Washington State Department of Transportation Superpave Implementation

Abstract

Washington State’s Department of Transportation (WSDOT) has investigated components of the Strategic Highway Research Program (SHRP)/Superpave technology to include performance-grade (PG) binder usage and specification validation, gyratory mix design, the Superpave Shear Tester, and field performance of Superpave mixes. With a focus on field performance, validation of the binder specification in respect to low-temperature cracking was accomplished using binder and field performance data from 28 projects. The results were encouraging: the original SHRP algorithm for binder selection correctly “predicted” field performance in 22 cases. The original SHRP algorithm was subsequently refined as part of FHWA’s Long-Term Pavement Performance (LTPP) Program. The LTPP algorithm used for binder selection correctly predicted field performance in 26 cases. Since 1993, of the 44 WSDOT projects with a Superpave technology component, 17 parallel Hveem and Superpave mix designs have been conducted. In 13, Superpave design asphalt content was equal to or greater than the Hveem design asphalt content, although the difference was insignificant. A conventional Hveem mix design was conducted on 18 projects using a PG binder (Hveem-PG). The remaining 26 projects were considered to be Superpave because materials selection and mix design were established in accordance with the Asphalt Institute’s SP-2, Superpave Level 1 mix design. According to WSDOT practice, the following indices trigger maintenance: pavement structural condition (PSC), rutting, or International Roughness Index (IRI). Although “young,” all 44 projects are performing well. The average values of rutting, PSC, and IRI are all below the “trigger” values. In rutting and PSC, performance of Hveem-PG and Superpave projects is virtually identical. However, ride quality of Superpave projects is rougher than that of Hveem-PG binder projects.