Stone Skeleton Asphalt: Field Trial U.S. 331, Luverne, Alabama: Fid-Bau Portal

Stone Skeleton Asphalt: Field Trial U.S. 331, Luverne, Alabama

G. C. Hurley / B. D. Prowell

Alabama Department of Transportation (ALDOT) developed Section 426, Stone Skeleton Asphalt (SSA), based on results from a laboratory concept study. The concept of stone skeleton asphalt was a mixture that would have similar performance characteristics to typical stone matrix asphalt (SMA) mixtures, but without some elements contained in SMA mixtures that increase its cost, such as modified asphalt binders, fibers, and mineral fillers. A trial project was let on U. S. Route 331 in Luverne, Alabama. A neat PG 67-22 binder was specified instead of the PG 76-22 specified in ALDOTs SMA mixes. Fiber was included in the design, but incrementally removed during placement. Samples of the produced mix were taken and results from the volumetric, draindown, and performance testing were gathered and analyzed. From these results, several conclusions were made. The removal of fibers reduced the VMA below the minimum specification for SSA during production. The rutting potential of the stone skeleton mixtures was generally not affected by the removal of fibers and reduction in asphalt content. All test sections had rut depths below the maximum specified value. Four out of the six test sections had TSR values that were slightly below the minimum specified value of 0.80. This matched observations from the mix design verification. None of the test sections had any issues with asphalt draindown. Field permeability values for the test sections correlated very well to in-place densities. In order to ensure an impermeable stone skeleton asphalt mixture, inplace densities should be approximately 95 percent of the theoretical maximum density. The stone skeleton mixes exhibited acceptable friction values.