MSCR: A Better Tool for Characterizing High Temperature Performance Properties

This article describes how the U.S. Congress budgeted $150 million for the Strategic Highway Research Program and $50 million was set aside to study asphalt materials in 1987. In 1993, the Performance Graded (PG) asphalt binder specifications were introduced as part of the Superpave system. These specifications were adopted as AASHTO M320. The PG system was primarily developed around unmodified asphalts, and G*/sin d (G-star divided by sine delta) became the high temperature specification parameter and rutting performance indicator in the specification. G*/sin d is determined by the Dynamic Shear Rheometer (DSR), one of the basic asphalt binder testing devices. In the Superpave system, the DSR is used to characterize the stiffness and elasticity properties of asphalt binders at high and intermediate temperatures. An asphalt sample is placed between two parallel plates and a torque is applied and the response measured. The test results are used to estimate the resistance to rutting and fatigue cracking. One of the objectives in the development of the PG asphalt binder specification was the use of performance-related criteria specific for a distress and related to climate and traffic loading. This objective implies that test measurements should be made at temperatures and loading rates consistent with conditions existing in the pavement. With this approach, the high temperature criteria stays the same for G*/sin d (1.00 kPa for unaged and 2.20 kPa for RTFO-aged binder) regardless of the location of the pavement, but the test temperature where this criteria must be met is derived from the actual pavement temperature.