Constitutive modeling of coupled aging-viscoelastic response of asphalt concrete

Oxidative aging is one of the most important processes affecting the time-dependent response of asphalt concrete materials, which subsequently results in brittleness of the material and fatigue damage. This paper proposes a mechanistic-based constitutive relationship of oxidative aging based on the evolution of a physically-based aging state variable. The aging constitutive relationship is coupled to the viscoelastic response of asphalt concrete by making both the creep compliance and relaxation time terms a function of the aging variable. The coupled oxidative aging-viscoelastic constitutive relationship is calibrated by analyzing the results of dynamic modulus testing conducted at different temperatures and frequencies on aged and unaged laboratory specimens with different air void contents ranging from 4% to 10%. The coupled constitutive relationships is validated against multiple repeated creep-recovery tests conducted on aged and unaged specimens at different stress levels, temperatures, and air void contents. The results illustrate the capabilities of the coupled oxidative aging-viscoelastic constitutive relationship to predict the viscoelastic response of aged asphalt concrete materials. Finally, the effect of oxidative aging on viscoelastic performance of aged pavement structures is investigated by: (a) applying a state-of-the-art experimental-computational method to determine oxygen diffusivity of asphalt concrete mixtures, and (b) conducting 2D finite element (FE) simulation of aged and unaged pavements.