Aging of asphalt has been an important subject area that has receivedextensive studies in recent years. Test results of short-term and long-term aging behavior of crumb rubber modifier (CRM) modifiedasphalt paving materials are presented. Eighteen combinations ofCRM modified binders in terms of CRM size, CRM content, and baseasphalt cement grade were studied by Brookfield viscometer test anddynamic shear rheometer test. The short-term aged binders were pre-pared using thin film oven test. Viscosity was measured at 350°F inthe Brookfield thermosel after mixing at 375°F for 2 hr. The testresults showed that the size and percentage of CRM affected the vis-cosity development in the modified binders: the smaller the CRM sizeand the higher the CRM content, the higher the viscosity measuredafter 2-hr reaction at 375°F. Short-term aging exerted more viscosityincrease in the CRM modified binders than in the unmodified binders.Modified binders showed less weight loss than unmodified binders.The modified binders showed higher complex modulus G* thanunmodified. Short-term aging increased G*, with modified bindersexhibiting higher increase. The Marshall mix design yielded variousCRM modified mixes. The result of indirect tensile strength testsshowed that short-term and long-term aging increased the measuredtensile strengths. The resilient modulus test results, in general, sup-ported the general understanding that aging tended to increaseresilient modulus.

A modification of the binder system in asphalt concrete hasbeen considered an attractive alternative for improving thequality of asphalt paving materials. In recent years, because ofenvironmental concerns, crumb rubber from scrap tires hasbeen studied extensively, either as asphalt binder or as rubberaggregate, for improving the engineering properties of asphaltconcrete.

With an aim to implement the Intermodal Surface TransportationEfficiency Act of 1991 the Ohio Department of Transportation(ODOT) has sponsored a research project to investigate the mixdesign and properties of asphalt-rubber binder and asphalt-rubber-aggregate mixtures. This paper will present the results of the por-tion of the study that pertains to the aging effects on modifiedbinders and asphalt-aggregate mixtures containing crumb rubberthrough wet and dry processes.

Aging of asphalt has been an important subject that has attractedextensive studies in recent years. Short-term aging is believed to becaused primarily by a loss of volatile components and oxidationduring the construction phase. Long-term aging, on the other hand,is believed to be a consequence of progressive oxidation of the in-place mixture in the field. Although, in general, aging may result instiffening (hardening) of the mixture, which may be beneficial fromthe viewpoint of load distribution and permanent deformation, it

can also result in embrittlement (i.e., increased tendency to crackand ravel) and loss of durability in terms of wear resistance andmoisture susceptibility (1).

Despite a relatively large amount of work done in the past toinvestigate the aging behavior of conventional asphalt-aggregatehot-mix, little information exists about the aging behavior of crumbrubber modified asphalt-aggregate mixes. Furthermore, there hasbeen no report of modified binder properties (aged and unaged) interms of viscosity measurement using thermosel and dynamic shearrheometer test results (i.e., complex shear modulus and phaseangle).

This paper presents the results of indirect tensile strength test andresilient modulus test on the various crumb rubber modifier (CRM)modified asphalt-aggregate mixes (wet and dry processes) forunaged, short-term aged, and long-term aged specimens. In addi-tion, the rubber modified binder properties for unaged and short-term aged (thin film oven test) are presented in terms of viscosity,complex shear modulus, and phase angle. The effects of aging arestudied in terms of the properties of the aged specimen over theproperties of the unaged specimen. Some preliminary observationsconcerning aging effects for both modified binders and mixes aregiven at the end of this paper.

OBJECTIVES

The primary objectives of this paper are as follows:

• To study the effect of short-term and long-term aging on vari-ous rubber modified asphalt concrete mixtures produced by wet anddry processes,

• To study the effect of short-term aging on the various CRMmodified asphalt binders via viscosity measurement using Brook-field viscometer and dynamic shear rheometer test, and

• To study a possible relationship of the aging behavior betweenthe rheological properties of the CRM modified binders and theCRM modified asphalt-aggregate mixes.

MATERIALS USED

Asphalt Cement

Three different grades of asphalt cement, AC-5, AC-10, and AC-20, were used in this study. These asphalt cements were mainlysupplied by Ashland Petroleum Co. in Canton, Ohio. Ecoflex, aproprietary rubberized asphalt manufactured by BITUMAR Inc.,Quebec, Canada, was also used.

