Parallel Plate Gap Height Effect on DSR Measurements of GTR Modified Binders

The current Superpave performance grade (PG) specification uses a dynamic shear rheometer (DSR) with a 1-mm gap height to determine the high-temperature performance grade of asphalt rubber binders. Concerns have arisen that rubber particulates exceeding 25% of the gap height might skew the measurements, leading to properties that do not accurately represent the actual behavior of ground tire rubber modified binders. This study explores adjusting the 1-mm gap height to better measure the high-temperature performance properties of these binders and minimize the interaction effects between the asphalt binder and rubber particles on PG grading results. A trial gap height of 2 mm was used to test three different ground tire rubber modified (GTR) binders. Both the standard Superpave high temperature grading test and the multiple stress creep and recovery (MSCR) test were employed. Results indicated a consistent trend for binders with 15% GTR, while notable differences were observed for 18% and 20% GTR. The 2 mm gap provided more stable results over several days compared to the 1 mm gap, likely due to reduced particle interaction at the increased concentration. Based on these findings, a 2 mm gap height is recommended for GTR modified binders.