Creep damage model and damage evolution of asphalt mixtures

To quantitatively evaluate the creep characteristics of asphalt mixtures, the mechanisms associated with both creep hardening and creep damage and deterioration throughout the creep process of asphalt mixtures were considered. Based on the fractional calculus theory, a relatively simple fractional creep damage model was developed. In this model, a fractional Maxwell model was used to describe the creep hardening mechanism, and the damage strain was used to represent the creep damage and deterioration mechanism. In addition, a damage evolution equation for asphalt mixtures was statistically derived. Uniaxial compressive creep tests were performed on AC-13 asphalt mixtures at different stress levels (0.179, 0.358, 0.448, 0.537, and 0.716 MPa). The nonlinear fitting was carried out using the Levenberg-Marquardt optimization algorithm to determine the parameters of fractional creep damage model as well as the damage evolution curves at different stress levels. To construct a unified damage evolution model for different stress levels, a method to statistically quantify the damage evolution of asphalt mixtures was proposed, and the evolution relationship between the creep damage and the damage strain was established. Research results show that the determination coefficients between the proposed fractional creep damage model results and the test results at different stress levels are all not less than 0.995, indicating that the proposed model is suitable for describing the entire creep process including the decay, stable, and accelerated creep stages. In the decay creep stage, the damage of asphalt mixture at different stress levels is less than 1.0×10^-3 and is negligible compared to the damage (0.8) at creep failure. In the stable creep stage, the damage gradually increases. Eventually, the asphalt mixture undergoes creep failure when the creep stress exceeds a certain value. The flow time depends on the applied stress level. The determination coefficient of evolution relationship between the creep damage and the damage strain fitted by the two-parameter Weibull distribution function is 0.992. This indicates that one damage evolution model can be developed for different stress levels. Its parameters are only related to material properties and temperature and are independent of the applied stress. 2 tabs, 10 figs, 32 refs.