University of California Pavement Research Center

This study evaluates the open-graded friction course (OGFC) mix design proposed by the National Center for Asphalt Technology (NCAT) in order to suggest revisions to California Test 368, Standard Method for Determining Optimum Binder Content (OBC) for Open-Graded Asphalt Concrete. Three asphalt types (PG 64-10, PG 64-28 PM, and asphalt rubber [AR]), three aggregate types (Sacramento, Watsonville, and San Gabriel) and three gradations (coarse, fine, and middle) that comply with Caltrans specifications of binder and the 1/2 in. OGFC gradation and aggregate quality were used in this study. The NCAT approach includes selection of optimum gradation, selection of optimum asphalt binder content, and evaluation of moisture susceptibility using a modified Lottman method in accordance with AASHTO T 283 with one freeze-thaw cycle. It was found that, regardless of binder and aggregate types, the optimum gradation selected per the NCAT approach—usually a coarse gradation with fewer fines—did not guarantee the success of an OGFC mix design. None of the mixes with coarse gradation, fabricated using the optimum asphalt binder content, simultaneously met the criteria for percent air-void content, draindown, and Cantabro loss. The resulting test data also show that binder type is the most significant factor affecting both draindown performance and Cantabro performance. This study proposes a volumetric-based OGFC mix design (1) to provide a better way to determine the initial binder content rather than basing it on the bulk specific gravity of the aggregate blend as suggested by NCAT; (2) to account for asphalt absorption; and (3) to allow direct selection of trial binder contents to prepare specimens for performance testing. Accordingly, an OGFC mix design procedure integrated with volumetric design and performance testing is proposed. A moisture susceptibility test in accordance with AASHTO T 283 is known to have considerable within- and between-variations of test results. Thus, the Hamburg Wheel-Track Device test seems to be a better candidate to evaluate moisture susceptibility. However, further study is required to establish how Hamburg performance results relate to field performance.