the university of mississippi, carrier 203, university, ms ... · yamini [2]. 2 superpave binder...

191Applied RheologyVolume 13 · Issue 4

Viscoelastic Characterization of Polymer-Modified AsphaltBinders of Pavement Applications

Waheed Uddin

The University of Mississippi, Carrier 203, University, MS 38677-1848, USA

Email: [email protected]: 001.662.915.5523

Received: 20.2.2003, Final version: 20.6.2003

Abstract:Rutting is a primary reason of premature deterioration of asphalt highway pavements. Pavements constructedwith polymer and other modifiers are showing improved performance. The virgin asphalt and modified asphaltbinders and mixes used on several test sections of the I-55 highway rehabilitation project in northern Missis-sippi are compared. The laboratory creep compliance data for these binders were measured at low temperaturesusing a modified test procedure adapted for the Bending Beam Rheometer device. Dynamic Shear Rheometerwas used at high service temperatures. The creep compliance data of the binder was used as an input to simu-late creep compliance behavior of the mix using a micromechanical model. The field evaluation confirms therelatively poor performance of the virgin asphalt section with respect to rutting, compared to modified bindersections.

Zusammenfassung:Fahrrinnenbildung ist ein Hauptgrund für die vorzeitige Alterung von Asphaltbelägen auf Autobahnen. Belägemit Polymeren und anderen Modifikatoren zeigen eine verbesserte Güte. Der unbehandelte Asphalt und modi-fizierte Zusatzstoffe und Mischungen, welche auf verschiedenen Testabschnitten des Sanierungsprojektes aufder I-55 im Norden Mississippis benutzt wurden, werden verglichen. Die im Labor ermittelten Daten der Kriech-nachgiebigkeit für diese Zusatzstoffe wurden bei niedrigen Temperaturen gemessen, wobei eine modifiziertesTestverfahren benutzt wurde, welches an das Biegebalken Rheometer angepasst ist. Das dynamische Scher-rheometer wurde bei hohen Temperaturen eingesetzt. Die Kriechnachgiebigkeitsdaten der Zusatzstoffe wur-den als Eingabedaten zur Simulation des Kriechnachgiebigkeitsverhaltens der Mischung benutzt, wobei einmikromechanisches Modell herangezogen wurde. Die Auswertung der Feldstudie belegt die relativ geringe Gütedes Primärasphaltes in Bezug auf die Bildung von Fahrrinnen, verglichen mit den Abschnitten, welche mit modi-fiziertem Bindemittel asphaltiert wurden.

Résumé:La première cause de détérioration prématurée des revêtements asphaltiques des autoroutes est la formationd’ornières. Les revêtements obtenus avec du polymère et d’autres adjuvants présentent des performancesaccrues. L’asphalte vierge, l’asphalte modifié avec des liants ainsi que des mélanges utilisés sur plusieurs sec-tions tests de l’autoroute I-55 dans le cadre d’un projet de réhabilitation de cette autoroute du nord du Missis-sipi, ont été comparés. Des données de relaxation en fluage ont été obtenues en laboratoire pour ces mélangesà basses températures, à l’aide d’un rhéomètre de fluage dont le protocole expérimental a du être modifié. Unrhéomètre de cisaillement dynamique a été utilisé pour l’obtention de données à hautes températures. Les don-nées de compliance en fluage obtenues pour l’asphalte modifié avec du liant ont été utilisées comme paramètresdans un modèle micromécanique pour simuler le comportement en fluage du mélange. L’évaluation faite surle terrain confirme les performances relativement pauvres de l’asphalte vierge comparé à l’asphalte modifiéavec du liant, en ce qui concerne la formation d’ornières.

Key Words: asphalt, binder, polymer-asphalt, laboratory, pavement, Superpave

© Appl. Rheol. 13 (2003) 191-199

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192 Applied RheologyVolume 13 · Issue 4

1 INTRODUCTIONAsphalt is predominantly used to construct pave-ments for roads, highways, and airports. Bothasphalt binder and asphalt-aggregate mixtureshow temperature- and time-dependent behav-ior. Rutting or permanent deformation is theleading cause of pavement deterioration in tem-perate and warm climatic regions of the worldand low-temperature cracking is a commonproblem in cold regions [1-4]. As the highwaypavement infrastructure has aged and deterio-rated, more pavements are in need of mainte-nance, rehabilitation, and reconstruction. This istraditionally done by placing hot-mix asphaltoverlay. The Mississippi Department of Trans-portation (MDOT) has adopted the SuperpavePerformance Grade (PG) binder specification andmix design system [1], developed by the Strate-gic Highway Research Program (SHRP), thatrelates the asphalt mix design to performance.Polymer-modified asphalt has been claimed toresist rutting and has been used in a side-by-sideexperimental study in Mississippi.

