A synthesis of theories and mechanisms of bitumen-aggregate adhesion including recent advances in quantifying the effects of water

A synthesis of research on bitumen-aggregate adhesion is presented within the framework of theories and mechanisms established in the general field of adhesion science. An effort is made to convey the material with perspective and to introduce terms and concepts that would aid in understanding the theories and associated mechanisms. The ability of these theories to place bitumen-aggregate adhesion and the effect of moisture on a quantitative level is considered and practical implications discussed. Experimental support is included where available. The theory of (weak) boundary layers, mechanical theory, electrostatic theory, chemical bonding theory, and thermodynamic theory are presented. While chemical bonding theory elucidates bitumen-aggregate adhesion on a microscopic level, thermodynamic theory conveniently provides a quantitative approach based on macroscopic measurements. Compatibility of different bitumen-aggregate pairs can be assessed if their surface free energies are known. In addition, surface energy in combination with other material properties serves as an important ingredient in performance prediction, including stripping potential. Thermodynamic theory also offers the potential to quantify the effect of pH of interface water on the adhesive bond. The authors suggest that the key to quantifying this effect should be sought in the relationship between pH and electron donor-acceptor properties of bitumen and aggregate surfaces.