Both coarse- and fine-graded hot-mix asphalt mixtures can be designed within the gradation control points recommended in Superpave (Registered trademark) mix design. Some states have begun to specify only coarse-graded mixtures (below the restricted zone), whereas other states are specifying only fine-graded mixtures (above the restricted zone). A study was conducted to compare coarse-graded Superpave mixtures with fine-graded Superpave mixtures in terms of resistance to rutting to determine whether restrictions on gradations (either coarse- or fine-graded mixtures) are justified. Fourteen mixtures were tested containing two nominal maximum aggregate sizes (9.5 and 19.0 mm), two coarse aggregates (granite and crushed gravel), and four fine aggregates (sandstone, limestone, granite, and diabase). Resistance to rutting of both coarse- and fine-graded mixtures was evaluated by three test methods: asphalt pavement analyzer, Superpave shear tester, and repeated load confined creep test. Statistical analyses of the test data obtained by the three performance tests indicate no significant difference in rutting resistance of coarse- and fine-graded Superpave mixtures. It has been recommended that mix designs not be limited to designing mixes on the coarse or fine side of the restricted zone.
Abstract