Detection and Monitoring of Cracks in Asphalt Pavement Under Texas Mobile Load Simulator Testing

The Texas mobile load simulator (TxMLS) is a newly developed accelerated pavement testing device used to evaluate pavement performance under real trafficking loads. This evaluation is performed by applying trafficking loads and monitoring surface distress, such as cracking and rutting, in conjunction with a number of other measurements of the pavement, such as those conducted with the falling weight deflectometer, multidepth deflectometer, strain gauge, pressure cells, and seismic (stress-wave) tests. A procedure for monitoring the progressive degradation of the asphalt surface was developed using the spectral-analysis-of-surface-waves (SASW) technique. This procedure was applied with the TxMLS and proved to be equally effective. SASW tests that were performed intermittently between trafficking phases on trafficked and untrafficked areas show (a) the effect of temperatures and frequencies on the asphalt moduli, (b) the importance of temperature and frequency corrections in analyzing the degradation of the asphalt surface layer, and (c) the long-term trends in surface-wave velocities (and hence, moduli) of the surface layer with increasing number of load applications. It was found that stiffness of the asphalt layer in the longitudinal direction was progressively reduced under trafficking. Concurrently, surface cracking progressively increased. The reduction in longitudinal stiffness occurred at a faster rate than the crack growth. In contrast, the reduction in the stiffness of the asphalt layer in the transverse direction was slower, probably because the main mode of cracking was transverse. The feasibility of using SASW testing as a predictor of degradation and imminent cracking was confirmed with these studies.