Performance Evaluations of Unbound Aggregate Permanent Deformation Models for Various Aggregate Physical Properties

Permanent deformation or rutting is the main performance indicator of unbound aggregate layers used in flexible pavements. This paper evaluates the prediction abilities of unbound aggregate base or subbase permanent deformation models in use or proposed for use in the Mechanistic– Empirical Pavement Design Guide (MEPDG) approach. Repeated load triaxial-type permanent deformation tests were conducted on three unbound aggregate materials—limestone, dolomite, and uncrushed gravel—commonly used for pavement base and subbase and subgrade replacement applications in Illinois. The test matrix was designed to evaluate effects of aggregate physical properties, including moisture content, gradation, types and amounts of fines, aggregate mineralogy, and particle shape, texture, and angularity. The laboratory-measured permanent deformations were compared with those predicted by four rutting models evaluated in this study. The permanent deformations predicted by the original 1989 Tseng–Lytton model and the 2006 El-Badawy model were generally in good agreement with the measured values. The current MEPDG rutting model and its enhanced version proposed in 2013 by Hashem and Zapata tended to overpredict permanent deformations and have a low sensitivity to changes in aggregate physical properties. In addition to enhancements recommended for the four evaluated models, a unified rutting model was developed; it used a shear stress ratio con-cept and imaging-based aggregate morphological indexes. With a single set of calibrated model parameters, the unified rutting model produced reasonably accurate permanent strain predictions for all unbound aggregate materials used in this study.