EVALUATING BMD CONVENTIONAL AND HIGH RAP SURFACE MIXTURES THROUGH FULL-SCALE ACCELERATED PAVEMENT TESTING The use of reclaimed asphalt pavement (RAP) is greatly influencing asphalt mixture production and development worldwide. While RAP usage has increased in recent years, its growth plateaued in the U.S. with a national average of around 20% in 2014. The economic and associated environmental benefits gained by using RAP in asphalt mixtures have since encouraged U.S. state agencies to introduce special provisions and specifications allowing its use at higher contents in mixtures. The challenges of using high RAP (HRAP) asphalt mixtures can be effectively addressed by incorporating recycling agents (RAs) and/ or softer binders within a performance-based framework such as the Balanced Mix Design (BMD) approach. This approach provides a tool to properly design and produce engineered asphalt mixtures, including those with higher RAP contents. The definition of HRAP mixtures remains specific to each state, and, in Virginia, HRAP mixtures are specifically defined as those containing RAP content exceeding 30%. Accelerated Pavement Testing: Virginia’s BMD Experiment Laboratory performance tests play a crucial role in the BMD process as they ensure the production of high-performing materials. In addition to performance testing and evaluation in the laboratory, accelerated pavement testing (APT) serves as a valuable tool to bridge the important and significant gap between models developed using laboratory material characterization and the actual long-term pavement performance monitoring. This technique involves applying wheel loading (typically surpassing the design load limit) to a pavement system to assess its response within a significantly condensed time frame. This provides insights into long-term pavement performance monitoring and analysis. Virginia is among the few states that have implemented the use of APT facilities for pavement evaluation purposes. In 2020, a collaborative experiment between the Virginia Transportation Research Council (VTRC), the Virginia Department of Transportation (VDOT), and Virginia Tech (VT) was planned and executed at the Virginia APT facility located at the Virginia Tech Transportation Institute (VTTI). A total of six experimental testing lanes were constructed for this study. These lanes featured the use of conventional and higher RAP contents, RAs, softer binder, and a warm mix asphalt (WMA) additive. Evaluated Mixtures The six evaluated mixtures were: • 30_C: a non-BMD mixture serving as a control and including 30% RAP content and PG 64S-22 asphalt binder (with S denoting standard traffic). This mixture was designed following the conventional Superpave mix design methodology through which the optimum binder content (OBC) was selected at 4% air voids. • 30_O: a BMD optimized version of mixture 30_C (with O denoting optimized) featuring the use of 30% RAP content and a PG 64S-22 at a relatively higher OBC when compared to mixture 30_C. • 45_HR: a BMD HRAP mixture containing 45% RAP and a PG 64S-22. The mixture design resulted in a much higher OBC as compared with all other evaluated mixtures. No softer binder or RA were used during the design and production of this mixture. • 45_HR_RA: a BMD HRAP mixture incorporating 45% RAP, a typical PG 64S-22, and an RA. • 45_HR_L: a BMD HRAP mixture featuring the use of 45% RAP and a softer binder, PG 58-28. • 60_HR_L_RA: a BMD HRAP mixture containing 60% RAP, a softer binder (PG 58-28), and an RA. The second through the sixth mixtures were designed following VDOT BMD specifications using Approach D (performance only). Equipment and Instrumentation The central component of the APT experiment was the Dynatest Heavy Vehicle Simulator (HVS) Mark VI shown in Figure 1a. The HVS automatically regulates surface temperature to ensure that the desired temperature is maintained at a depth of two inches from the surface. Jhony Habbouche, Ph.D., PE Stacey Diefenderfer, Ph.D., PE Brian Diefenderfer, Ph.D., PE Virginia Transportation Research Council 10 SPRING/SUMMER 2024
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