The recommended aggregate properties and criteria that appeared in the original Superpave mix design method included a restricted zone 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-micrometer size. The original basis for inclusion of the restricted zone requirement was to help reduce the incidence of tender or rutting-prone paving mixes. The restricted zone was presented in the Superpave mix design system as a recommended guideline and not a required specification, but it is commonly interpreted as being based on experience and appropriate to use, if at all possible, to produce acceptable paving mixes.
The objective of this research was to determine through evaluation of the performance properties of hot mix asphalt (HMA) if the restricted zone requirement in the Superpave mix design method was redundant with fine aggregate angularity (FAA) and volumetric mix criteria. The research team found that hot mix asphalt meeting Superpave mix volumetric and FAA requirements with gradations passing through the restricted zone performed similarly to or better than mixes with gradations passing outside the restricted zone. The team concluded that the restricted zone requirement is not necessary to ensure satisfactory performance when all other relevant Superpave design requirements are met, and it recommended changes to AASHTO MP2 that were adopted by the AASHTO Highway Subcommittee on Materials.
The final report has been published as NCHRP Report 464: The Restricted Zone in the Superpave Aggregate Gradation Specification.