Characterization of Healing in Sand Asphalt Mixtures Using a Thermomechanical Framework

Abstract

Asphalt mixtures are being used in almost 90 percent of the world’s airfield and highway pavements. Asphalt mixtures are know to exhibit non-linear mechanical response even at small deformation. Healing is one such non-linear response of asphalt mixtures in that the material exhibits “beneficial internal structure change” if allowed to rest after certain number of loading cycles. During the rest period of asphalt mixtures, relaxation of stresses due to the viscoelastic nature of the asphalt binder and healing of asphalt mixtures takes place. Relaxation of stresses for a nonlinear viscoelastic material is a tractable problem. However, the same cannot be said for healing of asphalt mixtures. Healing occurring during rest periods of an asphalt mixture is characterized by the change in the internal structure of the material. After the asphalt mixture has significantly healed, the subsequent response of the material to the next cycle of loading exhibits behavior that is more similar to that of a “stiffer” material than the original parent material. In this investigation, repeated load triaxial tests are conducted on sand asphalt mixtures. Appropriate rest periods are given and the healing of the material is quantified in terms of change in the deformation response. The constitutive model developed earlier by Krishnan and Rajagopal (2004) is appropriately modified to take into account the healing of sand asphalt mixtures. Healing is characterized by means of material functions that depend on the difference in the elastic response before and after the rest periods and also in the mapping between the natural configurations of the material before and after the rest periods. The model predictions were corroborated with the experimental investigations.