This comprehensive study included lab-oratory testing for creep compliance characteri-zation of asphalt layers, deflection testing, in situpavement performance evaluation, andadvanced finite element computer simulationsof an in-service asphalt highway pavement on I-55 highway near Grenada in northern Mississip-pi [1-2]. The analysis and results of laboratorybinder tests were conducted using Superpaveequipment for different asphalt binders used inthis study. The objectives of this paper are to: syn-thesize the laboratory test results of binders andmixes, compare the laboratory test results withthe field performance in terms of resistance torutting, and evaluate the effect of measured vis-coelastic properties of the asphalt layer on finiteelement dynamic response analysis of the con-structed asphalt pavement.

The pavement test sections were pavedin 1996 with virgin asphalt and eight other mod-ified binders including six polymers, two rubbers,and one chemical. Based upon the MDOT pave-ment design 76 mm thick (38 mm binder and 38mm surface) asphalt layer was removed andreplaced by the same thickness of new modifiedmix in each test section. The control and transi-tion sections were constructed using AC-30grade, the standard unmodified virgin asphalt

used by the MDOT in Mississippi [1]. The optimumbinder content of these mixes, determined usingthe Superpave level I mix design method, was 5.2% by weight of total mix. Other details of the con-structed sections are provided by Uddin andYamini [2].

2 SUPERPAVE BINDER TESTSThe Superpave binder PG specifications and mixdesign are recommended by the Federal High-way Administration (FHWA) for improved andlong life highway pavements [5, 6]. LaboratorySuperpave binder tests were conducted at theUniversity of Mississippi [7] using asphalt andmodified asphalt binder samples from I-55 testsections. The PG binder for the test was PG58-10(fast traffic), where 58-10 represents the high(58˚C) and low (-10˚C) pavement temperaturesfor northern Mississippi.

2.1 DYNAMIC SHEAR RHEOMETER TESTSDynamic Shear Rheometer (DSR) tests were con-ducted on all nine kinds of binders. In the DSR testoperation, the asphalt sample is compressedbetween two parallel plates, one of which is fixedand the other one oscillates, as shown in Fig. 1.The DSR test measures the complex shear mod-ulus, G*, and phase angle, d, of the binder. Thecomplex shear modulus is a measure of totalresistance of a material to deform when exposedto repeated pulses of shear stress. The phaseangle is an indicator of the relative amounts ofthe recoverable and non-recoverable deforma-tion.

The DSR tests were conducted at 46, 58,70, 82˚C for Novophalt and control asphalt sam-ples. The tests were also conducted at severaltemperatures 46, 52, 58, 64, 70, 76, 82˚C on Cry-opolymer. For the rest of the asphalt binder sam-ples the SHRP Spec Tests was conducted at 46˚C

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Figure 1:Dynamic Shear

Rheometer (DSR) principle.



only because of time constraints. The DSR ruttingfactor, G*/sind, represents a measure of the hightemperature stiffness or rutting resistance ofasphalt binders. The SHRP Superpave PG binderspecifies minimum values for G*/sind of 1000 Pafor original asphalt binder and 2200 Pa afteraging the binder using the Rolling Thin Film Oven(RTFO) procedure [7, 8].

The G*/sind results for all nine types ofbinders at 46˚C are shown in Fig. 2. The results ofG*/sind values were compared for controlasphalt, Novophalt and Cryopolymer at 46, 58,70, and 82˚C. At 46 and 58˚C Novophalt performsbetter than either control asphalt or Cryopoly-mer. At higher temperatures (70 and 82˚C), all ofthe three binders perform in the same way asshown in Fig. 3.

