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.

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Summary Gases can migrate into the cemented annulus of a wellbore during early gelation when hydrostatic pressure within the cement slurry drops. Different means to describe hydrostatic-pressure reduction have been proposed and reported in the literature. Among them, static gel strength (SGS) is the most widely accepted concept in describing the strength development of hydrating cement. The classic shear-stress theory uses SGS to quantify the hydrostatic-pressure reduction in the cement column. Approaches derived from the concept of SGS have contributed to understanding mechanisms of gas migration and methods of minimizing it. Unfortunately, these approaches do not accurately predict gas migration. Although SGS was originally adopted to describe the shear stress at interfaces, it has also been used to estimate the shear resistance required to deform slurry during the hydration period. Before early gelation, the hydrostatic pressure will overcome the formation gas pressure and prevent gas migrations. During gelation, the cement develops enough rigidity to withstand the gas invasion. This critical hydration period is defined as the transition time. API STD 65-2 (API 2010a) provides standards for determining the transition time by use of the concept of SGS. Current industry practice is to reduce the transition time, thereby lowering the potential for invading gas introducing migration pathways in the cemented annulus. This approach, although certainly helpful in reducing the risk for gas migration, does not eliminate its occurrence. Experimental results presented in this study demonstrate that the relationship between SGS and hydrostatic-pressure reduction is not linear. Characteristics of the transition-time endpoints depend on slurry properties and downhole conditions. Moreover, SGS is not able to characterize the gas-tight property of a cement slurry. When slurry gels, the mechanical properties are governed by its growing solid fraction. The gel can deform under shear loading, but gases and other fluids will need to break or fracture the bond between solids and push them aside for pathways to form within the cement/matrix domain at this point. To fully understand this process, the bond strength between solid particles and the compressibility of the cement matrix are needed. The bond strength and compressibility are mechanical properties dependent on the changing rigidity of the gelling cement. However, SGS does not address these important properties and, therefore, SGS is limited in its ability to predict gas-migration potential. A better means to characterize the cement/matrix strength by use of fundamental concepts and variables for replacing SGS is desired.

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