Evaluation of healing and constitutive modeling of asphalt concrete by means of the theory of nonlinear viscoelasticity and damage mechanics

Abstract

It has been proved by many researchers that existing fatigue failure criteria based on constant amplitude loading tests underpredict the fatigue life of asphalt concrete pavements. Unrealistic loading conditions in laboratory testing are the major sources of this discrepancy. Two major differences between laboratory and field loading conditions were addressed in this study: the existence of rest periods and the random sequence of load applications of varying magnitudes. Based on an extensive literature review, three mechanisms were identified as influencing the behavior of asphalt concrete subjected to multi-level repetitive loads interrupted by various durations of rest periods. They are: fatigue as a damage accumulation process, relaxation due to the viscoelastic nature of asphalt concrete, and chemical healing across crack faces during rest periods. Visual evidence of healing was achieved in this research by means of a scanning electron microscope analysis of fracture faces from Izod impact tests on samples of various asphalt grades and sources. The effort to evaluate the mechanism of chemical healing in the microcrack process zone is confounded by the concomitant occurrence of viscoelastic relaxation. Schapery's correspondence principle of nonlinear viscoelastic media was successfully used to separate viscoelastic relaxation from chemical healing. Application of the procedure of separating out the viscoelastic relaxation yields a method by which to quantify chemical healing in a damaged asphalt concrete body. Chemical healing as a function of the duration of rest periods is quantified using a healing index based on pseudo energy density. This healing index is presented for three asphalts of varied composition. As a result of the techniques applied to separate the relaxation and healing mechanisms, a uniaxial constitutive model was developed by employing the correspondence principle in concert with damage mechanics. The verification of this equation was successfully accomplished under realistic loading conditions, such as multi-level loading with various lengths of rest periods.