Development of Performance-Based Specifications for Asphalt Rubber Binder: Phase 2g Additional Testing of Five Plant-Sampled Binders and RHMA G Mixes

The work discussed in this interim report is part of a larger study, funded by the California Department of Transportation, with the objective of developing and recommending testing procedures and criteria for performance-based specifications of asphalt rubber binders used in gap-graded and open-graded mixes using current Superpave performance grade (PG) equipment. Work covered the testing of five plant-produced binders, the base binders used to produce them, and the gap-graded rubberized hot mix asphalt mixes produced with them. The following important observations from the binder rheology tests were made: Although the low-temperature performance grades appeared to be reasonable, the high-temperature grades appeared to be unrealistically high, while the intermediate-temperature grades appeared to be potentially lower than anticipated when compared to the base binders. A comparison of the concentric cylinder and parallel plate (3 mm gap) geometries indicated that the results between the two geometries are different and are likely to be higher than the precision and bias of the individual procedures. Precision and bias statements for these procedures had not been developed at the time of preparing this report. Consistent trends in results were observed between high-temperature PG/true grade, Delta TC, and non-recoverable creep compliance at 3.2 kPa. Observations from previous testing and during this phase of the study indicated that incompletely digested rubber particles—which have different sensitivities to temperature, aging, and applied stress and strain than the base asphalt binder—appeared to have a dominant influence on results and caused variability between results, regardless of the testing geometry used. Considering these incompletely digested particles as part of a homogenous binder may therefore not be appropriate when determining performance grades. Work is continuing in Phase 3 of this study to adjust testing procedures to account for the influence that these incompletely digested particles have on results. The proposed modifications to short- and long-term aging procedures (i.e., rolling thin film oven and pressure aging vessel) and to the bending beam rheometerspecimen preparation procedures developed in Phase 2 are considered to be more aligned with the original intent of the tests and will likely reduce the variability between replicate specimens during testing. Preliminary test results indicate that Fourier transformed infrared spectroscopy is a potentially valid method for quantifying rubber content in rubber-modified binders.