VAASPHALT.ORG 17 Background Most design procedures evolve, change and adapt over time. Asphalt mix design is no different. Just look through decades of state highway agency specification books and this will be obvious. What started out as a trial and error approach more than 125 years ago has evolved into a very complicated process. However, the basic ingredients in asphalt mix have not changed—fine aggregates, coarse aggregates and asphalt binder tossed in with some recycled materials and at times polymer modified binder. The evolution over the decades has been the recognition of how these basic ingredients and other additives in various proportions impact performance. During the Marshall Mix Design era (1980s and 1990s), much emphasis was placed on mixture volumetrics—specifically air voids or voids in total mix (VTM), voids filled with asphalt (VFA) and voids in mineral aggregate (VMA). Understanding the role of these mixture properties contributed to significant strides in mixture design. However, it became clear in the late 1970s and early 1980s that properties other than volumetrics must be considered to combat the asphalt rutting and bleeding experienced across the US. These concerns led to the asphalt mix design research efforts under the Strategic Highway Research Program (SHRP) in the mid-1980s. Ultimately, the outcomes from SHRP led to more stringent aggregate properties, a new asphalt binder grading system and design mixture compaction levels as a function of pavement loading. While envisioned as a three-level mixture design process (Level 1 recipe design, Level 2 lab performance design and Level 3 engineering properties design), limitations in lab testing equipment kept state highway agencies at Level 1. However, the lack of robust testing equipment and procedures did not deter researchers and agencies from pursuing Level 2 and Level 3 design processes. The Asphalt Mixture Performance Tester or AMPT device was developed through National Cooperative Highway Research Program (NCHRP) projects 9-19 (Superpave Support and Performance Models Management) and 9-26 (Simple Performance Tester for Superpave Mix Design)1 to achieve Level 3 designs for mixes where the results can be used in mechanistic/mechanistic-empirical flexible pavement design methodologies. While the AMPT has progressed since its continues on page 18 △ Most design procedures evolve, change and adapt over time. Asphalt mix design is no different. What started out as a trial and error approach more than 125 years ago has evolved into a very complicated process. " " Asphalt flushing. WHAT IF BMD IS A FAILURE?