Specifications for Aggregate Frictional Qualities in Flexible Pavements

Abstract

and granite) for their polish-resistant qualities. An alternative is to limit known polishing aggregates or those aggregates that have not historically demonstrated acceptable friction in the field. Illinois, Missouri, and Wisconsin are among the states bordering Iowa that limit the quantity of carbonate aggregates (commonly limestone and dolomite) in surface mixes for high-volume facilities. An important component of a frictional classification system is defining the properties used to distinguish an aggregate's frictional qualities. Some agencies qualify noncarbonates as meeting a minimum percentage of acid-insoluble residues, whereas others qualify material by source. The Iowa Department of Transportation (DOT) classification system defining the frictional qualities of aggregates is provided in Table 1 (3).Aggregates that provide the greatest frictional qualities may not be locally available. Therefore, specification limits should be established to meet friction demand while economically providing maximum use of local materials. A recent study of 23 test sites in Iowa and Indiana found that asphalt mixtures containing a large quantity of polishsusceptible aggregates should be blended with at least 20% coarse frictional aggregate. The study also suggests that a fineness modulus of the total blend that is greater than 4.6 will improve frictional properties (2). Verification of these conclusions is needed. To validate these conclusions, a desirable friction level must be established (2). RESEARCH OBJECTIVE AND APPROACHThis study was done to validate the classification system used by the Iowa DOT to establish frictional qualities of aggregates used in asphalt surface mixtures. An equally critical objective was to confirm whether desirable friction levels are being demonstrated in the field at the current minimum requirements for polish-resistant aggregates. To meet these objectives, the study was divided into two phases. The first phase defined a desirable level of friction for Iowa Interstates. Friction measurements were correlated with crash records on pavement sections for which asphalt mixture data were available. The data set spanned 30 years and contained mixtures with a wide range of frictional aggregate types, sizes, sources, and gradations at various quantities. The study scope was limited to Interstate pavements.The second phase used the desirable friction level to determine the adequacy of current specification limits. To carry this out, job mix formulas were evaluated by both the proportion of polishresistant aggregates and their ability to demonstrate desirable friction levels in the first 5 years of service. Limits were then modified as necessary to ensure desirable friction was achieved in the field.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 aggr...

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