Guidelines for Implementing the Multiple Stress Creep Recovery (MSCR) Test Method
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In recent years, the use of polymer-modified binders (PMBs), and warm mix asphalt (WMA) and reclaimed asphalt pavement (RAP) technologies has become important toward enhancing durability and strength of asphalt pavement, reducing the production temperature, and lowering the material costs. Unlike neat binders, PMBs are sensitive to the applied stress levels, and they exhibit nonlinear viscoelastic properties with respect to vital performance properties such as rutting. The American Association of State Highway and Transportation Officials (AASHTO) T 315 method works well for neat binders, but it has been found inadequate to characterize rheological properties of PMBs. Consequently, many state agencies often require expensive and time consuming performance grade (PG) “PG Plus” tests such as elastic recovery (ER) to characterize PMBs even though they are empirical and may not reflect the field performance. The newly balloted multiple stress creep recovery (MSCR) test (AASHTO TP 70) is expected to be a viable alternative to the aforementioned AASHTO T 315 and ER (ASTM D6084 and others) tests for measuring high temperature properties of rolling thin film oven (RTFO) aged asphalt binders. To this end, the current study developed and analyzed a MSCR database containing 163 samples of the following binders: (i) three commonly used Oklahoma Department of Transportation (ODOT) certified performance grade (PG) binders, namely, PG 64-22, PG 70-28 and PG 76-28, collected from 12 different sources; and (ii) the aforementioned binders from a selected source modified by different dosages of a WMA additive (Sasobit®). These binders were graded in accordance with AASHTO TP 70, and recommendations were made on how the MSCR test method could be incorporated in routine practice considering local climate and traffic conditions. The MSCR test parameters, namely, the non-recoverable creep compliance (Jnr) and MSCR % Recovery, along with other pertinent test data were then analyzed to develop acceptable limits without penalizing local suppliers and users.