Prediction of Compaction Temperatures Using Binder Rheology

Abstract

Guidelines for determining mixing and compaction temperatures of conventional asphalt mixtures were published by the Asphalt Institute in 1962. Since the adoption of the performance grading (PG) system in North America, the use of modified asphalts has been growing continuously. It is well recognized that those 1962 guidelines recommend excessively high construction temperatures that can result in damage to modified asphalts. But why doesn't the current system work? Is kinematic viscosity the wrong physical parameter? Are target viscosities too low? Or are the test parameters of 135 deg C and 6.8/s inappropriate? Many attempts have been made to develop more rational guidelines, but none have been universally adopted at this time. This study reviews the findings of those alternate methods and proposes a solution that requires measuring viscosity at various shear rates. The results show that most modified asphalts are shear thinning. Thus low-shear viscosity (LSV) could be a major factor in resistance of modified mixtures to compaction. A procedure for measuring LSV is described, and a discussion of the relevance of low-shear rates to gyratory compaction is presented. The study concludes by presenting recommendations for using LSV levels of 3,000 cP to estimate reasonable compaction temperatures for mixtures with modified binders. The study includes only laboratory testing. Field verification of the proposed guidelines is still needed.