The results in Fig. 2 and Fig. 3 show thatNovophalt may offer better resistance to ruttingat high service temperature. It is observed that,with the exception of Ultrapave, the modifiedasphalt binders and the control asphalt meet theSHRP PG specification requirements. This doesnot support the poor field performance of con-trol asphalt, compared to the modified binders.Therefore, it can be concluded that the SHRP SpecTest, originally designed only for virgin asphaltbinders may not be applicable to modifiedasphalt binders. The DSR test can be adopted toconduct creep compliance tests of these bindersat high temperatures representing in-serviceconditions in warm climatic regions.

2.2 BENDING BEAM RHEOMETER TESTThe Bending Beam Rheometer (BBR) is used toaccurately evaluate binder properties at lowtemperatures at which asphalt binders are toostiff to reliably measure rheological properties

using the parallel plate geometry of the DSRequipment. Used together, the DSR and the BBRtests provide rheological behavior of asphaltbinders over a wide range of in-service tempera-tures in cold to warm climatic regions. Fig. 4shows a close up of the BBR equipment. The BBRtest measures creep stiffness value at the lowestin-service temperature, S(t) as designated bySHRP, which is indicative of the susceptibility tolow-temperature cracking. To prevent this crack-ing, S(t) has a maximum limit of 300 MPa, asrequired by the Superpave PG binder specifica-tions. Since low-temperature cracking occursonly after the pavement has been in-service forsome time, this specification addresses theseproperties using binder aged in both the RollingThin Film Oven (RTFO) and Pressure Aging Vessel(PAV) devices.

The rate of change of binder stiffnesswith time is represented by the m-value, whichis the slope of the log stiffness versus log timecurve from the BBR test results, as shown in Fig.5a. A high m-value is desired because as the tem-perature decreases and pavement contractionbegins to occur, the binder will respond as amaterial that is less stiff. This decrease in stiff-

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Figure 2 (left above):Comparison of G*/sindvalues for asphalt andmodified asphalt bindersamples at 46˚C.

Figure 3 (left below):Comparison of G*/sindvalues for control asphalt,Novophalt, and Cryopoly-mer binders at varioustemperatures.

Figure 4 (right above):Bending Beam Rheometer(BBR) equipment.

Figure 5 a) (right below):Rate of change of binderstiffness with time,represented by the m-value.



ness leads to smaller tensile stresses in the binderand less chance for low-temperature cracking. Aminimum m-value of 0.3 after 60 seconds isrequired by the Superpave PG binder specifica-tion [7]. As shown in Fig. 5b, all binders show high-er than the required minimum m-value. The S(t)test results of all these binders satisfy the SHRPSuperpave PG binder specifications [7].

Fig. 6 compares the S(t) test results at -12˚C. Excessively high stiffness values, shown bysome polymer-asphalt binders (Novophalt, Kra-ton, and Multigrade), indicate that the asphaltmix produced using these binders may be sus-ceptible to low-temperature cracking, especiallyin the areas of cold climate. On the other hand,control asphalt and other modified asphaltbinders including Rouse Rubber, Sealoflex, andStyrelf may perform better.

3 BINDER CREEP COMPLIANCE TESTSUSING BBRAdditional laboratory tests were conductedusing BBR to evaluate creep compliance of theselected binders for longer duration of 3600 sec-onds. The creep compliance, as a function of time,is defined as:

(1)

where e(t) is the time-dependent strain under aconstant stress, s. The creep compliance is thereciprocal of the relaxation modulus, E.

A modified test procedure for the BBRequipment was adapted for asphalt and modi-

C t t( )

( )= εσ

fied binders at different temperatures. The mod-ified test was conducted by increasing the totaltest duration for a longer time of 3600 seconds.The BBRAN computer program was developed toanalyze and summarize the output generated bythe BBR machine.

Creep compliance and relaxation modu-lus values were calculated for several binders at atemperature of -12˚C. Tests at temperatures of -18,-12 and 0˚C were also conducted during this study.It is apparent from this study that the binder creepcompliance data at 0˚C may not be reliable usingthe BBR equipment. Therefore, future binder creepcompliance tests will be conducted at 0˚C usingthe DSR equipment. Fig. 7 shows the binder relax-ation modulus calculated from BBR tests at -12˚Cand at 1 second. The relaxation modulus value forSealoflex modified binder was similar to that ofthe control asphalt binder, an indication of betterperformance at low temperatures.

Fig. 8 plot for control asphalt bindershows an increase in the relaxation moduluswhen the test temperature is decreased. Fig. 9shows a comparison of the relaxation modulusfor control asphalt and modified binders at -12˚C

Figure 5 b) (left above):The m-value results at -12˚C.

