| A comprehensive study is conducted on the surface free energy concept, measurement, and its applications in asphalt pavement. Cohesive and adhesive bonding within the asphalt-aggregate system are directly related to the surface free energies of an asphalt-aggregate system. The thermodynamic changes in the surface free energy of adhesion and cohesion are related to the de-bonding of the interface between asphalt and aggregate and to cracks that may occur within the mastic, respectively. On the other hand, it is also true that thermodynamic changes in the surface free energy are required to heal a fracture between the surfaces of the asphalt and the aggregate or within the mastic. Two methods, the universal gas adsorption and the Wilhelmy plate, are used to measure surface free energies of aggregate and asphalt, respectively. The universal gas adsorption method can accommodate the peculiarity of the irregular shape, size, mineralogy, and rough surface texture of the aggregate. The Wilhelmy plate method measures both the surface free energy of wetting (healing) and the surface free energy of dewetting (fracture) from the advancing angles and receding angles, respectively. The surface free energy concept is very useful to model the fatigue and healing effect of the asphalt pavement.; The surface free energy concept can also help to select the most compatible asphalt-aggregate pair for mixtures in terms of adhesion and moisture resistance. Two moisture damage models based on major moisture failure mechanisms are proposed. The adhesion failure model was developed to analyze the adhesive fracture between asphalt and aggregate in the presence of water. The surface free energy of adhesion with or without the presence of water can be calculated from the surface free energies of asphalt, aggregate, and/or water. The moisture diffusion model was used to obtain the moisture diffusion characteristics of asphalt binders. The amount of moisture that permeates a binder is identified as a key factor in the moisture damage analysis. Finally, mechanics-based experiments were conducted on sand asphalt and asphalt mixtures, and the results have good agreement with the models. |
