University of California Pavement Research Center

In the United States, the Superpave Asphalt Binder Performance Grading (PG) system proposed by the Strategic Highway Research Program (SHRP) is the most common method used to characterize the performance-related properties of conventional and polymer-modified asphalt binders. Dynamic modulus (G*) and phase angle (δ) are the two main binder properties and they are measured using a dynamic shear rheometer (DSR) with parallel plate geometry and either a 1 mm or 2 mm gap between the plates. Since these Superpave parameters were developed for binders that do not contain additives or particulates, the California Department of Transportation (Caltrans) does not use them as asphalt rubber binder specification criteria. Instead, penetration and viscosity are used as acceptance of quality control; however, these parameters do not necessarily provide a satisfactory link between the measured binder properties and potential performance in the field over a range of operating temperatures. In California, current specifications require that crumb rubber particles used to produce asphalt rubber binder in the “wet process” must be smaller than 2.36 mm (i.e., 100 percent passing the #8 sieve), and typically these particles vary in size between 1 mm and 2 mm. As a consequence, when the parallel plate geometry is used to test this type of binder, the larger rubber particles can contact the plates; if this occurs, the rubber particle rheology can potentially dominate the results, which in turn may not be representative of the modified binder as a whole. To address this problem, a potentially more appropriate DSR testing protocol using concentric cylinder geometry was investigated in this study to explore an alternative means of determining the performance properties of asphalt rubber binders. In the first phase of the study, documented in this technical memorandum, a series of tests were undertaken to compare the two geometries and to assess which binder properties influence the results from the testing approaches. The interim results indicate that there is no significant difference between the concentric cylinder and parallel plate geometries in terms of the G*/sinδ after testing on a range of different binders and asphalt rubber binders with finer crumb rubber particle sizes (i.e.  <250 μm). However, the correlations between results from the geometries were increasingly weaker with increasing crumb rubber particle size, indicating some potential influence of larger sizes on the results of the testing using parallel plates. The concentric cylinder geometry resulted in relatively lower values of G*/sinδ compared to samples tested with the parallel plate geometry. This difference is provisionally attributed to large rubber particles touching both plates, and to edge effects issues. The proposed alternative approach to measuring the rheological properties of asphalt rubber binder is considered feasible, and that with its use the edge effect and trimming issues can be eliminated. However, the concentric cylinder method requires a longer testing time and a larger binder sample than the parallel plate test method. The testing will be continued to develop proposed revised quality control procedures for testing asphalt rubber binders used on Caltrans projects. ​