Development of guidelines for PG binder selection for Wisconsin

Abstract

In 1997, the Wisconsin DOT made the transition to use PG binders in place of the Penetration and Viscosity graded asphalts that had been historically used. The decision at that time to specify the PG 58-28 as the standard grade for use was based on this grade similarity to the asphalts that had been used previously, the wide availability of the material in the region, and the fact that there was little or no difference in cost. Although this binder grade has been widely used, it is well known that the climatic conditions in the state of Wisconsin vary between the north region and south region of Wisconsin. According to the Superpave binder specification (AASHTO MP1) other grades designed for colder climate need to be used. In fact, before the AASHTO MP1 was implemented, more than one penetration grade was already being used in Wisconsin. In the cold regions of Wisconsin the Pen 200-300 grade was used while in the south the grades of Pen 85-100 and 120-150 were used. Since the implementation of the Superpave binder specification in 1993 a large number of State Highway Agencies (SHAs) have found critical gaps in the PG grading system. Reviewing the published literature indicates that most of these gaps are somewhat related to the use of modified asphalts and to the fact that the existing PG grading system cannot discriminate between modification technologies, some of which are believed not be successful technologies in terms of adding value to the quality of binders. Last year (2001) the NCHRP report 459 was published, which focused on offering a revised system for testing and evaluation of asphalt binders using a more mechanistic testing system based on direct measurements of damage behavior of binders. This report is offered to address the implementation of the NCHRP 9-10 proposal and to show how it could be implemented on trial basis using the results of testing of a wide selection of asphalts currently used, or marketed, in the State of Wisconsin. The binders include unmodified and modified of several grades (PG 58-28, PG 64-22, PG 64-34, PG 70-22, PG 70- 28, and PG 76-28). The results of testing using the NCHRP9-10 protocols were used to select binders for specific climate, traffic, and pavement conditions. The existing knowledge of PG grading of the binders was used to derive specification limits that would meet specific climate and traffic conditions. No grade shifting was necessary because all testing is done at actual pavement temperatures and because a more mechanistic approach could be used to meet the traffic volume and speed requirements. The specification limits are tentative and mainly based on ranking of the PG grading system of the binders included. They serve as a starting point for future field validation and revision. This report is intended to describe the logic in deriving the limits and to offer initial specification limits to move us away from the Superpave plus and offer a logical alternative to compliment the existing PG grading system.