Evaluation of BMD Surface Mixtures with Conventional and High RAP Contents Under Laboratory-Scale and Full-Scale Accelerated Testing

About the report:

Balanced Mix Design (BMD) is an asphalt mixture design method that replaces some aspects of traditional volumetric design with performance testing for the most common distresses, such as rutting and cracking. This approach provides an opportunity to properly design and produce engineered asphalt mixtures, including those with higher reclaimed asphalt pavement (RAP) contents, recycling agents (RAs), fibers, polymer modified binders, and so on. The results of laboratory performance tests play a crucial role in the BMD process because they ensure the production of high-performing materials. In addition to performance testing and evaluation in the laboratory, accelerated pavement testing serves as a valuable tool to bridge the important and significant gap between models developed using laboratory material characterization and actual long-term pavement performance monitoring.

The purpose of this study is to evaluate the application of the BMD concept to design durable and longer lasting surface mixtures with A and D designations in Virginia, with a focus on relatively higher RAP contents, or HRAP mixtures, (that is, greater than 30% RAP). A and D mixtures are designated for traffic loads of 0 to 3 million and 3 to 10 million equivalent single axle loads, respectively. The scope of work consisted of laboratory and accelerated pavement testing of six surface mixtures incorporating a range of RAP contents (conventional and high), two binder grades, one RA, and one warm mix additive.

The study revealed that designing a dense-graded, unmodified surface mixtures with higher RAP contents is possible, using the current the Virginia Department of Transportation (VDOT) BMD special provision. The plant can produce those mixtures with no significant differences in aggregate gradations and asphalt binder content from the design. Furthermore, the study revealed that using RAs or a softer binder, or both, may be necessary when designing conventional and HRAP surface mixtures to meet BMD requirements. In addition, researchers need to collect more data to evaluate the relationships between the test results for non-reheat and reheat specimens for all BMD performance tests. Finally, the current selected BMD tests characterized the laboratory performance of mixtures similarly to the performance observed under accelerated pavement testing.

 This study recommends that VDOT should (1) consider allowing the use of other tools in addition to increasing binder content, such as RA or softer binder, or both, for the design and production of BMD surface mixtures with A and D designations, even at allowable RAP contents; (2) consider allowing mixtures with RAP contents of up to 45%, when properly controlled and where desired by the district, designed using current BMD specifications modified to allow additional tools like RA and softer binder; and (3) continue the efforts toward full implementation of BMD in Virginia for surface mixtures with A and D designations, using the currently selected performance tests.

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