Assessment of Top-Down Cracking Causes in Asphalt Pavements

Cracks observed at the surface may have several origins and causes. A crack may either begin at the bottom or at the top of an asphalt layer. Cracking originated at the top
of asphalt layers (top-down cracking) observed in temperate-climate countries is a degradation mechanism which has not been fully researched. In order to access this “new” degradation in Portugal, a sampling plan has been set in a high thickness pavement, including the extraction of cylindrical cores and slabs. Cracks were carefully identified and several laboratory tests were performed on grading, bitumen content and air voids, fatigue strength and stiffness. Pavement bearing capacity was also measured. The assessment and comparison of the results show, in a first approach, that all cracks are surface-originated and top layers contains excessive fine aggregates and voids. A second approach concerning modulus, fatigue strength and bearing capacity is less conclusive, which leaves a long way to go. Nevertheless, it seems that construction quality continues to be the most probable cause of top-down cracking.