This synthesis will be of interest to pavement designers, construction engineers, maintenance engineers, and others interested in avoiding or limiting moisture damage in asphalt concrete. Information is provided on physical and chemical explanations for moisture damage in asphalt concrete, along with a discussion of current practices and test methods for determining or reducing the susceptibility of various asphalt concrete components and mixtures to such damage. Moisture damage in asphalt concrete is a nationwide problem which often necessitates premature replacement of highway pavement surfaces. This report of the Transportation Research Board describes the underlying physical and chemical phenomena responsible for such damage. Current test methods used to determine the susceptibility of asphalt concretes, or their constituents, to moisture damage are described and evaluated. Additionally, current practices for minimizing the potential for moisture damage are examined.

Moisture damage in asphalt pavement reduces the service life of the pavement as well as increase permanent deformation. Moisture sensitivity in asphalt concrete mixtures is often associated with high concentrations of fine aggregate particles. In this study, the effects of baghouse fines, a source of fine mineral aggregate, on moisture sensitivity were examined. Two types of baghouse fines with different gradations were used in various concentrations in the laboratory production of hot-mix-asphalt samples. To determine the effects of the various baghouse fines contents, testing was performed to determine the tensile-strength-ratio of the different mixes. In order to prevent moisture damage in asphalt pavements, additives are often used to alter the interaction between the asphalt binder and the mineral aggregate. These additives can change the molecular charge of the binder or reduce the viscosity of the asphalt cement. In order to determine the effectiveness of the anti-strip additive in preventing moisture damage, the tensile-strength-ratio was also determined for specimens containing various additive and baghouse fines contents. The results of the tests showed a reduction in retained strength for the specimens without additive as compared to the specimens containing additive, demonstrating the effectiveness of the additive in preventing moisture damage. To assess the rutting resistance of the various asphalt mixtures, an Asphalt Pavement Analyzer test was performed. Half of the laboratory compacted specimens were moisture conditioned and tested submerged, while the other half was tested dry. Results indicate an increase in rut depth with the removal of anti-strip additive from the mix, indicating the effectiveness of the additive in preventing moisture damage. Finally the specimens were tested in the Superpave Shear Test Machine. Frequency Sweep and Repeated Shear tests were performed for each mixture with half of the samples again conditioned. The Frequency Swee.

Incompatibility between aggregate type and asphalt binder, presence of standing water or water under pressure in the pavement layers, and improper construction practices are some of the many factors that influence moisture susceptibility of flexible pavements. The TRB National Cooperative Highway Research Program's NCHRP Synthesis 595: Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements documents practices used by state departments of transportation to prevent or to minimize moisture damage in hot-mix asphalt pavements.