Comparison of Concentric Cylinder and Parallel Plate Geometries for Asphalt Binder Testing with a Dynamic Shear Rheometer

Abstract

In California, 35% of the asphalt concrete placed by the California Department of Transportation must, by law, contain recycled tire rubber. Asphalt rubber binder (produced in what is known as the wet process) is the main binder type used in rubberized hot-mix asphalt in California. The rubber particles vary in size, but 100% must pass the 2.36-mm No. 8 sieve. Because of the presence of these relatively large rubber particles in the binder, the current parallel plate testing system (with either 1- or 2-mm gaps specified in the Superpave® performance grading system) is not appropriate for use to measure the binder properties. These large rubber particles would be more likely to contact the plates, and the resulting measurement could be dominated by the rheology of the rubber particles rather than by the binder. Consequently, a modified dynamic shear rheometer testing geometry, a concentric cylinder, was investigated and compared with traditional parallel plates with a view toward the use of this new geometry for quality control on projects that used asphalt rubber binder. In the first phase of the investigation, summarized in this paper, the two geometries were compared by testing conventional and polymer-modified binders to determine the complex shear modulus (G*) and phase angle (d). The results indicated no statistically significant difference between the two geometries. On the basis of these results, testing has begun of asphalt rubber binders with a range of particle sizes with the use of the two geometries.