Abstract
Numerous Washington State Department of Transportation (WSDOT) paving projects have experienced a cyclic occurrence of premature failure of open-textured asphalt concrete (AC) pavement sections by fatigue cracking, raveling, or both, generally called “cyclic segregation” or “endof-load segregation.” This resulted in an initial study in which mat temperature differentials were observed during laydown. In turn, this led to the current study and the reported results. Pavement temperature differentials result from placement of a cooler portion of the hot-mix mass into the mat. This cooler mass generally constitutes the crust, which can develop during hot-mix transport from the mixing plant to the job site. Placement of this cooler hot mix can create pavement areas near cessation temperature that tend to resist proper compaction (they may also exhibit tearing or roughness or appear to be open textured). These areas were observed to have decreased densities and a higher percentage of air voids (higher air voids). Four 1998 WSDOT paving projects were examined to determine the existence and extent of mat temperature differentials and associated material characteristics. An infrared camera was used to identify cooler portions of the mat, which were then sampled along with normal-temperature pavement sections. Gradation and asphalt content analysis showed no significant aggregate segregation within the cooler areas. However, these cooler portions of the mat consistently showed higher air voids than the surrounding pavement. On the basis of numerous studies that have related AC deterioration and high air voids in a mix, it is known that the areas of a mat with higher air voids may experience premature failure compared with the time to failure of the mat as a whole.
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