The aggregate specification for Superpave ® hot-mix asphalt (HMA) mixtures includes a restricted zone that lies along the maximum density gradation between the intermediate size (i.e., either 4.75 or 2.36 mm, depending on the nominal maximum size of the aggregate) and the 0.3-mm size. The restricted zone forms a band through which gradations were recommended not to pass. The restricted zone requirement was http://gulliver.trb.org/publications/nchrp/nchrp_rpt_464-a.pdf / http://gulliver.trb.org/publications/nchrp/nchrp_rpt_464-b.pdfadopted in Superpave to reduce the incidence of tender or rut-prone HMA mixes. Although the restricted zone was included in Superpave as a recommended guideline and not as a required specification, some highway agencies interpret it as a requirement. According to many asphalt paving technologists, compliance with the restricted zone criteria may not be desirable or necessary to produce paving mixes that give good per-formance in terms of rutting. Some highway agencies and suppliers can provide examples of aggregate gradations that pass through the restricted zone, but produce paving mixes that have performed well. This report may be accessed by Internet users at This research project was undertaken to evaluate the effect of the Superpave restricted zone on permanent deformation of dense-graded HMA mixtures on the basis of a statistically planned and properly controlled laboratory experiment. The project’s primary objective was to determine under what conditions, if any, compliance with the restricted zone requirement is necessary when HMA meets all other Superpave requirements such as fine aggregate angularity (FAA) and volumetric mix criteria for the specific project. The following factors were evaluated: two coarse aggregates, ten fine aggregates, two nominal maximum size mixes (i.e., 9.5 and 19.0 mm), five aggregate gradations, and three compactive efforts (i.e., Ndesign =75, 100, and 125). Of the five gradations used, three pass through the restricted zone and two (i.e., the control group) fall outside of the restricted zone. 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 was used. The Superpave shear tester and a repeated load confined creep test were used 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. Mixes meeting Superpave and FAA requirements with gradations that violated the restricted zone performed similarly to or better than the mixes having gradations pass-ing outside the restricted zone; therefore, the restricted zone requirement is redundant for mixes meeting all Superpave volumetric parameters and the required FAA. It has been recommended to delete references to the restricted zone as either a requirement or a guideline from the AASHTO specification (AASHTO MP2) and practice (AASHTO PP28) for Superpave volumetric mix design. (A)
Samenvatting