The work discussed in this interim report is part of a larger study, funded by the California Department of Transportation. The study objective focuses on 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 in this phase covered the testing of 19 plant-produced binders and the base binders used to produce them. Plant-produced gap-graded rubberized hot mix asphalt mixes from five of the projects were also tested. 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 comparedto the base binders. Fourteen of the binders tested with concentric cylinder geometry and 13 tested with parallel plate geometry had PGs higher than the maximum grade of 82°C listed in the AASHTO M 320 standard. Grades higher than 82 are considered to be unrealistically high and probably not a true indication of likely high-temperature performance (i.e., rut resistance under heavy loads on hot days).

--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. These results indicate that the two geometries cannot be used interchangeably.

--No consistent trends in results were observed between any of the parameters tested.

--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 rheometer specimen 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.