Significance of Restricted Zone in Superpave Aggregate Gradation Specification

TRB Transportation Research Circular E-C043: Significance of Restricted Zone in Superpave Aggregate Gradation Specification presents the historical basis of the restricted zone in hot-mix asphalt (HMA) mixes and summarizes the published research conducted on the restricted zone to determine its significance within the Superpave gradation specification.

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 echnologists did not agree with its inclusion in Superpave.