Experimental Researches On Visco-elastoplastic Constitutive Model Of Asphalt Mixture

In recent years, asphalt concrete is widely used as a pavement structural layer. However, pavement damage is getting worse and worse, with the increasing volume of road loading and the focus of transport converting to the heavy direction. The research on the constitutive model of asphalt mixture is very important for understanding the deformation law of the road pavement, predicting the permanent deformation of pavement structure, and suppressing the road failure. Based on the compression and creep tests, the visco- elastoplastic constitutive model of asphalt mixture is proposed by combining theoretical and numerical fitting method in the paper, and the analytical method of determining the nonlinear parameters included in the proposed model is also presented. Predictions of the model are compared with experimental results, a good agreement shows that the model is verified validly. At last, in the paper, the effect of the gradation on the mechanical behavior of asphalt mixture is also studied. The main contents include:(1) The visco- elastoplastic differential constitutive models of asphalt sand are proposed. Based on the creep behavior of asphalt sand, the total deformation is decomposed into two components of viscoelastic and viscoplastic deformation. The viscoelastic deformation is described by Burgers model, and the viscoplastic deformation is described by the parallel model consists of a slider and a sticky pot. Then, by combining the two submodels, three visco- elastoplastic constitutive models based on the two components is obtained. Furthermore, the total deformation is decomposed into three components of the viscoelastic, viscoplastic and elastoplastic deformation. By using different submodels to describe them separately, the fourth visco- elastoplastic constitutive model based on the three components is also established. Analyzing the predictions of the above four models in different stress levels, the optimal model is selected to predict experimental results at different temperatures and load stress levels. By comparing its predictions with experimental results, it shows that the model can well describe the creep deformation characteristics of three stages of asphalt sand.(2) The visco- elastoplastic integral constitutive model of asphalt sand and mixture is proposed. Based on the separation of the total deformation to viscoelastic and viscoplastic deformation, by adopting Shapery nonlinear model to describe viscoelastic deformation and Uzan model to describe viscoplastic deformation, a modified Shapery integral model is proposed to establish a nonlinear visco- elastoplastic constitutive relationship. And also a data processing method for determining the nonlinear parameter of the model is presented. Different types of experiments are designed to extract the different deformation components, for example, creep-recovery tests determine the viscoelastic parameters, and viscoplastic parameters can be obtained by multiple cyclic creep-recovery experiments. By assuming that creep compliance is of exponential for time, and using the obtained model, the deformations of asphalt sand and mixture at different stress levels are predicted. Compared with the Shapery model predictions, the proposed model shows a better agreement with experiment results, especially in high stress levels.(3) The effect of the gradation on the mechanical behavior of asphalt mixture is studied by compression and creep experiments. Analysis on the composition theory of asphalt mix and size differentiation of coarse and fine aggregates is conducted, and then the effect of composition of coarse and fine aggregates on the mechanical behavior is studied. By using the compressive strength and creep rate as two indicators, it shows, while keeping the content of the coarse and fine aggregates constant, gradation changes of fine aggregate (the aggregate which size is less than 2.36mm) have little effect on the mechanical behavior of asphalt mixture, however, and gradation changes of coarse aggregate (the aggregate which size is larger than the 2.36mm) have a significant effect. Therefore, in the research and analysis, the particle size 2.36mm can be set as the dividing line of coarse and fine aggregates, and then the effect of fine aggregate gradation changes on mechanical behavior of asphalt mixture can be ignored, which is the basis of research on the mechanical behavior of asphalt mixture with the two-step method.