Assessment of Various Approaches to Determining Binder Bending Beam Rheometer Low Temperature Specification Parameters from Dynamic Shear Rheometer Test

Abstract

The current asphalt binder performance grading system employs Dynamic Shear Rheometer (DSR) testing to determine high and intermediate temperature rheological properties. In recent years, the ability to measure DSR instrument compliance has allowed researchers to reliably measure low temperature binder properties as well. Low temperature characterization using DSR requires substantially smaller amount of binder as compared to the currently employed binder testing method, Bending Beam Rheometer (BBR). For these reasons and the possibility of using one piece of equipment for full characterization of asphalt binders, previous research has investigated DSR as an alternative to replace BBR testing by determining equivalent creep stiffness (S) and slope (m-value) from shear complex modulus. Different methods have been proposed to determine BBR specification parameters from DSR data and their viability has been evaluated primarily for virgin binders. The objective of this paper is to further assess the applicability of different methods to determine S and m-values from DSR data for four neat binders as well as extracted and recovered binders from eighteen different mixture samples. The variables within the mixtures include lab versus plant production, aggregate size and gradation, binder PG and source, and recycled material type and content. The methods employ different interconversion methods, ranging from exact interconversions to regression-based estimates. The shear relaxation modulus or creep stiffness and slope are correlated to S and m-value measured from BBR testing. The study also investigates the impact and differences due to use different interconversion methods. The results show that the Christensen approximate interconversion is adequately able to predict parameters from DSR results that are equivalent to S and m-value determined from BBR testing. The exact interconverted shear creep stiffness and shear relaxation modulus using generalized Maxwell model are compared to lab measured S and m values, results show that a linear relationship exists between these parameters. Finally, a simple equation is developed to enable estimation of BBR S and m-value from a single point measurement of complex shear modulus and phase angle. This contribution is expected to have a practical use by providing a platform to estimate low temperature specification parameters from a single point DSR measurement.