The effects of moisture and aging conditions on the cohesive and adhesive debonding of asphalt mixture using thermodynamical and mechanical methods

Abstract

The two mechanisms of bitumen-aggregate adhesive and cohesive failure are generally affected by the bitumen's characteristics and its compatibility with aggregates. Bitumen aging is one of the most critical factors with the most destructive impact on both these mechanisms, especially when associated with moisture conditions. By capturing asphalt mixtures' behavior in critical states, i.e., aging and moisture penetration, it is possible to choose the most compatible bitumen-aggregate combination, offer helpful solutions to improve the properties of aged bitumen by rejuvenating it with additives, and eventually, change the rate of occurrence of both failure mechanisms. Accordingly, this study investigated adhesive and cohesive failures under the impact of moisture and aging conditions with mechanical and surface free energy (SFE) methods and provided helpful solutions using additives to minimize these two major failures. While the efficacy of antistripping additives like Wetfix BE and Teragrip AN is recognized, their performance mechanism under combined environmental stressors remains poorly understood. This study provides a novel mechanistic evaluation by subjecting asphalt mixtures to simultaneous aging and moisture conditioning. The research advances the field by correlating fundamental SFE properties with direct mechanical pull-off strength measurements. This integrated approach allows for the first time to deconvolute the adhesive and cohesive failure mechanisms and quantitatively evaluate how each additive modifies the bitumen-aggregate interface thermodynamics to enhance durability under severe field-realistic conditions. This study used the rolling thin-film oven and pressure aging vessel tests to simulate the environmental conditions and bitumen aging, Universal Sorption Device and sessile drop tests to measure the SFE elements of aggregate and bitumen, and the Pull-Off test to simulate the failure mechanism of bitumen and aggregate. To this end, 36 combinations of asphalt mixture, comprising two bitumens (PG 58-22 and PG 64-16), two aggregates with basic and acidic properties (limestone and granite), and two rejuvenators (Teragrip AN and Wetfix BE ), were utilized. The results revealed that despite the adhesion reduction due to moisture and aging, the two additives successfully compensated for the decline in adhesion, which can also be ascribed to positive changes in the debonding (stripping) free energy. Moreover, despite the high rate of changing of cohesion due to aging, these additives played an undeniable role in cohesion. Finally, statistical analyses indicated that the correlation coefficient between the cohesion Pull-Off in the dry state and the cohesion free energy increased from 0.84 (without additives) to above 0.9 (modified states), suggesting the high accuracy of the laboratory data when using the additives.