Critical Considerations toward Better Implementation of the Multiple Stress Creep and Recovery Test

In recent years, many studies have dealt with the inadequacy of the current Superpave performance grading (PG) specification for accurately capturing the performance properties of modified binders. An alternative binder testing mode that has gained much attention in recent years is the repeated creep and recovery (RCR) test, and more specifically, the multiple stress creep and recovery (MSCR) test. As with any new test procedure, a number of concerns and questions remain with regards to interpretation of results in terms of performance and sufficiency of the number of cycles and stress levels used, as well as representative temperature and stress levels to be used. This paper investigates the aforementioned issues using a set of modified and unmodified binders. The results show that the standard 10 cycles per stress level are not sufficient to reach a stable steady-state creep behavior for modified binders, thus the addition of 20 conditioning cycles are recommended, for a total of 30 cycles for each stress level. It is also shown that using the last five cycles to determine an average $J_{nr}$ value is a better choice for reporting and analysis due to reaching a more stable and consistent state of creep. Furthermore, the current stress levels’ use of 0.10 and 3.20 kPa are deemed insufficient for accurate representation of binder stress state in the pavement, thus a higher stress level step is recommended for addition to the test procedure.