Issues Related to Pavement Rehabilitation Design Options in the New Mechanistic- Empirical Pavement Design Guide

Abstract

Since the DARWin-ME is expected to be implemented by State DOTs for rehabilitation and new pavements analysis and design in the near future, it is important to highlight the practical issues related to the execution of these options. Several issues were encountered while executing the DARWin-ME rehabilitation options. These concerns are related to structural and material characterization of the existing pavement conditions. More specifically, the issues are related to: (a) characterizing the existing Portland cement concrete (PCC) pavement structure for unbonded overlays, (b) modeling of the HMA interlayer for unbonded overlays, (c) using backcalculated subgrade modulus of resilience (MR), and (d) utilizing Falling Weight Deflectometer (FWD) testing. Typical PCC elastic moduli for existing PCC layers range between 3 and 6 million psi. For unbonded overlays, the elastic modulus is used to characterize the existing PCC condition. However, a limiting value of 3 million psi is suggested by the developers of the DARWin-ME. Counter-intuitive results were observed when higher values of elastic modulus were used to represent field conditions. It was also found that the asphalt interlayer structural and material properties have insignificant effect on the equivalent PCC slab thickness. Furthermore, the current performance models in the DARWin-ME are calibrated using backcalculated subgrade MR values. Generally, the values are much greater than ones used for AASHTO 93 designs. In addition, DOT's cannot use the AASHTO 93 MR values in the DARWin-ME directly because the values are internally reduced by the software. Finally, because of critical impact of existing pavement conditions on the rehabilitation design, use of FWD to characterize the existing pavement structure has become more important. This paper documents the above mentioned issues and their impacts on pavement performance. Moreover, remedies are discussed to ensure reliable and accurate results by using the DARWin-ME overlay options.