Figure 6 (left below):Comparison of measured

S(t) values for asphaltbinder samples at -12˚C.

Figure 7 (right above):Binder relaxation modulus

for control asphalt andmodified binders at 1 second

from BBR tests at -12˚C.

Figure 8 (right middle):Relaxation modulus forcontrol asphalt binder.

Figure 9 (right below):Relaxation modulus for

control asphalt andmodified binders at -12˚C.



test temperature. Fig. 9 shows that the asphaltmodulus attenuation is relatively more both inthe short-term and long-term ranges at -12˚C testtemperature. This test temperature is well belowthe low-temperature regime in northern Missis-sippi. The binder creep compliance test resultsand elastic properties of aggregate were used topredict creep compliance of the compacted mixusing the micromechanical analysis approach [9,10], as discussed in the following section.

4 MICROMECHANICAL MODEL TOPREDICT MIX CREEP COMPLIANCEThe micromechanics approach for material mod-eling examines the interaction of the constituentmaterials directly on a microscopic scale [9 - 11].The micromechanics approach used in this studyis based upon the application of Aboudi’s“method of cells” micromechanical analysis topredict viscoelastic response of resin matrix com-posites [9, 12]. The coupling of the method of cellswith the time-stepping algorithm provides anaccurate model for the analysis of compositematerials displaying viscoelastic behavior.

The most attractive advantage of usingthe micromechanics approach, incorporated inthe ASPHALT program [10], is that overall prop-erties of the mix may be determined from theknown properties of the constituents. The time-dependent behavior of asphalt or modifiedbinder, as a nonlinear viscoelastic component(binder creep compliance data and bulk modu-lus) is characterized by a time-stepping algo-rithm through the use of the hereditary integralform of the constitutive equations. The aggre-gate is characterized as a linearly elastic materi-al by its Young’s modulus and Poisson’s ratio.

Fig. 10 shows mix relaxation modulusresults predicted by the micromechanical analy-

sis for the control asphalt and several modifiedbinders at -12˚C.

5 TEMPERATURE CONTROLLEDSTRESS-STRAIN TESTS ON COMPACTEDASPHALT MIXThe stress-strain behavior of asphalt and modi-fied asphalt highway mixes is dependent on timeof loading and test temperature. For accuratelymodeling asphalt mixes it is important to recog-nize the temperature- and time-dependent vis-coelastic material behavior that can be charac-terized in the laboratory by simple static creeptests [10]. The environmental controlled AsphaltTesting Machine (ATM) was used to conductthese stress-strain tests at different tempera-tures on compacted asphalt mix specimens pro-duced using the U.S. Army Corps of EngineersGyratory Testing Machine (GTM). The GTM wasdeveloped to produce laboratory test specimensby a kneading compaction process using anappropriate gyration angle to simulate the fieldwheel rolling condition [13]. The principle of gyra-tory compaction has been adopted in the SHRPSuperpave asphalt mix design system. SeveralGTM compacted specimens were fabricatedusing the binders from the I-55 test site [1].

The ATM tests were conducted on theGTM specimens at temperature of 10˚C for the

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Test Temperature = -12.0 oC

Binder

Figure 10 (left):Mix relaxation moduluspredictions from theASPHALT program.

Figure 11 (right): Comparisonof lab relaxation modulusresults for virgin asphaltand modified asphaltmixes at 10˚C.



control asphalt and several modified binders andat 25˚C for the control asphalt. In this paper, onlythe results of the tests conducted at 10˚C are pre-sented for comparing the behavior of the controlasphalt with the behavior of the modifiedbinders. The test procedure consisted of an appli-cation of a constant load on the surface of theasphalt compacted mix specimen. This load wasapplied for 3600 seconds and then released tomeasure the permanent deformation of theasphalt specimen. The data acquisition equip-ment reads the displacement measured by twovertical Linear Variable Displacement Transduc-ers (LVDTs).

Fig. 11 shows a comparison of the relax-ation modulus calculated from the test datameasured for the first 1800 seconds. The relax-ation modulus values are normalized by the ini-tial relaxation modulus calculated at 0.5 secondsfor the virgin asphalt mix. These plots show thetime-dependent properties and viscoelasticbehavior of the mix specimens. The modifiedasphalt mixes perform better than the unmodi-fied control asphalt mix. More rutting of theasphalt section after four years, measured in thefield, confirms this finding.

