The recommended aggregate properties for Superpave hot mix asphalt (HMA) mixtures include a restricted zone which lies along the maximum density gradation between the intermediate size (either 4.75 or 2.36 mm, depending on the nominal maximum size of the aggregate) and the 300 μm size; it forms a band through which gradations were not recommended to pass. The restricted zone requirement was adopted to help reduce the incidence of tender or rut-prone HMA mixes. Although recommended as a guideline and not a required specification, the restricted zone is commonly interpreted as being based on experience and appropriate to use, if at all possible, for producing acceptable HMA mixes.
According to many paving engineers, compliance with the restricted zone criteria may not be desirable or necessary in every instance to produce paving mixes that give good performance. Some highway agencies and suppliers can provide examples of aggregate gradations that pass through the restricted zone, but produce paving mixes that are performing well. Research has supported this position.
This research project was undertaken to evaluate the effect of the Superpave restricted zone on permanent deformation of dense-graded HMA mixtures. The following factor-level combinations were evaluated: two coarse aggregates, ten fine aggregates, five 9.5 mm nominal maximum aggregate size gradations, and one compactive effort. Of the five gradations used, three pass through the restricted zone and two fall outside the restricted zone (control). Permanent deformation characteristics of mixes meeting Superpave volumetric requirements were evaluated by two different types of tests: empirical and fundamental. For the empirical test, the Asphalt Pavement Analyzer (APA) was used. The Superpave shear tester (SST) and a repeated load confined creep test were utilized as fundamental tests. Test results from the three mechanical tests were analyzed statistically to evaluate the effect of the five gradations on permanent deformation of the HMA mixtures. Based upon the analysis of data, mixes having gradations passing through the restricted zone did not necessarily have lower rut resistance compared to mixes having gradations outside the restricted zone.