Development of Flexible Pavement Rut Prediction Models from the NCAT Test Track Structural Study Sections Data

Abstract

This research study was attempted to address two of the most important aspects of mechanistic-empirical (M-E) pavement design. As M-E design continues to advance toward full implementation by state agencies, there is a need to assess the accuracy of the load-response models under dynamic truck loading. The load response model is a core component of flexible pavement M-E design and the common practice is to use a layered elastic approach to predict pavement responses under load. Concerns regarding accuracy of this type of model arise when considering unbound materials exhibiting non-linear behavior, viscoelastic hot-mix asphalt (HMA) materials and dynamic loads applied by moving traffic. Despite this, layered elastic models continue to be the state-of-the practice for most pavement design and analysis applications. Considering this, one of the objectives of this study was to assess the accuracy of a layered elastic model with respect to measured pavement responses under live truck traffic.

Specifically, eight test sections at the National Center for Asphalt Technology (NCAT) Test Track were instrumented to measure vertical pressures in the unbound base and subgrade layers. The test sections consisted of various HMA thicknesses and used modified and unmodified asphalt binders. Material properties were established using backcalculation of falling weight deflectometer (FWD) data. The test sections were then simulated with the layered elastic computer program, WESLEA. Comparisons between theoretical and measured pavement responses were made over a wide range of environmental conditions and the two different truck load configurations. The measured responses were generally within 15% of theoretical with a strong correlation between the two sets of data.

After validating the load response model, an effort was made to develop a rut prediction model that can accurately predict field rutting. HMA layer rutting was the only source of rutting observed in all eight sections. During the development of the rut prediction model, two different approaches were evaluated and compared. First, a vertical strain-based rut model was built by relating the measured rutting to the vertical strain on the top of granular layers and the number of truck axle passes. In the second approach, rutting was linked with maximum shear strain in the HMA layer and the number of truck axle passes. The model coefficients were analyzed for both approaches and their validity was evaluated. It was concluded that the shear strain model predicted rutting realistically and the model coefficients distinguished rutting in polymer-modified and unmodified asphalt sections.