Redefining the Failure Mode for Thin and Ultrathin Whitetopping with 1.8- × 1.8-m Joint Spacing

Bonded whitetopping is a thin concrete overlay on a distressed asphalt pavement. The existing design procedures for bonded whitetopping assume the failure mode is a function of the overlay thickness. It has been traditionally assumed that the failure mode for thin whitetopping [overlay thickness greater than 102 mm (4 in.) but less than 152 mm (6 in.)] is transverse cracking, and the failure mode for ultrathin white-topping [overlay thickness between 51 mm (2 in.) and 102 mm (4 in.)] is corner cracking. However, the performance of in-service whitetopping overlays indicates that the actual failure mode is dictated more by slab size than by overlay thickness. For both thin and ultrathin whitetopping with 1.8-m (6-ft) joint spacing, cracks initiate at the bottom of the overlay at the intersection of the transverse joint and the wheelpath and propagate longitudinally. At times, these cracks will continue to propagate in the longitudinal direction and intersect the adjacent transverse joint; at other times, they will turn on a diagonal and propagate toward the lane–shoulder joint. To verify this failure mechanism observed in the field further, a three-dimensional finite element model subjected to environmental and wheel loads was developed. The results support the proposed failure mode, showing that the critical tensile stress is indeed in the wheelpath and at the bottom of the portland cement concrete overlay. This type of failure results in a longitudinal crack.