One of the concerns about the Superpave Binder Specification (AASHTO M-320, Specification for Performance Graded Asphalt Binders) has been the applicability of the specification tests for modified asphalt binders. Specifically, the high temperature parameter — G*/sin d — has been shown to relate poorly to rutting for many “premium grade” modified asphalt binders. To address this concern, the Federal Highway Administration and members of the Asphalt Binder Expert Task Group have developed a new test procedure and recommended specification requirements to replace the current high temperature specification parameter.
The multiple stress creep-recovery (MSCR) test can be conducted using the same sample and dynamic shear rheometer (DSR) equipment as the AASHTO M320 specification test. The MSCR test is performed using the DSR by applying a controlled shear stress of 0.1 kPa using a haversine load for 1 second followed by a 9-second rest period. During each cycle, the asphalt binder reaches a peak strain and then recovers before the shear stress is applied again. Ten creep-recovery cycles are conducted at 0.1 kPa shear stress immediately followed by ten cycles using 3.2 kPa shear stress. The new high temperature specification parameter is determined by dividing the non-recoverable (or permanent) shear strain by the applied shear stress. The result is called the non-recoverable creep compliance, or Jnr.
At this time, the Jnr parameter is only intended to replace the RTFO G*/sin d parameter; the Original G*/sin d parameter remains as a specification requirement. To determine the RTFO Jnr value, the asphalt binder is tested at the environmental temperature, not the current “grade-bumped” temperature. Thus a PG 76-22 binder will most likely be tested at 64°C instead of 76°C. In the current specification, in an environment where a PG 64-22 binder would normally be used, PG 70-22 and PG 76-22 binders would be used for heavy and very heavy traffic loading, respectively. In the proposed specification, all traffic loading is accounted for by lowering values of Jnr (resulting in a stiffer binder), while keeping the test temperature the same. Grades may be designated as the base environmental grade followed by a letter designating standard (S), heavy (H), or very heavy (V) traffic. Thus, in the preceding example:
PG 64-22 becomes PG 64-22S
PG 70-22 becomes PG 64-22H
PG 76-22 becomes PG 64-22V.
For more information or to get a quote on performing the multiple stress creep-recovery (MSCR) test for asphalt binders, please contact either Mike Anderson or Gary Irvine of the Asphalt Institute Lab.