Development of Relationships Between SHRP Asphalt Test Parameters and Structural Mixtures for Mechanistic Analysis and Rehabilitation Design of Flexible Pavement

By Mang Tia, Byron Ruth, and Reynaldo Roque

The University of Florida


Problem Statement

The recent Strategic Highway Research Program (SHRP) in the asphalt area has resulted in the development of performance based asphalt binder and asphalt-aggregate mixture specifications. The Florida Department of Transportation (FDOT) is committed to implementing these new tests and specifications. While these new tests and specifications offer potential for improved pavement performance and management, there has been very little experience with them. There was a need to evaluate these new test methods with regard to their applicability for use as direct input for developed for mechanistic pavement analysis and design program previously developed for FDOT. SHRP test methods and specifications did not address pavement rehabilitation nor mechanistic analysis. Consequently, there is a need to relate asphalt properties obtained from these tests to mix behavior at the critical thermal load conditions where pavement failures are most likely to occur.

A laboratory study was conducted to evaluate the relationship between the results of the SHRP binder tests and those of the conventional binder tests, and the applicability of these tests for prediction of asphalt mixture properties. The scope of the study covers six different base asphalts from five different refinery sources and three types of modifiers, namely a ground tire rubber (GTR), styrene-butadiene rubber (SBR) and styrene block copolymer.

Objectives

The objectives of this research study were to develop and evaluate the relationships between the results of SHRP binder tests and the results of other conventional asphalt tests that have been commonly used by the FDOT, and to develop and evaluate the relationships between the results of the SHRP binder tests and the cracking resistance of the asphalt mixtures under Florida conditions. The study also evaluated the aging characteristics of selected modified asphalts and mixtures which are likely to be used in Florida, and evaluated the SHRP Performance-related tests for asphalt binders and modified asphalt binders. Modifications were made to the computer program Rehabilitation Evaluation and Design of Asphalt Pavement Systems (REDAPS), used for mechanistic analysis and design of flexible pavements by FDOT using nondestructive testing techniques.

Findings

The standard rolling thin film oven test (RTFOT) was used to simulate the short term aging of the binders during the hot-mixing process, and the SHRP pressure aging vessel (PAV) process at 100C was used to simulate the additional aging of the asphalt binders in the field. The original and the aged binders are evaluated by both the conventional and SHRP binder tests, which include (1) penetration, (2) absolute viscosity, (3) Brookfield rheometer, (4) Schweyer rheometer, (5) Frass breaking point, (6) bending beam rheometer, (7) dynamic shear rheometer, and (8) direct tension tests.

Asphalt mixtures were compacted by the gyratory testing machine (GTM) and aged according to the SHRP proposed short term oven aging (STAO) procedure and long term oven aging 9LTOA) procedure. These aged and unaged asphalt mixtures were evaluated by resilient modulus, indirect tensile strength, and indirect tensile creep test at 0, -10, and -20°C. Asphalt residues were recovered from the broken samples and evaluated by the binder tests.

The results show that the G*sin and G*values as measured by the SHRP dynamic shear rheometer correlate well with the results of the penetration and viscosity tests on the same binders at similar temperatures. The creep stiffness as measured by the bending beam rheometer at low temperatures correlates well with the Frass breaking point temperature. However, the failure strain as measured by the direct tension tester at low temperatures correlates poorly to the Frass breaking point temperature.

The mixture stiffness as determined by the SHRP indirect tensile creep test at -10°C was found to be linearly related to the creep stiffness of the binder at the same temperature. The tensile strength, failure strain and fracture energy of the mixtures as measured by the indirect tensile strength test at -10°C were found to be not related to the binder creep stiffness at the same temperature. The resilient modulus was found to be close to the stiffness of the mixture at 1 second as determined by the indirect tensile creep test.

Conclusion

The test results indicate that asphalt modifications with SBR or GTR could improve both the high temperature and low temperature characteristic of the asphalt binders. The findings support the use of GTR and SBR as asphalt additives for improved resistance to rutting and low temperature cracking.

This research project was conducted by Mang Tia, Byron Ruth, and Reynaldo Roque, at the University of Florida. For more information on the project, contact Gale Page, Project Manager, at (352) 642-3208.

The University of Texas at Austin  •  UT's Cockrell School of Engineering