6 COMPARISON OF FIELD EVALUATIONWITH SUPERPAVE BINDER TEST RESULTSField rutting measurements were made on peri-odic basis by the MDOT staff for all nine types ofasphalt binder sections on highway I-55. Fig. 12 isthe box plot of the mean rut depth measured onthe outside wheel path (OSWP) for the controlasphalt and modified asphalt binders sections inNovember 1999. It shows that the largestamount of rutting (about 8 mm) occurred in thecontrol asphalt section and the minimum ruttingoccurred in the Rouse Rubber section and fol-

lowed by the Sealoflex, Styrelf, Kraton andNovophalt polymer-modified asphalt binder sec-tions.

The mean rut depth measured in theoutside wheelpath in November 1999 has beencorrelated with the rheological properties ofthese binders. The G*/sind values for tempera-tures of 46, 76˚C for original binders, 76˚C forRolling Thin Film Oven (RTFO) aged binders, and22˚C for Pressure Aging Vessel (PAV) have beenconsidered. These results were obtained fromthe tests conducted at the University of Missis-sippi [7] and from the MDOT State Study 123report [8]. From the plots of these properties andfield rut depth measurements in 1999, it isobserved that the highest correlation existsbetween the mean rut depth and the DSR ruttingfactor G*/sind at a temperature of 76˚C for bothoriginal binders as well as the RTFO aged binders.The BBR binder test result at -12˚C for Rouse Rub-ber shows relatively higher relaxation moduluscompared to the control asphalt, Sealoflex, Kra-ton, and Novophalt binders.

7 BACKCALCULATION OF IN SITUMODULUS VALUESIn this study nondestructive evaluation (NDE) ofthe pavement sections was conducted by mea-suring surface deflection data using the fallingweight deflectometer (FWD), operated by theMississippi Department of Transportation. Thedeflection data were measured in fall of 1997 and1998 at peak loads of around 4083 kgf (9000 lbf)and higher in the outer wheel path, about 1m (3ft) from the pavement edge [1]. The FWD deflec-tion data measured at the last drop height werenormalized to 4083 kgf (9000 lbf) peak load foranalysis.

A comparative study using several back-calculation programs was done [1]. Based on theresults of this study, the PEDD modulus backcal-culation program was selected because itensures the uniqueness of backcalculated mod-ulus results by predicting seed modulus valuesusing in-built nonlinear deterministic equations.The I-55 pavement structure model consists of a76 mm (3 in) asphalt overlay on top of a 190.5 mm(7.5 in) asphalt layer, a 305 mm (12 in) cementtreated base (CTB) and lime treated subgrade(LTS) layer, and a semi-infinite subgrade layer. Insitu effective modulus values were backcalculat-

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Figure 12:Mean rut depth data

measured in 1999.



ed by the PEDD program using the FWD data col-lected on 9th September 1998 at in situ test tem-peratures in the range of 21.7 to 26.1˚C (71 to 79˚F).The in situ modulus results for the overlay layerare summarized in Fig. 13.

The in situ Young’s modulus values ofthe overlay backcalculated from the dynamicdeflection data also show smaller modulus forthe control asphalt compared to all other binders.The modulus values of Rouse Rubber andSealoflex pavement layers are closer to the con-trol asphalt modulus indicating a desirablebehavior at the low test temperature for in-ser-vice pavements in northern Mississippi. Howev-er, the lower modulus value of the controlasphalt layer at the test temperature of 26.1˚C(79˚F) makes it more susceptible to rutting dur-ing the warmer in-service temperatures, whichmay exceed 50˚C (122˚F) during hot summer days.

8 3D-FE SIMULATION USING VIS-COELASTIC MATERIAL PROPERTIESKey limitations of most backcalculation pro-grams are the assumptions of linear elastic,homogenous and isotropic material propertiesfor the pavement layers and static loading. Threedimensional-finite element (3D-FE) analysisenables for the evaluation of the three-dimen-sional state of stress and strain in a continuum.Dynamic, static, and nonlinear analysis can beconducted using 3D-FE models [14].

