The Strategic Highway Research Program’s (SHRP) asphalt research was primarily aimed at the properties of asphalt binders and paving mixes and their effect on asphalt pavement performance. The study of aggregate properties, including gradation, was intentionally excluded from the asphalt research program. Yet the SHRP researchers had to recommend a set of aggregate properties and an aggregate gradation specification without the benefit of experimentation so that a comprehensive Superpave mix design system could be formulated.
Although the restricted zone was included in Superpave as a recommended guideline and not a required specification, some highway agencies are interpreting it as a requirement.
Many asphalt technologists believe that the compliance with the restricted zone criteria may not be desirable or necessary in every case to produce asphalt mixes with good performance. If highly angular aggregates are used in the mix it is likely that the mix will not exhibit any tenderness during construction and will be rut-resistant under traffic, although its gradation may pass through the restricted zone.
The effect of restricted zone on mix performance should be evaluated on the basis of a statistically planned and properly controlled experiment. This is the subject of the National Cooperative Highway Research Program Project 9-14 “Investigation of Restricted Zone in the Superpave Aggregate Gradation Specification” undertaken by the National Center for Asphalt Technology in May 1998. The primary objective for this research is to determine under what conditions compliance with the restricted zone requirement is necessary when the asphalt paving mix meets all other Superpave requirements such as fine aggregate angularity (FAA) and volumetric mix criteria for the project.
Since the restricted zone is applied within the fine aggregate sieve sizes, the shape and texture of the fine aggregates are the most important factors affecting the performance of hot-mix asphalt mixtures. Therefore, the approach taken in identifying and selecting fine aggregates for use in this study was to select aggregates with varying values of fine aggregate angularity. Of the five gradations proposed to be used in this project, three will violate the restricted zone while two will reside outside the restricted zone.
Three gradations will violate the restricted zone as follows: going straight through the zone; crossing the restricted zone; and humping on the 0.6- and 0.3-mm sieves within the zone.