Performance Checks for Unbound Aggregate Base Permanent Deformation Prediction Models under Dynamic Stress States Induced by Moving Wheel Loading

Permanent deformation (rutting) is the most critical load-associated distress that develops in unbound aggregate layers significantly affecting their performance. Despite past research work has focused on estimating rutting of unbound aggregates using a variety of prediction models, few of them can be applied with adequate confidence to actual field conditions, i.e., dynamic stress states due to moving wheel loads and varying aggregate source properties. This paper aimed to evaluate these rutting models by comparing computed permanent deformation to that measured in advanced laboratory repeated load triaxial (RLT) tests. The data source analyzed was from a series of laboratory RLT tests conducted to characterize two crushed aggregate materials for their permanent deformation trends when subjected to moving, complex gear loading of next generation aircraft. Both constant and variable confining pressure tests were performed using an advanced RLT testing device to clearly account for the effects of dynamic stress states, including stress ratio, stress magnitude, and stress path loading slope (representing rotating principal stress directions) on permanent deformation accumulation. The applicability of several commonly used unbound aggregate rutting models to dynamic stress states was evaluated using this comprehensive laboratory database. The comparison of model predictions against laboratory RLT test results indicated the need for further enhancement of these models. Finally, rational modifications were suggested to improve their predictive ability, which would provide an improved analysis and design of flexible pavements for the rutting distress.