Effects of Aggregate Properties on Flexible Pavement Rutting in Alabama

Pavement rutting which results in a distorted pavement surface is the accumulation of permanent deformation in all or a portion of the layers in a pavement structure. The objective of this study was to examine the effects of aggregate properties on the rutting of asphalt concrete mixtures. This objective was accomplished by analyzing rutting data from the Alabama Highway Department's pavement condition database. Field sites were evaluated and sampled and laboratory tests performed on aggregate from field samples. Analyses of the Department's pavement condition database indicate that rutting in Alabama is increasing, and that this increase is attributable to either increased loading intensity or increased asphalt concrete rutting susceptibility. The analyses also indicate that rutting varies geographically and that this variation can be explained by quality of locally available aggregate. Those areas with crushed stone and angular natural sands are less susceptible to rutting. Analyses of data from field test sites indicate that permanent deformation causing rutting is generally confined to the top 7 to 10 cm (surface and binder courses). There was little evidence that lower base/subbase courses or subgrade were significant contributors to rutting. A rate of rutting of 5.0 × 10-4cm/√ESAL or 2.5 × 10-7cm/ESAL delineated good and poor performing pavements. Aggregate properties studied include: gradation, coarse aggregate percent fractured faces, and fine aggregate particle shape and texture. The correlation coefficients for most of these properties were low, however, there appeared to be definite trends. The poor correlations show that rutting is a very complicated process that is affected by multiple factors, and demonstrates the importance of both aggregate properties and asphalt content during material selection and mix design.