In this study, the LS-DYNA finite elementcode [15] was used to analyze and verify the back-calculated modulus values from the PEDD back-calculation program shown in Fig. 13. The 3D-FEhalf model developed for the I-55 test sectioncontained 7313 solid brick elements and 8528nodes. The boundary conditions for this 3D-FEmodel were fixed at the bottom and roller sup-ports were used on the sides.

Material type 6 in the LS-DYNA finite ele-ment program was used to simulate viscoelasticbehavior. The viscoelastic properties for the LS-DYNA input file are: the bulk modulus (B), theshort-term and long-term shear modulus (G0and G1), and the decay parameter, b. The ele-ment’s shear modulus value G(t) and the bulk

modulus, B, are expressed by the following rela-tionships [15]:

(2)

(3)

where E is the Young’s modulus and ν is the Pois-son’s Ratio.

These properties were calculated fromthe ATM tests conducted at 10˚C on the GTMcompacted specimens. From the relaxation mod-ulus plots G0, G1, and b were calculated. The G0value was calculated at the time of 0.5 secondsand G1 at the time of 3000 seconds.

A comparison between the elastic analy-sis and the viscoelastic analysis was done for thecontrol asphalt and Sealoflex pavements sub-jected to the FWD impulse loading. Detailedinput data and results are not shown here forbrevity. The Sealoflex modified binder plot showslarger deflections from viscoelastic analysis,compared to the control asphalt plot, as shownin Fig. 14. This implies smaller in situ Young’smodulus for the modified binder pavement layer.These results indicate that correct properties,appropriate material models, and 3D-FEresponse analysis are needed for correct perfor-mance modeling of asphalt pavements.

9 CONCLUSIONS AND RECOMMENDA-TIONSSeveral modified asphalt binder and controlasphalt samples from I-55 highway overlay pro-ject site were tested using the Superpave DSRand BBR equipment to measure and analyze con-

B E=−3 1 2( )ν

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Figure 13 (left): The PEDDbackcalculated in situYoung’s modulus values forthe overlay.

Figure 14 (right):Comparison of FWDdeflection time historiesfrom 3D-FE elastic andviscoelastic dynamicanalysis.


trol asphalt and modified asphalt binder proper-ties. Creep compliance and relaxation modulusdata for different binders were measured usinga modified BBR test procedure. The performanceof the modified asphalt binders was better thanthe virgin asphalt. This was confirmed by morerutting measured on the control asphalt sectionafter four years of traffic.The mix creep compliance and relaxation modulusresults predicted using the micromechanicsapproach also indicate poor performance of thecontrol asphalt section compared to Rouse Rubber,Sealoflex, and other modified binder sections. Thecorrect time- and temperature-dependent charac-terization of asphalt materials is imperative foradvanced computer modeling and simulation.These improved laboratory material characteriza-tion and analysis efforts are needed for more accu-rate and meaningful structural response analysis ofasphalt pavements and prediction of their perfor-mance for resistance to rutting.

ACKNOWLEDGMENTSThe work reported herein was conducted as apart of the research studies funded by the Mis-sissippi Department of Transportation, U.S.Department of Transportation Federal HighwayAdministration, and Center for Advanced Infra-structure Technology at the University of Missis-sippi. The author acknowledges the efforts ofseveral research assistants and graduate stu-dents including Ben Sale, Yamini Nangiri, andSergio Garza.

DISCLAIMERThe contents of this paper reflect the views of theauthor who is responsible for the facts, findings,and data presented herein.

REFERENCES[1] Uddin W: Improved Asphalt Thickness Design

Procedures for New and Reconstructed HighwayPavements. Report FHWA/MS-DOT-RD-99-122Final Report-State Study SS 122. MississippiDepartment of Transportation (1999).

[2] Uddin W and Nanagiri Y: Performance of Poly-mer-Modified Asphalt Field Trials in MississippiBased on Mechanistic Analysis and Field Evalua-tion. IJP- International Journal of Pavements 1(2002) 13-24.

[3] Monismith CL and Tayebali AA: PermanentDeformation (Rutting) Considerations inAsphalt Concrete Pavement Sections. Proceed-ings, Association of Asphalt Paving Technolo-gists 57 (1988) 414-463.

[4] Ayers Jr.M and Witczak MW: Resilient ModulusProperties of Asphalt Rubber Mixes from FieldDemonstration Projects in Maryland. In Trans-portation Research Record 1492, TRB, NationalResearch Council, Washington, D.C. (1995) 96-107.

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