Mechanistic modeling of healing in asphalt mixtures using internal stress

Abstract Healing of a material refers to a process of the restoration of original material properties to a damaged material. There are currently phenomenological methods and mechanism-based methods to characterize healing in asphalt materials. The phenomenological methods are based on the change of some kind of material property due to healing. The mechanism-based methods describe healing as a combination of a wetting process and a diffusion process. In addition to these two types of models, the paper aims at presenting a mechanical view of the healing process and developing a mechanistic approach to model healing in asphalt mixtures. A new perspective of studying healing is proposed in this paper by relating it to the recovery of a material, based on which the mechanics of healing is well explained. The relationship between healing and recovery is clarified by defining the apparent recovery and true recovery. The apparent recovery of a bulk damaged material is driven by the apparent internal stress. The true recovery of the intact material is driven by the true internal stress. The apparent recovery involves two simultaneous processes: the true recovery and healing. By studying the energy redistribution around cracks during healing, it is found that the true internal stress and the interfacial force of attraction are the two driving forces for healing. Under their actions, the direct result of healing is the decrease of damage density as a result of the growth of the contact area between crack surfaces. The energy-based mechanistic (EBM) approach is used to conduct mechanical analysis on the healing process and model healing by the progression of damage density. The creep and step-loading recovery (CSR) test is utilized to measure the apparent and true internal stresses. By using the EBM approach to analyze the CSR test data, cracking is modeled in the creep phase of a CSR test, and healing is modeled in the recovery phase of the same CSR test. The analysis produces a damage density progression curve in the both phases. The initial damage when healing starts is represented by the damage density at the end of the creep phase; the damage at any time during the healing process is represented by the damage density in the recovery phase. The difference between the initial damage when healing starts and the damage at an arbitrary time point during recovery thus quantifies the extent of healing that occurs during this rest period.