Effect of Superpave Defined Restricted Zone on Hot Mix Asphalt Performance

The effect of the Superpave defined restricted zone on HMA rutting performance was evaluated. One gradation that violated the restricted zone (TRZ) and two gradations that did not violate the restricted zone (BRZ and ARZ) were evaluated. Mixes evaluated represented a range of maximum aggregate sizes (MAS), design traffic levels, and aggregate types. Three laboratory tests, Asphalt Pavement Analyzer, Rotary Loaded Wheel Tester, and Marshall test, were used to evaluate the rutting performance. From the analysis, it was found that mixes having gradations violating the restricted zone performed similarly to or better than the mixes with gradations passing outside the restricted zone with respect to laboratory rutting tests. This conclusion was drawn from the results of experiments with 12.5 mm, 19.0 mm and 25.0 mm MAS gradations at Ndesign values of 100, 75, and 50 gyrations. This conclusion is confirmed and supported by a recently completed National Cooperative Highway Research Program project NCHRP 9-14: “The Restricted Zone in the Superpave Aggregate Gradation Specification.” The results also showed that rutting performance of mixes having gradations below the restricted zone, which was commonly recognized to be rut-resistant, appears more sensitive to aggregate properties than do mixes having gradations above or through the restricted zone.