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