Characterization of microdamage and healing of asphalt concrete mixtures

Asphalt concrete mixture is the most commonly used material in pavement construction today because of its high quality engineering performance such as elasticity, stability, durability, and water resistance. Asphalt concrete exhibits creep and relaxation behavior. Its magnitude normally depends strongly on temperature, stress level and loading duration. Attempts to predict the crack initiation and propagation in fracture of asphalt concrete are complicated by all such variables. Although extensive research has been done, the fundamental mechanism of fatigue fracture and permanent deformation still awaits discovery and general predictive theory remains to be worked out.; Pseudo strain concept in the correspondence principle has been extended to nonlinear viscoelastic behavior of asphalt mixtures by introducing a nonlinear reference modulus. It has been demonstrated in this research that dissipated pseudo strain energy is a damage parameter. Along with pseudo stiffness, it can be successfully used to quantify the fatigue microdamage and tertiary microdamage of permanent deformation. Pseudo strain energy increases and pseudo stiffness decreases consistently with an increasing number of loading cycles under strain-controlled mode of loading, indicating the microdamage is occurring during the fatigue test. The significant recovery of pseudo stiffness after introducing rest periods indicates the healing effect due to rest periods.; Modified Paris' law in which the J-integral rather than stress intensity factor is used is applied to quantify the microdamage initiation during fatigue and permanent deformation tests. In general, the mixtures with hydrated lime have a better healing effect and then have a longer microdamage fatigue life than those of the mixture without hydrated lime by significantly enhancing the healing effect.; The effects of rest periods on microdamage fatigue life extension due to healing of microcracks are significant. It has been demonstrated that the longer the duration of rest period is, the more significant healing effect is observed, and the more number of rest periods are introduced, the more fatigue life extension is expected.; The effects of two additives on permanent deformation have been evaluated by the rate of change of permanent strain and pseudo strain energy. Both methods can be successfully used to characterize the tertiary damage of permanent deformation in the asphalt concrete mixtures.