R. Y. Liang, Department of Civil Engineering, Center for InfrastructureMaterials and Rehabilitation, University of Akron, Akron, Ohio 44325-3905. S. Lee, Hyundai Institute of Construction Technology, HyundaiEngineering and Construction Co., Ltd., Korea.

TRANSPORTATION RESEARCH RECORD 1530 11

Short-Term and Long-Term Aging Behavior ofRubber Modified Asphalt Paving Mixture

ROBERT Y. LIANG AND SUCKHONG LEE

12 TRANSPORTATION RESEARCH RECORD 1530

Aggregate

The aggregates used were crushed limestones (No. 57 and 9-D perODOT specification) purchased from a local aggregate supplier(Akron Crushed Limestone). The aggregates were first oven-driedbefore being sieved to separate various particle sizes. The follow-ing sieve sizes were used: 3/8 in., 1/4 in., No. 4, No. 8, No. 16, No.50, and No. 200 sieve. Table 1 provides information on aggregategradations used in preparing rubber modified asphalt-aggregatemix according to wet process and generic dry process.

CRM

Most CRM used in this study was obtained from Baker RubberInc. in Chambersberg, Pennsylvania. Different sizes of CRMwere available, including WRF 1/4 in., WRF 10, and WRF 30.An ultrafine CRM was obtained from Goodyear Tire Rubber Co.in Cleveland, Ohio.

BINDER TESTS OF ASPHALT-RUBBER

Eighteen different asphalt-rubbers, which consist of combinationsof AC-5 or AC-10 with three different CRM sizes and various

CRM contents, were mixed at elevated temperature to study thereaction between asphalt cement and CRM.

The method of preparation of rubber modified binders is as fol-lows. First, the asphalt cement and crumb rubber modifier wereheated to 375°F separately. Next, the two were mixed and allowedto react at 375°F for 2 hr. Finally, the rubber modified binders wereretrieved from the oven and inserted into the thermosel and allowedto cool to 350°F, at which point the viscosity was measured. TheBrookfield DV-II+ viscometer with either RV3 or RV4 spindle wasused in this study. The rotational speed of the spindle was 12 RPM.Viscosity measurements at 350°F for CRM modified binders after2 hr of reaction at 375°F are summarized in Table 2. It can be seenthat increasing percentage of crumb rubber in the modified binderincreases the viscosity, measured at 350°F after the modifiedbinders has reacted at 375°F for 2 hr. Furthermore, the size ofcrumb rubber affects the viscosity measured; the finer the crumbrubber, the higher the viscosity developed.

Figures 1 and 2 are prepared to demonstrate differences in thedevelopment of viscosity of CRM modified binders because of dif-ferent sizes of CRM. Figure 1 shows reaction behavior of finer(WRF 30) CRM, in which viscosity quickly reaches a maximumvalue at the early stage of the reaction period. Thereafter, the vis-cosity decreases slightly and remains fairly constant over a 24-hrreaction period. On the other hand, Figure 2 shows reaction behav-ior of coarser (WRF 10) CRM, in which viscosity continuouslyincreases even after 24 hr of reaction period. Again, the viscosityreported in these two figures was measured at 350°F although thereaction of binder and CRM was at 375°F.

Figure 3 shows reaction curves for five different CRM (WRF 10)contents. Note the significant influence of crumb rubber content onthe viscosity increases as a result of extended reaction period.

Figure 4 shows the variations of viscosity versus temperature ofmodified binders. The binders were allowed to react at 375°F for 2hr and then brought out in room temperature for cool down. Theviscosity was measured at three temperatures: 200°F, 250°F, and300°F. This figure gives an indication of how much viscositychange would occur as a result of temperature change in the modi-fied binders. The similar slopes of the lines in Figure 4 indicate thatmodified binders do not pose any more temperature susceptibilitythan the unmodified binders.

TABLE 1 Aggregate Gradations Used and FHWARecommendations for Different Graded Mixtures

TABLE 2 Viscosity (in cp) of Rubber Modified Binders Measured at 350°F after 2 hr of Reactions at 375°F

Liang and Lee 13

lag between applied stress and the resulting strain, is an indicator ofthe relative amount of recoverable and nonrecoverable deformation.For perfectly elastic material, an applied load coincides with animmediate strain response, and the time lag is zero. A viscous mate-rial such as asphalt has a relatively large time lag between load andresponse, leading to a phase angle that approaches 90 degrees.

FIGURE 1 Viscosity versus reaction time for finer CRM (WRF30) at 350°F.

