Thermodynamics of healing of asphalt binders with free energy

Inspired by self-healing attributes, one of the key topics in the field of pavement engineering is changing the way that asphalt materials behave for prolonged service life and environmental savings. Extensive experimental approaches, models and technologies have been developed to characterize the evolution of healing influenced by internal and external factors. The purpose of this study is to explore the local kinetics of asphalt binders that energetically favor autonomous self-healing as well as its mechanism using free energy method. With consideration of driving forces including internal stress and interfacial forces of attraction (or surface energy) responsible for energy redistribution during the rest period, the healing of an asphalt binder was first modeled as a function of thermodynamic parameters. New healing indexes were then defined by the total free energy change for healing (ΔGH) and fast-rate response ratio (RH) of initial healing in duration to the total rest time. Thirdly, a fatigue-healing test was performed on #70 asphalt binder and SBS-modified asphalt binder to examine the potential and reactivity of healing affected by different damage levels. Finally, the effectiveness of proposed thermodynamic healing indexes in describing healing ability was discussed by considering their relations with properties measured by mechanical analysis. Results show that more negative ΔGH indicates a greater tendency to spontaneous healing while less negative ΔGH suggests a poor healing potential as crack propagates to a certain length. In contrast to #70 asphalt binder, SBS-modified asphalt binder exhibits a superior healing potential on a shorter crack length scale. RH gradually increases with the increase of crack length for both #70 asphalt binder and SBS-modified asphalt binder, implying limited mobility of molecules. By contrast with mechanistic healing indexes assessing properties before and after healing (i.e. the degree of healing), the thermodynamic healing indexes provide a new vision regarding healing potential, which reveals the true ability of a material to autonomously repair damaged areas.