United States Department of Transportation
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TRIS Online Accession Number:00793261
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ISBN:0965231046
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Abstract:Current research at Iowa State University on behalf of the Iowa Department of Transportation has focused on the volumetric state of hot-mix asphalt (HMA) mixtures as they transition from stable to unstable configurations. This has traditionally been addressed during mix design by a minimum voids in the mineral aggregate (VMA) requirement, based solely upon the nominal maximum aggregate size. The current research addresses three maximum aggregate sizes (11 mm, 12.5 mm, and 9.5 mm), three gradations (coarse, fine, and dense), and combinations of natural and manufactured coarse and fine aggregates. Specimens are compacted using the Superpave Gyratory Compactor (SGC), conventionally tested for bulk and maximum theoretical specific gravities, and physically tested using the Nottingham Asphalt Tester (NAT) under a repeated load confined configuration. The results clearly demonstrate that the volumetric conditions on an HMA mixture at the stable/unstable threshold are influenced by the maximum aggregate size, gradation and aggregate shape and texture. The currently defined VMA criterion, while significant, is seen to be insufficient by itself to correctly differentiate sound from unsound mixtures. Under current specifications, many otherwise sound mixtures are subject to rejection solely on the basis of failing to meet the VMA requirement. The results of the current research project suggest a new set of volumetric design parameters that explicitly take into account such factors as aggregate gradation, shape, and texture.
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