Evaluation of moisture susceptibility of asphalt mixes containing RAP and different types of aggregates and asphalt binders using the surface free energy method

The surface free energy (SFE) measurement of asphalt binder and aggregate is considered a reliable mechanistic framework for evaluating the moisture-induced damage potential of asphalt mixes. In the present study, the SFE method was used to evaluate the effects of asphalt binder type, Reclaimed Asphalt Pavement (RAP) and its amount, and aggregate type on the moisture-induced damage potential of asphalt mixes. The SFE components (non-polar, acid and base) of a PG 64-22 and a PG 76-28 (polymer-modified) asphalt binders, blended with different amounts of RAP binder (0%, 10%, 25% and 40%) were measured in the laboratory using a Dynamic Contact Angle (DCA) analyzer. Also, the SFE components of six types of aggregates, namely limestone, rhyolite, sandstone, granite, gravel, and basalt were used in this study. The SFE components of limestone and rhyolite aggregates were measured using a Universal Sorption Device (USD), while those of the sandstone, granite, gravel, and basalt aggregates were obtained from the literature. The energy ratio parameters estimated based on the spreading coefficient, the work of adhesion, and the work of debonding were used to assess the moisture-induced damage potential of different combinations of asphalt binders and different RAP binder contents and aggregates. The SFE test results indicated that the acid SFE component of PG 64-22 and PG 76-28 asphalt binders increase with the addition of RAP binder, while the base SFE component remains almost unchanged. Also, the wettability and the work of adhesion of both PG 64-22 and PG 76-28 asphalt binders over different types of aggregates increased with an increase in RAP content (by 25% and more). Based on the energy ratio parameters, it was found that the resistance to moisture-induced damage increased with an increase in RAP content for both PG 64-22 and PG 76-28 asphalt binders and all types of aggregates, specifically when higher RAP contents were used. Moreover, it was found that the higher the total SFE of the aggregates, the lower the energy ratio parameter values. Therefore, a high total SFE component of aggregate may result in a high moisture-induced damage potential of the mix. The results presented herein are expected to be helpful in mechanistically assessing the moisture-induced damage potential of asphalt mixes, produced with polymer-modified and non-polymer-modified asphalt binders, containing RAP.