Models and solutions for permanent deformation and thermal cracking of flexible pavements

This research deals with modeling permanent deformation and thermal cracking of flexible pavements. A three-dimensional non-linear finite element algorithm is developed to predict the permanent deformations and residual stresses of flexible pavements. Important factors such as moving loads and material non-linearity are considered. Special numerical techniques, which offer some numerical advantages, are also incorporated to treat the involved non-linear boundary conditions and unbounded domains. The finite element program is used to study some problems of practical interest, such as: the effect of moving loads on permanent deformations and residual stresses, the effect of wheel-wheel interaction and the analysis of the stress histories of subgrade materials.; The thermal cracking model reflects the multi-scale nature of the thermal cracking of flexible pavements: micro-cracking on the meso-scale and damage localization or redistribution on the macro-scale. Randomly distributed fictitious cracks are introduced to represent the material heterogeneity and damage on the meso-scale. Friction is recognized as the mechanism leads to stress redistribution and, therefore, damage localization on the macro-scale. When the problem is assumed to be one-dimensional and Coulomb friction assumption is adopted, a semi-analytical numerical scheme is developed for both elastic and visco-elastic pavements. The formation of stress-free open cracks is due to the combination of continuous crack growths and unstable jumps, which involves a nonlinear stability analysis. Equilibrium solutions and stability conditions are obtained. Numerical simulations demonstrate that, with the aid of the constraining friction force, heterogeneity of the material triggers the localization of damage and leads to stress-free open cracks distributed at spacings on the order of the macro-scale. The thermal cracking model is also used to study the effects of major factors, such as material homogeneity, material brittleness, material viscosity, frictional constraint and rate of cooling, and leads to conclusions consistent to the field observations on real pavement structures.