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Minnesota Department of Transportation
This report presents the results of an investigation into the use of the Superpave asphalt mix design methodology at the local government level in Minnesota. In the project, researchers combined low-cost natural sand with locally available aggregates from four sources: limestone, quartzite, and partially crushed river gravel, and granite. They evaluated coarse and fine aggregate gradations, along with the use of two asphalt grades. It was difficult to achieve the Superpave volumetric requirements of voids in mineral aggregate (VMA) and voids filled with asphalt (VFA) at 4% air voids, regardless of the gradation. A target air void content of 3% satisfied the VFA requirement, even though the VMA requirement could not be fulfilled. The fine aggregate gradations produced densities indicating that the mixtures might be tender during construction, but not necessarily be susceptible to rutting. The coarse-graded mixtures did not show the tenderness problem, but did show that they might be susceptible to rutting. Resilient modulus testing showed little or no difference in the mixtures, regardless of aggregate source or gradation. The difference in resilient modulus due to asphalt grade was apparent only at the intermediate temperatures, and not at the highest or lowest test temperatures. Moisture sensitivity testing showed that all the mixtures studied had adequate durability. Indirect tensile creep and APA rut testing indicated that resistance to low temperature cracking and rutting may be improved by decreasing the lower PG binder grade and increasing the upper PG binder grade.
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