FIGURE 2 Viscosity versus reaction time for coarser CRM(WRF 10) at 350°F.

FIGURE 3 Viscosity of asphalt rubber (AC-10 base) versusreaction time at 350°F.

Short-term aging of modified binders was carried out accordingto thin film oven test (TFOT) (ASTM D1754). Viscosity of themodified binders after a 2-hr reaction period at 375°F was mea-sured at 275°F, before and after TFOT. The measured viscosity val-ues are summarized in Table 3. As evidenced from the calculatedratio, CRM modified binders show larger increase in viscositycompared with unmodified binders. Furthermore, it also indicatesthat the higher the CRM content, the higher the viscosity increasesbecause of short-term aging.

Weight loss of various CRM modified binders due to TFOT issummarized in Table 4. By and large, weight loss does not appearto be a major problem for CRM modified binders. In fact, CRMmodified binders show less weight loss than unmodified binders.

Dynamic shear rheometer test (AASHTO TP5) provides a meansfor measuring the complex shear modulus (G*) and phase angle (δ)of asphalt binder. The method is applicable to asphalt binders havingthe value of the complex shear modulus from 100 to 10 MPa, whichis typically found for binders in the temperature between 5°C and85°C. This test method is intended for determining the linear vis-coelastic properties of asphalt binders as required for SUPERPAVEspecification (2). The complex shear modulus (G*), defined as theratio of the maximum shear stress (τmax) to the maximum shear strain(γmax), is a measure of the total resistance of a material to deformingduring repeated shearing. The phase angle (δ), defined as the time

FIGURE 4 Temperature susceptibility of asphalt rubbers (AC-10 base).

TABLE 3 Viscosity (in cp) Measured at 275°F for Samples Beforeand After TFOT

14 TRANSPORTATION RESEARCH RECORD 1530

Three different temperatures (40°C, 50°C, and 60°C) and onelogarithmic frequency sweep (1 rad/sec to 100 rad/sec) wereselected for conducting dynamic shear rheometer (DSR) tests onaged and unaged rubber modified binders. The applied shearstresses were as follows: 120 Pa for unaged binders, 220 Pa forshort-term aged binders, and 5 KPa for the rubber modified binders,both aged and unaged. The DSR test results are summarized inTable 5 and Table 6 for unmodified and modified binders, respec-tively. Some observations can be made from these two tables. First,CRM in the binder increases the complex modulus. The higher theCRM content, the higher the complex shear modulus. In addition,the short-term aging appears to increase complex shear modulus forunmodified and modified binders, although the increase in complexshear modulus is more significant for modified binders particularlyat 40°C to 50°C. The increase in complex shear modulus becomesrather insignificant at 60°C.

The dynamic storage moduli G′ (elastic component) and thedynamic loss moduli G″ (viscous component) at 60°C are shown inFigures 5 and 6, respectively. It can be seen that short-term agingincreases G′ and G″, although the increase is more significant at40°C than at 60°C. The ratio of G′ over G″ seems to increase with

both aging processes. Addition of CRM to binders shows moreelastic response at higher temperature range.

SUMMARY OF MARSHALL MIX DESIGN RESULTS

The Marshall mix design procedure (3) was followed to developappropriate mix design for different processes (wet process and dryprocess) and gradations (dense and gap-gradation). The optimummix based on Marshall procedure is summarized in Table 7.

AGING METHODS (FORCE-DRAFT OVEN AGING)

The method for aging the asphalt-rubber-aggregate mixtures fol-lowed the procedures in a work by Von Quintus et al. (4). Forshort-term aging, the compacted specimen was heated in the force-draft oven at 275°F for 8 hr. For long-term aging, the compactedspecimen was heated in the force-draft oven at 140°F for 2 days,and then the temperature was increased to 225°F for an additional5 days.

TESTS FOR ASPHALT-RUBBER-AGGREGATE MIXES

Indirect Tensile Strength Test Results

The results (an average of two specimens) of the indirect tensilestrength test for different mixes and ages are summarized in Table 8.Generally speaking, the rubber modified mixes showed smaller indi-rect tensile strength than the conventional hot mix asphalt (HMA)(AC-20). However, it should be pointed out that AC-20 was used inconventional HMA; AC-5 was used in rubber modified mixes.Therefore, direct comparison of indirect tensile strength betweenthese mixes should be avoided. The increase in indirect tensilestrength, as a result of short-term and long-term aging, is evident by

TABLE 4 Weight Loss Because of TFOT

TABLE 5 Results of Dynamic Shear Rheometer Test for Unmodified Bindersat Different Temperatures and Ages (Frequency = 10 rad/sec)

TABLE 6 Results of Dynamic Shear Rheometer Test for CRM Modified Binders atDifferent Temperatures and Ages (Frequency 5 10 rad/sec)

FIGURE 5 G′ versus CRM content at 60°C. FIGURE 6 G″ versus CRM content at 60°C.

