Specifications for Aggregate Frictional Qualities in Flexible Pavements

Asphalt mixtures can be engineered to meet a pavement facility's surface friction demands. Researchers recommend laboratory testing on mixtures and aggregates to ensure desirable friction in the field. Some state highway agencies use this testing, and other agencies specify properties related to the aggregates. Aggregate structure, source, grain size, and chemistry have been shown to affect surface friction, yet these effects have been little validated in the field. The Iowa Department of Transportation (DOT) classifies frictional qualities of aggregates on the basis of grain size, hardness, and abrasion. Mixtures are assigned one of three designations that specify the frictional class and amount needed to satisfy the facility's demands. A case study analyzed the effectiveness of the classification system and validated material specification limits for flexible Interstate pavements. A comprehensive review of mixture designs and friction measurements from the past 30 years showed that friction was highly sensitive to aggregate classification and could be improved with modifications to specification limits. A correlation between average ratio of wet-to-dry crashes and corresponding friction numbers was developed from more than 14,000 crash records. The developed model provided a desirable minimum friction level and was validated with a multimethod approach. The analysis showed the average ratio of wet-to-dry crashes remained constant when friction levels exceeded 38. Historically, this level was achieved when 30% or more of the coarse aggregate and 25% or more of the fine aggregate was a crushed quartzite, granite, or steel slag. Iowa DOT specifications for asphalt mixtures have been updated to reflect these findings.