Friction Testing of Tack Coat Surfaces

In response to litigation because of an accident case involving tack coat on a construction project, a field trial was evaluated to determine frictional characteristics of tack-coated surfaces. When a literature search on this topic returned little information, state Department of Transportation materials engineers were surveyed to determine the state of the practice with respect to tack coat operations. On the basis of the survey responses and the existing litigation, a field trial was conducted to evaluate friction numbers on tack coat materials. Variables included residual asphalt content (three levels); test time (three levels); combinations of wet, dry, and flushed surface conditions (seven levels); and replicate testing. It was found that at typical residual asphalt rates reported by states and specified in Louisiana, reduced friction capability existed for up to 7 h after application. With the friction numbers obtained, traffic should be maintained only at controlled low speeds if at all. However, the residual asphalt content appeared heavier than in typical practice. At residual application rates that were typical of practice, friction properties were produced that would allow traffic at moderate speeds. After several days, friction numbers returned to the original condition because of traffic or weather abrasion.