending on therecycling percentage, suggesting that direct comparisonbetween the mixes made with a different binder grademay be inappropriate. Also, further study withrubberized RAP from the field is needed to validatethese findings.

AcknowledgementsThis study was supported by the Asphalt Rubber Technol-

ogy Service (ARTS) of the Civil Engineering Department,Clemson University, Clemson, South Carolina, USA. Theauthors wish to acknowledge and thank South Carolina’sDepartment of Health and Environmental Control (DHEC)for their financial support of this project.

ReferencesAASHTO. 1999. Standard method of test for soundness of aggre-

gate by use of sodium sulfate or magnesium sulfate. AASHTOstandard T 104. American Association of State Highway andTransportation Officials, Washington, D.C.

AASHTO. 2003. Standard method of test for resistance to degrada-tion of small-size coarse aggregate by abrasion and impact inthe Los Angeles machine. AASHTO standard T 96. AmericanAssociation of State Highway and Transportation Officials, Wa-shington, D.C.

AASHTO. 2007. Standard method of test for Superpave volumetricmix design. AASHTO standard M 323. American Association ofState Highway and Transportation Officials, Washington, D.C.

AASHTO. 2008. Standard method of test for specific gravity andabsorption of coarse aggregate. AASHTO standard T 85. Amer-ican Association of State Highway and Transportation Officials,Washington, D.C.

Albritton, G.E., Barstis, W.F., and Gatlin, G.R. 1999. Constructionand testing of crumb rubber modified hot mix asphalt pavement.Report No: FHWA/M.S.-DOT-RD-99–115. Federal HighwayAdministration, Washington, D.C.

ASTM. 2006. Standard practice for preparation of test specimensof asphalt-stabilized soils. ASTM standard D4423-99(2006).American Society for Testing and Materials, West Consho-hocken, Pa.

Bischoff, D., and Toepel, A. 2004. Tire rubber in hot mix asphaltpavement. Wisconsin Department of Transportation, Madison,Wisc.

Crockford, W.W., Makunike, D., Davison, R.R., Scullion, T., and

Billiter, T.C. 1995. Recycling crumb rubber modified pave-ments. Texas Transportation Institute Research Report 1333–1F.Texas A & M University, College Station, Tex.

Gunkel, K.O’C. 1994. Evaluation of exhaust gas emissions andworker exposure from asphalt-rubber binders in hot mix asphaltmixtures Part I: exhaust gas emissions results. Wildwood Envir-onmental Engineering Consultants, Inc. and Michigan Depart-ment of Transportation.

Huang, B., Mohammad, L.N., Graves, P.S., and Abadie, C.2002. Louisiana experience with crumb rubber-modified hot-mix asphalt pavement. Transportation Research Record, 1789:1–13.

Kandhal, P.S., and Cooley, L.A., Jr. 2003. Accelerated laboratoryrutting tests: evaluation of the asphalt pavement analyzer, na-tional cooperative highway research program report 508. Trans-portation Research Board, National Research Council,Washington, D.C.

Kandhal, P.S., and Mallick, R.B. 1997. Pavement recycling guide-lines for state and local governments: participant’s referencebook [online]. Federal Highway Administration, Washington,D.C. 98(042). Available from www.fhwa.dot.gov/pavement/recycling/98042/#s1 [accessed 1997].

Kim, K.W., and Burati, J.L. 1993. Use of GPC chromatogram tocharacterize aged asphalt cement. Journal of Materials in CivilEngineering, 5(1): 41–52. doi:10.1061/(ASCE)0899-1561(1993)5:1(41).

Kim, K.W., Lee, S.-J., and Amirkhanian, S. 2006. Estimation ofrutting characteristics of waste tire rubber-modified asphalt bin-der using GPC. In Urban Transport XII: Urban Transport andthe Environment in the 21st Century, Proceedings of UrbanTransport 2006, Prague, Czech Republic, 12–14 July 2006.WIT Transactions on The Built Environment, Vol.89(1767).WIT Press, Southampton, UK. pp.15–24.

Kliewer, J.E., Bell, C.A., and Sosnovske, D.A. 1995. Investigationof the relationship between field performance and laboratoryaging properties of asphalt mixtures. In ASTM STP 1265:Proceedings of the symposium on Engineering Properties ofAsphalt Mixtures and the Relationship to Their Performance,December, 1994. American Society for Testing and Materials,West Conshohocken, Pa. pp. 3–20.

Lee, S.-J., Amirkhanian, S., and Shatanawi, K. 2006. Effects ofcrumb rubber on aging of asphalt binders. In Proceedings of As-phalt Rubber 2006, Palm Springs, Calif., 25–27 October 2006.Vol. 3. pp. 779–795.

Liang, R.Y., and Lee, S. 1996. Short-term and long-term aging be-havior of rubber modified asphalt paving mixtures. Transporta-tion Research Record. Transportation Research Record, 1530:11–17. doi:10.3141/1530-02.

McDaniel, R., and Anderson, R.M. 2001. Recommended use of re-claimed asphalt pavementin the superpave mix design method:technician’s manual. National Cooperative Highway ResearchProgram, Transportation Research Board, National ResearchCouncil, Washington, D.C. Report 452.

Shen, J., Amirkhanian, S., and Lee, S.-J. 2005. Effects of rejuve-nating agents on recycled aged rubber modified binders. Interna-tional Journal of Pavement Engineering, 6(4): 273–279. doi:10.1080/10298430500439319.

Shen, J., Amirkhanian, S., Lee, S.-J., and Putman, B.J. 2006.Recycling of laboratory-prepared RAP mixtures containingcrumb rubber modified binders in HMA.. TransportationResearch Record, 1962: 71–78. doi:10.3141/1962-09.

Shen, J., Amirkhanian, S., and Miller, J.A. 2007a. Effects ofrejuvenating agents on superpave mixtures containing re-claimed asphalt pavement. Journal of Materials in Civil Engi-

1316 Can. J. Civ. Eng. Vol. 35, 2008

# 2008 NRC Canada

neering, 19(5): 376–384. doi:10.1061/(ASCE)0899-1561(2007)19:5(376).

Shen, J., Amirkhanian, S., and Lee, S.-J. 2007b. HP-GPC charac-

terization of rejuvenated aged CRM binders. Journal of Materi-als in Civil Engineering, 19(6): 515–522. doi:10.1061/(ASCE)0899-1561(2007)19:6(515).

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