Effect of Asphalt Film Thickness on Short- and Long-Term Aging of Asphalt Paving Mixtures

Abstract

It is generally believed that an asphalt paving mixture should have an adequate asphalt film thickness around the aggregate particles to ensure reasonable durability (resistance to aging) of the mixture. The minimum asphalt film thickness generally recommended ranges from 6 to 8 μm. However, no significant background research data are available in the literature to support these recommended minimum asphalt film thicknesses. Some states specify minimum asphalt film thickness for mix designs. This study was undertaken to quantify the relationship between various asphalt film thicknesses and the aging characteristics of the asphalt paving mix so that an optimum film thickness desirable for satisfactory mix durability could be established. Mixes prepared with asphalt binder film thickness ranging from about 4 to 13 μm were subjected to accelerated aging using Strategic Highway Research Program (SHRP) procedures to simulate both short- and long-term aging. Both the aggregate (RD) and the asphalt cement (AAM-1) used in this study were obtained from the SHRP Materials Reference Library. The aged, compacted mix was tested for tensile strength, tensile strain at failure, and resilient modulus. The aged asphalt cement was recovered and tested for penetration, viscosity, complex modulus, and phase angle. Aging indexes were obtained from these tests, and the relationship between film thickness and the aged mix/aged asphalt cement properties were determined using regression analysis. For the particular aggregate/asphalt cement combination used in this study, it was found that accelerated aging would occur if the asphalt binder film thickness was less than 9 to 10 μm in an asphalt paving mixture compacted to 8 percent air void content.