Finite element analysis of flexible pavement considering cross-anisotropy

This research studied the effects of cross-anisotropy of the asphalt concrete (AC) and unbound aggregate base (UAB) layers on the flexible pavement response and performance by using the finite element (FE) method. Five sections of conventional flexible pavement that represent high and low volume road sections were studied. A subroutine code written by the author was implemented in the FE analysis to characterize nonlinear cross-anisotropic and nonlinear isotropic properties of the unbound aggregate base. Four models were considered, which consisted of linear isotropic AC and UAB (LI-LI); linear isotropic AC and nonlinear isotropic UAB (LI-NI); linear cross-anisotropic AC layer with a ratio of horizontal modulus to vertical modulus of nAC = 0.7 and nonlinear cross-anisotropic UAB (LA0.7-NA); and linear cross-anisotropic AC with nAC= 0.5 and nonlinear cross-anisotropic UAB (LA0.5-NA). The subgrade in all models was assumed linear isotropic material. The response variables obtained from the finite element analysis were the surface deflection, the vertical and horizontal strains at the centerline of loading, and the tensile strain at the bottom of the AC layer. The pavement performance was evaluated in terms of the number of load repetitions to bottom-up fatigue cracking and permanent deformation of each pavement layer. The performance variables were calculated using empirical models from the literature and the finite element results. The pavement response and performance for the four material models were compared for the five sections considered. The results showed that the effect of considering cross-anisotropy of the asphalt concrete and unbound aggregate base was significant in terms of fatigue cracking and considerable in terms of permanent deformation. It was found that applying an initial horizontal residual stress in the UAB was important when modeling flexible pavements to obtain reasonable results in the response variables such as surface deflection, vertical strain at the centerline of loading and tensile strain at the bottom of the AC layer. The cross-anisotropy of AC and UAB materials and an initial horizontal residual stress in the UAB layer should be considered in the mechanistic analysis of flexible pavements.