Effects of superpave aggregate gradation on permanent deformation

Abstract

In order to improve the performance, durability, and safety of United States roads, the Strategic Highway Research Program (SHRP) was established by Congress in 1987 as a five year research program. SUPERPAVE[] (Superior Performing Asphalt Pavements) is the final product of the SHRP research effort. Superpave is a complete mix design system with three major components: asphalt binder specification, mixture design, and analysis system. SHRP research effort mainly concentrated on properties and testing of asphalt binder. As a result, some aspects of Superpave aggregate specifications are not universally accepted, being questioned by the agencies or the industries. One of the most controversial components of the Superpave mix design is the aggregate gradation restricted zone. This restricted zone was adopted in order to reduce premature rutting. Some agencies question the validity of the restricted zone stating that specified gradations of many current mixes pass through the restricted zone, but the mixes have performed well for years in actual field conditions. These mixes have been prepared mainly with high quality manufactured aggregates. The purpose of this thesis is to evaluate the restricted zone effect using three different manufactured aggregates: granite, limestone, and river gravel. As the restricted zone is a component of Superpave, the blends prepared met all the Superpave criteria, except the restricted zone in some of them. The mixtures designed have been tested in the laboratory to evaluate their relative resistance to permanent deformation. Four types of tests have been run: simple shear at constant height, frequency sweep at constant height, repeated shear at constant stress ratio, and repeated shear at constant height. From the analysis of these tests it has been found that there is no relationship between the restricted zone and permanent deformation when crushed aggregates are used in the mixture design.