Mechanistic-Empirical Asphalt Overlay Thickness Design and Analysis System (0-5123-3)

The placement of an asphalt overlay is the most common method used by the Texas Department of Transportation (TxDOT) to rehabilitate existing asphalt and concrete pavements. The type of overlay and its required thickness are important decisions that TxDOT engineers make on a daily basis. To perform well, an asphalt overlay must have a balance of both good rut and crack resistance. Furthermore, overlay performance is highly influenced by many factors, such as existing pavement conditions, traffic loading, and environmental conditions. It has also recently become common practice to use two different materials in an overlay, the first being a crack resistant level up course and the second being a wearing surface. The properties of both overlay types have a big impact on performance. The main objective of the Research Project 0-5123 was to develop a comprehensive mechanistic-empirical (M-E) asphalt overlay design system to assist TxDOT engineers to make these design decisions. The design system developed incorporates models for both rutting and reflection cracking of the proposed overlay. The Paris' law-based reflection cracking model was evaluated and recommended for use in this study. This model requires the use of both stress intensity factors (SIF) and fracture properties (A and n) for predicting crack propagation caused by both traffic loading and thermal effects. For practical implementation of the SIF concept, a total of 34 SIF regression equations were developed based on more than 1.6 million finite element computations. The required fracture properties can be easily determined using the Overlay Tester. The proposed reflective cracking model was calibrated using performance data from three HMA overlay field case studies and then verified using the California's Heavy Vehicle Simulator test results. To predict asphalt overlay rutting, the well-known VESYS layer rutting model was used and later calibrated using the field rutting data from the National Center for Asphalt Technology (NCAT) test track 2006. The material properties required for this model are obtained from repeated load tests. The reasonableness and accuracy of the calibrated rutting was further verified by the field rutting data from NCAT test track 2000. Finally, the calibrated reflective cracking and rutting models were integrated into an asphalt overlay thickness design and analysis program. To assist in implementation, default values of material properties have been provided for all the overlay types currently used in Texas. The program provides the designer with a tool to evaluate on a project-level basis the impact of load transfer efficiency (LTE) on predicted life and to determine what level of LTE must be repaired in order to achieve adequate performance. In summary this study has developed a comprehensive overlay thickness design and analysis system based on solid engineering principles. The software package developed in this study has been provided to TxDOT. Based on the evaluations made in this study its predictions appear rational and reasonable. This system is ready for state-wide pilot implementation.