In recent years, the asphalt industry has been increasingly evaluating mixture testing for a variety of purposes. This article assesses the Micro-Deval (MD) test for its ability to evaluate compacted dense-graded asphalt mixtures. Historically, the MD test has been used to evaluate durability characteristics of loose aggregates, so this investigation deviates from the equipment’s intended purpose. MD testing was performed in traditional manners (i.e., submerged in water), as well as absent water. The investigation benchmarked MD testing against Cantabro mass loss and the Illinois Flexibility Index Test with an emphasis on use of the protocols during mixture production. Four evaluation criteria were used for assessment (level of rationality of test results, equipment cost, noise during testing, and time to achieve test results). Results of this investigation were that the MD testing protocol was not optimally suited for assessing compacted dense-graded asphalt. Wet MD tests (traditional test manner) were highly variable and provided no appealing test outputs. Operationally, the MD test was one of the noisier tests evaluated; it required an intermediate amount of time to conduct a test but had the lowest equipment cost. With all factors considered, the other tests considered are believed to be more promising than the MD for assessing dense-graded asphalt.
In recent years, the asphalt paving industry has been strained by numerous factors including increased asphalt binder costs, funding that has not kept up with material costs, increased societal pressure to recycle, and deteriorating pavement networks. Mix design should account for the market in which it is used, which is very different now than when today’s volumetric mix design practices were developed (many of the aforementioned factors were less present). Given this reality, a statewide database of all 1,452 approved mix designs in Mississippi from 2005 to 2018 was compiled and analyzed, and the objective of this paper is to present findings, trends, and unintended consequences of exclusive reliance on volumetrics. With volumetrics-only mix design, asphalt content is primarily controlled by voids in mineral aggregate (VMA), which is influenced by aggregate bulk specific gravity (Gsb). Minor Gsb deviations (i.e., within AASHTO d2 s limits), can significantly affect VMA, so much so that 99% of Mississippi’s mixes could be failing VMA while reported VMA passes. This allows mix manipulation and economization, with 0.8% asphalt content reductions possible while still meeting volumetric requirements. Recycled materials can exacerbate this issue, and common approaches to increase asphalt content (decreasing design gyration level or using finer gradations) are ineffective with fixed VMA requirements. Overall, the mix design database analysis agrees with numerous smaller studies but does so with an entire state’s actual practice. This presents a compelling case that volumetrics-only mix design has limitations, and supports ongoing efforts to reintegrate mechanical tests.
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