Effect of Axle Load Spectrum Characteristics on Flexible Pavement Performance

Abstract

The Mechanistic–Empirical Pavement Design Guide (MEPDG) uses performance models to predict cracking and rutting in flexible pavements. A unique mechanism controls the initiation and accumulation of each distress, but each mechanism can have several causes. Axle repetitions and loads are the main causes of all load-related distress types. MEPDG incorporates axle load spectra to characterize axle loading for a site and uses them to calculate pavement response and damage accumulation. These load distributions have a bimodal shape, and a mixture of two continuous distributions can be used to model them. In this paper, closed-form solutions are developed to estimate the characteristics of a mixture of bimodal axle load distributions. The observed axle load spectra from 14 sites in different states were used to relate load distribution characteristics to predicted flexible pavement performance. The overall mean and other characteristics of a bimodal axle load distribution explained the variations in expected flexible pavement performance. Cracking, surface rutting, and ride quality are related to the fourth root of the fourth moment of axle load distributions. Rutting in the hot-mix asphalt layer is strongly associated with the overall mean, but in base and subbase layers it is related to the 95th percentile load of axle load spectra. These findings imply that cracking, rutting, and roughness growth in flexible pavements are caused mainly by axle load distributions having heavier tails with infrequent extreme loads. Heavier loads appear to cause more cracking; a higher number of load repetitions is more critical in developing additional surface rutting in flexible pavements.