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Experimental Assessment of Aggregate Surfacing Materials
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2007-06-30
[PDF-1.06 MB]
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Abstract:An extensive suite of geotechnical laboratory tests were conducted to quantify differences in engineering properties of three crushed aggregates commonly used on Montana highway projects. The material types are identified in the Montana Supplemental Specifications as CBC-6A, CBC-5A, and CTS-2A. Results from R-value tests and direct shear tests performed on large samples (12 in by 12 in) indicates the CBC-6A aggregates generally exhibited the highest strength and stiffness of the three material types. In terms of strength parameters, there was no statistically significant difference between CBC-5A and CTS-2A materials. The CBC-6A and 5A materials exhibited similar average Rvalues, which were both slightly greater than the CTS-2A materials. Overall, the CTS-2A materials generally exhibited the lowest average strength and stiffness. Although this material was the poorest performer when compared to the two CBC materials, it still exhibited relatively high strength and stiffness. Drainage capacity was quantified by conducting multiple saturated constant head permeability tests on 10-inch-diameter samples of each material type. The CBC-6A and CTS-2A materials had the highest average permeability values, while the CBC-5A materials had the lowest. Permeability was shown to depend more on the fine fraction void ratio (ef) than on aggregate type or maximum particle size. An analytical method of predicting permeability based on ef was developed, which will allow MDT designers to estimate permeability based on gradation and state of compaction. Overall, the CBC-6A materials were generally the best performers in this study. In general, the CBC- 5A aggregates exhibited the second highest strength and stiffness, but also had the lowest drainage capacity. The CTS-2A aggregates exhibited the lowest strength and stiffness, but had relatively good drainage capacity. The ability to substitute CTS-2A material for CBC aggregates depends on the relative importance that is assigned to strength, stiffness, and drainage in the pavement design model.
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