Limits on Adhesive Bond Energy for Improved Resistance of Hot-Mix Asphalt to Moisture Damage

The loss of physical adhesion between the aggregate and the asphalt binder is one of the important mechanisms that accelerate moisture damage in hot-mix asphalt pavements. In this study, two parameters related to bond energy—adhesive bond energy between the aggregate and the asphalt and reduction of free energy when asphalt debonds from the aggregate surface in the presence of moisture—were quantified with surface energies of both materials. Threshold values of these parameters to identify asphalt–aggregate combinations susceptible to premature moisture damage were derived by comparison of the values of these parameters with observed field performance for several mixes. Results show significant differences in bond energies developed between various aggregates and a given binder. This finding illustrates the importance of binder–aggregate compatibility and the sensitivity of calculated bond strength to surface energy measurements. Asphalt binders from different sources with the same performance grade were also found to develop different bond energies with any given aggregate. The results show that binders differ in their sensitivity to changes in aggregate source in terms of the developed bond energy. The methodology of using surface energy and concomitant bond energy calculations to assess the moisture sensitivity of asphalt concrete mixes is discussed as well as the advantages of using this technique compared with conventional mechanical tests.