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3The laboratory evaluation of the relationship between thick- ness, density, and permeability was divided into two parts. Part 1 evaluated the relationship of lift thickness, air voids, and per- meability in a controlled, statistically designed experiment. This part looked at varying the lift thickness in the gyratory compactor and determining density; the experimental vari- ables included three aggregates, four gradations, three nomi- nal aggregate sizes for Superpave mixes, and three nominal aggregate sizes for SMA mixes. The aggregate properties are shown in Table 1. Only one asphalt binder was used for this study, a PG 64-22. After the mix designs were performed for these mixes, they were compacted in the Superpave gyratory compactor (100 gyrations) to heights of 2.0, 3.0, and 4.0 times the t/NMAS. The effect of t/NMAS on density was then deter- mined. The plan was to select the t/NMAS that gave optimum density; but, as will be shown later, the results from the Super- pave gyratory compactor data did not provide a conclusive answer; hence, additional work was needed to better establish the appropriate ratio. It was then decided to look at many of the same mixes with a vibratory compactor, to establish whether the vibratory com- pactor would better simulate field compaction and would provide more conclusive results The experimental variables included two aggregates, three gradations, two nominal aggre- gate sizes for Superpave, and three nominal aggregate sizes for SMA. These mixtures, which had already been designed in the first part, were compacted at three thicknesses using three compactive efforts with the vibratory compactor. The density results were determined, and again the results did not identify a definitive minimum ratio. It was then decided that additional work was needed if an acceptable answer was to be obtained. The third attempt at the effect of t/NMAS on compaction was to look at a field study during the rebuilding of the National Center for Asphalt Technology (NCAT) test track. During this work, the layer thicknesses were varied and compacted under similar conditions. Seven mixes from the track were constructed on a paved surface adjacent to the track to look at the effect of layer thickness on density. A general description of these seven mixtures is provided in Table 2. For this part of the study, seven mixes were com- pacted at layer thicknesses varying from two to five times the t/NMAS. For some of these seven mixes, one side was compacted with a vibratory roller and the other sided was compacted with vibratory and rubber tire rollers. The test data were evaluated, as shown later, and provided reason- able results. Another part of the study for Part 1 looked at the effect of lift thickness on permeability. The air voids were controlled at 7 percent and the thickness varied. The permeability results were then determined. These variables were evaluated: two aggregate types, three gradations, two Superpave NMAS, three SMA NMAS, and three t/NMAS. Part 2 of Task 3 looked at the permeability of cores obtained from the NCHRP 9-9 project. This project contained 40 sec- tions with varying aggregate types, NMASs, thicknesses, and design gyrations. The results were evaluated to determine the effect of gradation, NMAS, thickness, and design gyration on permeability. It was assumed that this information would help to determine the in-place air voids at which permeability would become a problem. Both field and lab permeability were measured. CHAPTER 3 RESEARCH APPROACH
4Section NMAS Gradation Asphalt Type Aggregate Type 1 9.5 mm Fine-Graded Superpave Unmodified Granite and Limestone 2 9.5 mm Coarse-Graded Superpave Unmodified Limestone 3 9.5 mm SMA Modified Granite 4 12.5 mm SMA Modified Limestone 5 19.0 mm Fine-Graded Superpave Unmodified Granite and Limestone 6 19.0 mm Coarse-Graded Superpave Unmodified Granite 7 19.0 mm Coarse-Graded Superpave Modified Limestone Aggregate Type Property Test Method Granite Limestone Crushed Gravel Coarse Aggregate Bulk Specific Gravity AASHTO T-85 2.654 2.725 2.585 Apparent Specific Gravity AASHTO T-85 2.704 2.758 2.642 Absorption (%) AASHTO T-85 0.7 0.4 0.9 19.0 mm 14, 0 10, 0 4, 0 12.5 mm 16, 0 6, 0 16, 2 Flat and Elongated (%), 3:1, 5:1 9.0 mm ASTM D4791 9, 1 16, 3 19, 2 Los Angeles Abrasion (%) AASHTO T-96 37 35 31 Coarse Aggregate Angularity (%) AASHTO TP56-99 42.9 43.0 44.0 Percent Crushed (%) ASTM D5821 100 100 80 Fine Aggregate Bulk Specific Gravity AASHTO T-84 2.678 2.689 2.610 Apparent Specific Gravity AASHTO T-84 2.700 2.752 2.645 Absorption (%) AASHTO T-84 0.3 0.9 0.5 Fine Aggregate Angularity (%) AASHTO T-33 (Method A) 49.4 45.7 48.8 Sand Equivalency (%) AASHTO T-176 92 93 94 TABLE 1 Physical properties of aggregate TABLE 2 Mix information for field density study
