Report on the Prediction of Surface-Initiated Longitudinal Wheel Path Cracking in Asphalt Pavements

Almost all mechanistic flexible pavement design procedures implicitly assume that fatigue cracks originate at the bottom of the bound layers and propagate upwards towards the surface of the pavement. However, observations from cracked roads in many countries have shown that the cracking originated from the surface of the pavement rather than at the base, particularly for thick flexible constructions. Many researchers have been trying to confirm that surface-induced cracking can be explained by the presence of tensile and/or shear stresses (strains) at (or near to) the pavement surface. A standard analysis of a pavement structure loaded by a uniform vertical contact stress shows that small horizontal stresses and strains can be generated due to negative curvature effects although it should be noted that the maximum (principal) strain at the pavement surface (at approximately 45o to the surface) can be of a similar magnitude to the horizontal tensile strain at the base of the asphalt layers. In addition, the inclusion of the complex three-dimensional contact stress distribution generated between the tyre and the pavement surface has been shown to produce high local values of horizontal tensile stress and strain at the pavement surface which could drive surface-induced cracking. However, other factors such as ageing of the surface material and temperature-induced stiffness gradients are also likely to be important.