PREDICTION AND EVALUATION OF MOISTURE EFFECTS ON ASPHALT CONCRETE MIXTURES IN PAVEMENT SYSTEMS

Water-induced damage of asphalt concrete mixtures have produced serious distress, reduced performance, and increased maintenance for pavements in the United States, as well as other areas in the world. This damage mainly occurs due to stripping of asphalt cement from aggregate and in some case possibly due to softening of the asphalt matrix. In an attempt to reduce the magnitude of the problem, various antistripping additives have been incorporated into asphalt mixtures. Unfortunately, there has been no reliable way to evaluate their potential effectiveness or to evaluate proposed aggregate-asphalt combinations to determine their water susceptibility.; In response to the above problems research results in this report describe how tests can be used to determine the extent, nature, and severity of moisture-related damage to asphalt concrete mixtures used in pavements. In addition, the causes and mechanisms creating deterioration are discussed and related to those mixture and environmental factors associated with moisture damage.; Included in the report are evaluations of several testing techniques for distinguishing between aggregate-asphalt combinations which are susceptible to moisture damage and those which are not. Test methods included: (1) Static Indirect Tensile Test on dry and wet cylindrical specimens, (2) Repeated-Load Indirect Tensile Test on dry and wet specimens, (3) Texas Freeze-Thaw Pedestal Test, and (4) Texas Boiling Test. Results of these evaluations show that both the Texas Freeze-Thaw Pedestal Test and the Boiling Test can be used to differentiate between known stripping and nonstripping asphalt mixtures. In addition, the tests can be used to evaluate the individual components of mixtures to determine which are water susceptible.; The authors suggested that the Texas Boiling Test, which is simple and easy to conduct, be used for initial short-term screening. The Texas Freeze-Thaw Pedestal Test should be used for final and long-term evaluations. However, if the mixture has high air voids, it should be evaluated using the Static Indirect Tensile Test on dry and wet specimens.; Finally, the report includes a discussion of potential treatments which could alleviate the adverse moisture effects on pavement: adding antistripping agents or lime slurry, pretreatment of stripping-prone aggregates, and design or construction control, etc. A limited study using the Texas Freeze-Thaw Pedestal Test and the Boiling Test was performed to evaluate the effectiveness of antistripping additives in enhancing the bond between asphalt and aggregate.