Development and Validation of Viscoelastic-Damage Model for Three-Phase Permanent Deformation of Dense Asphalt Mixture

Permanent deformation of asphalt mixture under repeated load can be divided into three phases: decelerating phase, stationary phase, and accelerating flow phase. Most of the existing models cannot fully describe deformation characteristics during the three phases. In this paper, the characteristics of three-phase permanent deformation are considered as the results of a competition between damage and hardening, in which a damage variable and a hardening variable are introduced to modify Burger’s model. First, the series-wound dashpot of the Burger’s model is modified by Usan’s hardening variable, and the undamaged viscoelastic properties are derived from rheological theory. Secondly, Kachanov’s equation for damage evolution is adopted to establish the constitutive model by using Lemaitre’s effective stress principle. Then a repeated load consisting of a haversine loading period and rest period is employed to simulate the actual vehicle loading on the asphalt pavement. Third, a viscoelastic-damage mechanical model is proposed to characterize the permanent deformation of asphalt mixture under repeated load. The proposed model is validated by conducting uniaxial permanent deformation tests on four kinds of asphalt mixtures. The model parameters are obtained by using the principle of least square. The proposed model is capable of describing the whole three-phase permanent deformation of the asphalt mixtures. The proportion of residual viscoelastic strain to permanent strain decreases very quickly as load cycle increases and only accounts for $2\sim 3\%$ during most of the loading period. It is indicated that residual viscoelastic strain is significantly less than the permanent strain, thus the residual viscoelastic strain can be ignored and the proposed model could be simplified to a concise format.