SIGNIFICANCE OF RESTRICTED ZONE IN SUPERPAVE AGGREGATE GRADATION SPECIFICATION

Asphalt technologists recognized it for many years prior to Superpave (Registered trademark) -- a characteristic hump in a mix gradation within the intermediate sieve sizes which could cause a hot-mix asphalt (HMA) mix to be prone to tenderness and rut susceptibility. The Strategic Highway Research Program (SHRP) gave it a name -- the restricted zone -- and included it in Superpave as a recommended guideline. Some transportation agencies interpreted it as a requirement. However, based on experiences with mixes violating the restricted zone, many asphalt technologists did not agree with its inclusion in Superpave. The authors of this Circular present the historical basis of the restricted zone in HMA mixes and summarize the published research conducted on the restricted zone to determine its significance within the Superpave gradation specification. The Transportation Research Board Committee on General Issues in Asphalt Technology (A2D05) finds that this information should be of interest to materials and pavement engineers and others responsible for designing HMA pavements.