Characterization of microdamage healing in asphalt concrete with a smeared continuum damage approach

The fatigue of asphalt concrete (AC) mixtures is a major source of distress in pavement structures. In most laboratory fatigue studies, testing is carried out by means of continuously repeating load cycles. However, under real traffic conditions, AC is subjected to a succession of load pulses as the traffic passes. Between these loads the material is not subjected to external forces and undergoes changes related to relaxation and healing, which together act to increase the overall fatigue life relative to what is observed under continuous loading. In the study reported in this paper, the effects of a rest period on the fatigue response in uniaxial, on-specimen, displacement-controlled loading was studied at 4.4°C, 21.1°C, and 38°C. Loading consisted of a sinusoidal pulse-rest history with rest periods of 1, 5, or 10 s. Mixtures were used with the same aggregate structure and constituent materials hut different asphalt and air void (AV) contents. It was found that the introduction of even short rest periods substantially improved the fatigue response of AC. A viscoelastic damage model for smeared healing was derived and characterized with the measured data. This model was compared with a damage-only continuum formulation and used to identify the impacts of healing, apart from any viscoelastic relaxation effects. It was found that greater AC and lower AV content yielded more favorable healing behaviors. The findings also suggested differences in the active healing mechanisms in pulse-rest and block-rest loading, typically used to study healing in AC. These differences are discussed.