Coarse Superpave Mixture Sensitivity

Asphalt-aggregate mixture sensitivity has long been recognized for its potential negative impact on performance if variables including the asphalt content (AC) and the percent that passes the 0.075-mm sieve (p0.075) are not carefully controlled during construction. Typical production standard deviations for AC and p0.075 are 0.3 and 0.9 percent, respectively. This mixture sensitivity analysis shows that variations of this magnitude can result in significant changes in coarse Superpave mixture volumetrics. Three coarse mixtures designed for the WesTrack project were examined, including the original coarse mixture and the replacement mixture. The coarse mixture was most sensitive to both AC and p0.075, contradicting the relatively worse performance of the replacement mixture. The replacement mixture was relatively insensitive to p0.075, and the third mixture was extremely sensitive to reductions in p0.075. Results also suggest that mixture sensitivity may affect optimum AC by up to 1.0 percent. When measured sensitivities were compared with those predicted from models calibrated in the field, the potential for extreme sensitivity was demonstrated by errors in predicted changes in air voids that ranged from 0 to 98 percent because of small changes in AC or p0.075. These findings are disturbing when one considers the current industry trend toward the use of coarse Superpave mixtures. The conclusions drawn warrant a recommendation that mixture sensitivity analyses are needed in the Superpave volumetric mix design process. A mixture sensitivity analysis that is useful for identification of sensitive mixtures and that serves as an excellent field quality management tool is presented.