Reflective and thermal cracking modeling of asphalt concrete overlays

Although asphalt concrete overlay systems represent a rapid and economical alternative for the repair of deteriorated pavements, reflective cracking continues to be major cause of premature deterioration of these systems. A better understanding of the complex mechanisms behind reflective cracking in asphalt overlays must first be obtained before significant advances in reflective crack prevention and mechanics-based overlay design can be fully realized. Traditional modeling approaches have not provided a direct means for the study of crack initiation and propagation in pavements. The cohesive zone fracture modeling approach provides a rational means for modeling cracking in structural systems consisting of quasi-brittle materials, as a finite length scale associated with the fracturing process is considered. A bi-linear cohesive zone model (Song et al., 2006) was used in the simulation of cracking in three field pavement sections studied in a recent NSF GOALI project. Detailed field performance data, especially crack maps from visual surveys were obtained and compared to the numerical simulation results. The temperature boundary conditions were generated using the Enhanced Integrated Climatic Model developed by Dempsey et al. (1990) based upon air temperatures obtained from National Weather Service data-bases. Viscoelastic bulk and cohesive fracture material properties for these pavement sections were obtained by laboratory testing of specimens fabricated from 150-mm field cores, in accordance with a new, efficient testing suite (Wagoner et al., 2006). A series of numerical simulations were performed using finite element models, which provided new insights towards the mechanisms of cracking in asphalt concrete overlays under thermal and mechanical loads. A series of finite element analyses were performed with hypothetical overlay configurations in an effort to demonstrate the concept of a "simulation-guided" interlayer/overlay design process, which allows the direct consideration of initiating and propagating cracks in one or more overlay layers.