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Evaluating MSCR Testing for Adoption in ADOT Asphalt Binder Specifications

Asphalt binder is a product of the petroleum refining process and holds asphalt roadway pavements together. The use of modifiers in asphalt, such as polymer, has become more prevalent as a method to improve pavement performance and longevity. Arizona Department of Transportation (ADOT) engineers require laboratory test parameters that can quantitatively identify the presence of such modifiers and relate asphalt binder test properties to an asphalt mixture's roadway performance. The current ADOT asphalt binder specifications and associated laboratory tests are based on American Association of State Highway and Transportation Officials (AASHTO) M320, a national specification that grades an asphalt binder based on its performance over a specified ambient temperature range at the pavement site. However, this specification is limited because it does not consider a pavement location's traffic levels and it is based largely on research conducted using non-modified asphalt binders. AASHTO has addressed this concern by developing M332, a new specification that accounts for traffic loads and can grade certain modified asphalt binders. The M332 specification is based on a new test method referred to as the multi stress creep recovery (MSCR) test. There is a nationwide push to adopt the new specification and test standard; according to an Asphalt Institute survey (AI, 2015), 20 states have or will soon implement M332 and another 16 states are considering adoption. Arizona evaluations of the MSCR test are hindered by the fact that some key binder types that Arizona uses were missing from AASHTO's validation work of the MSCR test (D'Angelo 2009, DuBois et al., 2014, Greene et al., 2015). ADOT engineers need to understand the potential benefits and limitations of the MSCR test as it relates to Arizona conditions if ADOT decides to adopt the M332 specification. For example, a preliminary study by Arizona State University (ASU) (Stevens et al., 2014) was unable to predict whether the MSCR test parameter was a better indicator of asphalt pavement rutting resistance than the existing M320 test parameter. Confirming which test parameter is the better indicator of rutting resistance is important because this parameter could lead to the formulation of a stiffer binder than required, which may lead to accelerated pavement damage, such as cracking or fatigue. The ASU study suggested that approximately 30 percent of the time the current M320 specification delivers a stiffer binder than the M332 system would deliver for higher grade binders commonly used in Arizona. The critical information to determine the full implications of using M332 is whether the MSCR test parameter captures the performance potential for ADOT binders (unmodified and modified) better than the M320 test parameter, and if there are undesirable consequences (accelerated pavement aging and fatigue damage) associated with changes in the MSCR test parameter value.