Improved Methodology for Mix Design of Open-Graded Friction Courses
This study presents an improved methodology for the mix designs of open-graded friction courses (OGFC). The methodology has been enhanced by the development of an Excel macro in order to suggest revisions to California Test 368, Standard Method for Determining Optimum Binder Content (OBC) for Open-Graded Asphalt Concrete. In addition to the development of the Excel macro, one of the primary objectives of this study was to evaluate the effect that fines content has on mix performance, which cannot be identified by the “break point sieve” concept or by volumetric properties. The proposed OGFC mix design includes two phases: Phase I: Volumetric OGFC Mix Design and Phase II: Performance Testing. The tasks required to perform Phase I include the determination of material volumetric properties such as specific gravities, voids in coarse aggregate in dry-rodded condition (VCADRC), and asphalt absorption. These must be performed so it becomes possible to select three trial binder contents for fabricating specimens for performance testing. The main purpose of Phase II is to decide the optimum binder range (OBR) according to the results of draindown, Cantabro, and Hamburg Wheel-Track Device (HWTD) tests. Two aggregates (Watsonville and Sacramento), three binders (PG 76-22 PM, PG 64-28 TR, and PG 64-10), two gradations (Coarse and Fine) designed to verify the fines content, and three trial binder contents obtained from Phase I were used in the Phase II testing. It was found that an increase of fines content is significant in reducing Cantabro loss, preventing draindown, minimizing the variation of Superpave gyratory compaction curves, and producing more consistent HWTD test results. Hence, it is suggested that the fines content should be part of the OGFC performance specifications. This study also demonstrated the accuracy of the measured air-void contents of Superpave gyratory-compacted specimens that were fabricated with height control rather than gyration control and with binder contents calculated based on the volumetric equation, VCADRC. A preliminary comparison indicated that the proposed mix design produces similar binder contents for conventional and asphalt rubber binders with similar gradations, and that unreasonably low binder contents it may produce indicate a fines content that is too high. This improved OGFC mix design together with the Excel macro developed provides a rational, accurate, and convenient methodology for determining OBR. However, further studies are required to establish the proper performance specifications that relate to field performance.