CHEMICAL AND MECHANICAL PROCESSES OF MOISTURE DAMAGE IN HOT-MIX ASPHALT PAVEMENTS

Historically, six contributing mechanisms to moisture damage have been identified: detachment, displacement, spontaneous emulsification, pore pressure-induced damage, hydraulic scour, and the effects of the environment on the aggregate-asphalt system. However, it is apparent that moisture damage is usually not limited to one mechanism but is the result of a combination of processes. It has become necessary to seek a more fundamental understanding of the moisture damage process by carefully considering the micromechanisms that influence the adhesive interface between aggregate and asphalt and the cohesive strength and durability of the mastic. Factors that influence the adhesive bonds in asphalt mixtures and the cohesive strength of the mastic in the presence of water are presented and discussed as a fundamental approach to calculating adhesive bond strength in asphalt mixtures in the presence of water on the basis of surface free energy measurements. The adhesive bond that determines the durability of asphalt mixtures in the presence of water is described in this paper to be based on a nonuniform distribution of charges in the asphalt and on the aggregate surface. The polar compounds in the asphalt that react with the aggregate polar sites determine the strength and durability of the adhesive bond. Several processes are presented that affect this bond. The effect of aggregate mineralogy, surface properties, and the pH at the water-aggregate interface is discussed.