| This dissertation is devoted to understanding and phenomenologically characterizing the mechanisms underlying permanent deformation in asphalt concrete. To capture the combined effect of both deviatoric stress and confining pressure, a series of cyclic creep and recovery tests are performed with 3 deviatoric stress levels (70, 100, 130 psi) and 4 confining pressures (0, 10, 20, 30 psi). Experimental observations and hypotheses are made regarding the variation in material's hardening/softening state and time dependence of viscoplastic deformation.;The previous viscoplastic models developed at NC State University and their concepts are revisited and evaluated against the obtained experimental data. Focus has been put on the convolution-type model proposed by Subramanian (2011) and the incrementalshift model developed by Choi (2013). The convolution-type model features a rather complex structure and a highly computational demand. It is first simplified through mathematical speculations based on experimental evidences and then the additional variable is introduced to account for effects of the confining pressure on permanent deformation. The improved model is able to represent the permanent deformation phenomenon through a single viscoplastic relaxation spectrum and a stress state dependent parameter. The incremental-shift model is also extended to incorporate the confinement effect so that eventually it can serve as a feasible material evaluation tool in the laboratory and also as a convenient model in the finite element analysis of pavement structures. Both models are successfully verified through random loading tests.;The rutting predictions on the two full-scale pavement tracks (FHWA ALF and NCAT) using the in-house developed finite element program Layered ViscoElastic pavement analysis for Critical Distresses (LVECD) reveal the importance of incorporating the shear effect in pavement performance simulations. The inadequacy of representing the field shear mechanisms via triaxial testing is pointed out. An exploratory study using the WesTrack shear based transfer function is conducted, which further emphasizes the necessity and enhances the confidence of taking the shear contribution into account. |
