Investigation of mechanisms of top-down fatigue cracking of asphalt pavement

Various types of responses have been selected in the past as the driving forces of the top-down cracking. However, the most appropriate candidate for top-down cracking is the horizontal tensile strain at the pavement surface, considering the horizontal tensile strain at the bottom of asphalt layer has been historically used as the critical response for bottom-up cracking in the ME design. In this study, the mechanisms of top-down cracking are investigated based on the horizontal tensile strain at the surface. A multi-layered elastic computer code, APRA, was utilized to compute the horizontal strains at the pavement surface. APRA uses the Lucas algorithm and integration, summation and extrapolation methods to accurately determine the surface responses. The results show that traffic loading may induce significant horizontal tensile strain in the transverse direction at the surface, which is a major cause of the top-down cracking. The critical location of the tensile strain is just outside the tyre edges. It is pointed out that although the newly developed Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the horizontal strain at the surface for the prediction of top-down cracking, the analysis locations adopted in the MEPDG may not be able to identify the maximum tensile strain. It is observed in this study that the maximum tensile strain at the surface increases rapidly with temperature. For the pavement with cement-treated base, the top-down cracking is the dominant type of fatigue cracking, while the bottom-up cracking is unlikely due to the strong support from the base. For the pavement with granular base, the bottom-up cracking is more likely to occur at medium or low temperatures, while the cracking is more likely to initiate from the top at high temperatures. The likelihood of top-down cracking occurrence increases with increasing temperature and asphalt layer thickness.