16 TRANSPORTATION RESEARCH RECORD 1530

the tabulated ratio in the table. With exception of gap graded, wetprocess with the modified binder (AC5+15 percent WRF 30) anddense grade with Ecoflex, rubber modified mixes exhibit smallerincrease in ratio because of longer aging period when comparedwith conventional HMA.

Resilient Modulus Test Results

The numerical values of the measured total resilient modulus fordifferent mixes with different aging are summarized in Table 9. Ingeneral, it may be observed that short-term and long-term agingtend to have more effect on modified mixes than conventionalHMA. This observation was consistent with observations made inthe binder test.

SUMMARY OF RESULTS

• Increase of viscosity of CRM modified binders depends onCRM sizes and percentage of CRM in the binder. The smaller theCRM size, the higher the viscosity increase measured at 350°Fafter 2 hr of reaction at 375°F. The larger the CRM content, thelarger the increase in viscosity as well. However, the reaction pat-tern appears to be different: the reaction occurs longer in large sizeCRM modified binders than in smaller size CRM modifiedbinders.

• Increase of viscosity (measured at 275°F) due to short-termaging (TFOT) is more pronounced for CRM modified binders thanfor unmodified binders. The higher the CRM content, the higher theincrease in viscosity due to TFOT.

TABLE 7 Results of Marshall Mix Design for Different Processes

TABLE 8 Results of Indirect Tensile Strength Test for Various Mixtures and Aging Conditions

Liang and Lee 17

• CRM modified binders seem to have less weight loss afterTFOT than the unmodified binders.

• CRM modified binders exhibit higher complex shear modu-lus G* than unmodified binders. Short-term aging increases G*for unmodified and modified binders, although the increase ismore pronounced for modified than for unmodified, particularlyat 40°C and 50°C. The increase due to short-term aging becomesinsignificant at 60°C.

• CRM modified binders exhibit higher G′/G″ ratio. The ratio ofG′/G″ increases with increasing CRM content. Higher G′/G″ indi-cates more elastic response at the test temperature and thus betterrutting resistance. Therefore, it seems that CRM modified binderwould provide better mix for resisting permanent deformation.

• Optimum mix designs for various types of process (e.g., wetand dry) have been determined based on the Marshall procedure.The indirect tensile test results on these mixes showed that short-term and long-term aging will increase indirect tensile strength ofthe mixes. The conventional HMA showed much more differencebetween short-term aged mixes and long-term aged mixes, whencompared with CRM modified mixes.

• The test result of the effect of aging on the resilient moduluswas less consistent. However, in general, aging tends to increasethe resilient modulus.

ACKNOWLEDGMENT

The results presented in this paper were part of Mixture Design andPerformance Prediction of Rubber Modified Asphalt Pavement inOhio sponsored by FHWA and ODOT. This support is gratefullyacknowledged. Also, the second author acknowledges financialsupport from the Hyundai Institute of Construction Technology,Hyundai Engineering & Construction Co., Ltd., Korea.

REFERENCES

1. Bell, C. A., Y. Abwahab, and M. E. Christi. Laboratory Aging ofAsphalt-Aggregate Mixtures: Serviceability and Durability of Construc-tion Materials. Proc., 1st Materials Engineering Congress, ASCE, Den-ver, Colo., 1990, pp. 254–262.

2. McGennis, R. B., S. Shuler, and H. U. Bahia. Background of SUPER-PAVE Asphalt Binder Test Methods. Report FHWA-SA-94-069. U.S.Department of Transportation, Jan. 1994.

3. Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types.Manual Series 2. Asphalt Institute, Lexington, Ky., 1993.

4. Von Quintus, H. L., J. A. Scherocman, C. S. Hughes, and T. W.Kennedy. NCHRP Report 338: Asphalt-Aggregate Mixture AnalysisSystem. TRB, National Research Council, Washington, D.C., 1991, 185 pp.

TABLE 9 Results of Resilient Modulus Test for Various Mixtures and Aging Conditions (Mpa)