Superpave Binder Implementation - Oregon Dot
SUPERPAVE BINDER IMPLEMENTATION - OREGON DOT
Leahy, RB;Cramer, SB
Oregon Department of Transportation (ODOT) has specified performance-based asphalts (PBAs) since 1991. Developed by the Pacific Coast Conference on Asphalt Specifications (PCCAS) in 1990, the PBA concept uses conventional test methods for classification and facilities binder selection based on climatic conditions. The Conference plan was to use the PBA concept and conventional tests as an interim approach which would eventually be replaced with the Strategic Highway Research Program performance grade (SHRP PG) specification and supporting tests. As a first step in the SHRP implementation/validation effort ODOT has evaluated its commonly used PBA grades in terms of the SHRP (now called Superpave) protocols. The limited binder evaluation to date suggests the following equivalencies: PBA-2s may be classified as PG 64-16 or PG 64-28; PBA-3s as PG 58-34 or PG 64-28; PBA-5s as PG 64-22; and PBA-6s as PG 58-28 or PG 64-34. There was not always agreement with regard to PG classification between the research results and the supplier data, nor was there always agreement among the suppliers. As described by the number of different performance grades for a particular PBA, PBA-2 appeared to be the least consistent whereas PBA-5 and PBA-6 appeared to be the most consistent. Comparison of Superpave PG and conventional binder test data indicates that there was no relationship between the high temperature performance grade and kinematic viscosity. However, there was a moderate relationship between the high temperature performance grade and absolute viscosity. However, there was a moderate relationship between the high temperature performance grade and absolute viscosity as values of explained variation (R-squared) of the unaged and rolling thin film oven (RTFO)-aged binders were 0.38 and 0.52, respectively. The relationship between the low temperature performance grade and penetration at 4 deg C and 25 deg C were significantly higher with values of explained variation for the RTFO-aged binders of 0.80 and 0.84, respectively. The diversity of Oregon's climate suggests that as many as 13 to 14 binder grades might be "needed" at the 98% level of reliability, although many grades overlap. Realistic constraints and practical considerations such as readily available binder sources and state-maintained-road-miles associated with a particular performance grade led to the recommendation that four PG binders be specified: PG 58-22 and PG 64-22 west of the Cascades; PG 58-28 and PG 64-28 in the central part of the state; and PG 64-28 in the eastern part of the state. Preliminary economic analysis suggests that implementation of the PG system could provide substantial savings. Because of Oregon's extensive use of open-graded friction courses, additional work must be done to determine what effects, if any, the PG classification might have on this mix type in terms of field performance.