Using the Dynamic Shear Rheometer for Low-Temperature Grading of Asphalt Binders

The Superpave Performance Grading (PG) framework or a variation of this framework is often used as a purchase specification in the United States and as a tool to evaluate the expected performance of modified and unmodified asphalt binders in many countries around the world. A cornerstone of the PG framework is the low-temperature grade of the asphalt binder that is assessed using a bending beam rheometer (BBR) using maximum stiffness and a minimum rate of deformation as criteria. Several previous studies from other researchers have demonstrated the feasibility of using the dynamic shear rheometer (DSR) with a 4-mm parallel plate geometry to measure the rheological properties of asphalt binders at low temperatures. This article builds on these studies to evaluate the feasibility of using a DSR to replace the use of a BBR for low-temperature grading of the asphalt binder using two different approaches. The first approach was to measure complex shear modulus of different asphalt binders using the DSR, convert these measurements to BBR equivalent stiffness and m-value using principles of linear viscoelasticity, and compare the two pairs of parameters from each method. The second approach was to use the complex shear modulus and phase angle measured using the DSR in lieu of using the stiffness and m-value measured using the BBR. Although the results from both methods show feasibility of using the 4-mm plate with the DSR to estimate low-temperature properties, the second approach of measuring and using the DSR-based complex shear moduli and phase angle is a more viable alternative for routine use.