VAASPHALT.ORG 21 WHAT IF BMD IS A FAILURE? others have not performed well at all. Without immediate failures, the bad performers are not identified without tying the job mix formula to measured field data many years after the mix was produced. By understanding the overall lab performance of mixes and establishing actual baselines, each existing or new mix design can be assessed. The contractor and the agency can evaluate ways to improve lab performance outcomes that may improve performance in the field that would benefit the agency. The second success can be seen in reduced costs to the taxpayer. These reductions may be in terms of initial cost, i.e., producing the asphalt, or long-term in lower life cycle cost. In a performance-based BMD approach, the actual performance of the mixture is paramount. Except for certain criteria such as design and production VTM as well as top-sized aggregate (function of lift thickness and pavement design), contractors are free to design mixes that meet or exceed the performance criteria. For some contractors, that may change the amount of recycled asphalt pavement (RAP) or asphalt shingles (RAS) used in a mix. This could lead to cost savings for the contractor and agency. The contractor may be able to use more local aggregates and not have to truck in special materials to meet gradation bands and volumetric properties. Again, this will be a cost savings. A combination of recycled materials, specialty binders and mix additives could result in lower costing or better performing materials. Whether it is lower costs or longer performance by eliminating poorer performing mixes, the cost to the taxpayer should be reduced. Finally, success can be seen in terms of sustainability. Performance-based BMD mixes will be more sustainable, especially in urban areas where RAP and RAS are abundant. Incorporation of RAP and RAS beyond the current limits in most state agency specifications will reduce the demands for virgin aggregates and virgin asphalt binders. In terms of aggregates alone, this will extend the life of quarries and reduce the amount of energy needed to blast, haul, crush and truck materials to asphalt plants. Binder additives such as softeners and rejuvenators will allow aged asphalt binder in RAP and RAS to be better utilized in new mixes. Mix additives such as ground tire rubber, crumb rubber and various fibers can be part of performance designed mixes. Many of the additives are products of other waste streams or by-products. And while the research is still being conducted and consensus conclusions still not obtained, plastics may be in future asphalt mixes. To understand the impacts on performance, BMD will be an assessment tool. Don’t Miss the Forest for the Trees Will BMD result in better mixes? The hopeful answer to that question is “yes.” However, it will be many years before an agency will be able to definitively answer that. Even if the newmixes meet certain criteria during design and production, will the change in actual performance be measurable? Given the number of factors that contribute to the actual performance of a mix, improvement may be a nebulous term. In the meantime, agencies should not miss the forest for the trees. Proper implementation of a performance-based BMD approach will have long term benefits through knowledge gained, reduced costs, and improved sustainability. References 1. “TechBrief: Asphalt Mixture Performance Tester,” FHWA 2013, https://www.fhwa.dot.gov/pavement/asphalt/pubs/ hif13005.pdf 2. “Facilitating Balanced Mix Design Implementation,” NCHRP 20-44(27) Project Description, https://apps.trb.org/ cmsfeed/TRBNetProjectDisplay.asp?ProjectID=4920
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