A Study on Cross-anisotropy of Unbound Layers of Pavement Considering Nonlinearity

Abstract

The effects of cross-anisotropy of unbound layer materials on the stress-strain response of a pavement under Falling Weight Deflectometer (FWD) test load are determined at three temperatures employing a dynamic Finite Element Model (FEM) in ABAQUS. Viscoelastic behavior of Asphalt Concrete (AC) is characterized by laboratory dynamic modulus test. Nonlinear elasticity of base material is incorporated through a User Defined Material (UMAT) subroutine. Cross-anisotropy is introduced by changing the ratio of horizontal to vertical stiffness (n-value) of unbound layers. FEM model is validated using field collected stress-strain under FWD test on the instrumented pavement section at mile post 141 (MP 141) on Interstate 40 (I-40) near Albuquerque, New Mexico. It is observed that tensile strain at the bottom of the AC layer is influenced by the base layer cross-anisotropy whereas vertical strain in this layer is barely affected by the cross-anisotropy of the unbound layers. However, vertical strains in the unbound layers are significantly affected by cross-anisotropy. Therefore, it can be postulated that fatigue damage by tensile strain at the bottom of AC layer, and rutting by vertical strain should be evaluated for unbound layer cross-anisotropy in the pavement design. Overall, strain responses due to cross-anisotropy are highly sensitive to temperature.