The asphalt industry has a well-established history of using new technologies and innovative materials in flexible pavement cross-sections. Examples include warm mix asphalt (WMA), recycled asphalt shingles (RAS), reclaimed asphalt pavement (RAP), recycled tire rubber (RTR) and cold recycled mixtures. These, and other technologies currently in development, offer significant economic, environmental, and engineering advantages over conventional materials if used properly within a structural design framework. The primary structural design approaches in the U.S. are empirical and mechanistic-empirical. At last count, the empirical approach is in use by 28 state DOTs with the majority of those using the 1993 AASHTO Design Guide. The remaining states have implemented or are transitioning to the new M-E approach adopted by AASHTO or have their own state-specific empirical or M-E method. A major challenge every agency faces when making the transition to M-E design is local calibration of the empirical transfer functions that predict pavement performance. Calibration is typically accomplished with a combination of laboratory testing and field-data collection from existing pavements which can take years to complete at relatively great cost. This assumes that existing pavement sections that vary in age and conditions are available from which to extract performance data. Calibration for new pavement materials or technologies is further complicated by the fact that field and lab data may be sparse or non-existent. This fact greatly limits the deployment of the technology or new material until it is fully tested, and transfer functions are fully calibrated. The pace at which new pavement materials and technologies are developed requires more rapid methods for calibrating transfer functions. To this end, calibration should take no longer than one year to be efficient and effectively support technology deployment. This research project aims to identify and develop methodologies to rapidly calibrate M-E transfer functions for new materials and technologies. Since construction of full-scale trial pavement sections, as a matter of routine practice, would be too time consuming, it is anticipated that calibration would rely on extensive laboratory testing. Further, it is expected that the transfer functions calibrated with the new methods could be used in the AASHTO M-E framework.
The AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) and accompanying software (AASHTOWare© Pavement ME Design) represents a major improvement in structural pavement design based on decades of research and development. While the MEPDG has significant advantages over its predecessor (1993 AASHTO Design Guide) it is still limited by the empirical transfer functions which require verification, calibration and validation. The initial national calibration performed on Long-Term Pavement Performance (LTPP) sections represents relatively old sections that did not include more modern materials and technologies. Therefore, local calibration is recommended by AASHTO, even for conventional materials, and ultimately required for any new materials that have yet to be evaluated. Calibration typically involves laboratory testing and field study. Under NCHRP Projects 1-37A and 1-40, the MEPDG was “globally” calibrated across LTPP sites in North America. As documented in the AASHTO Guide for Local Calibration of the Mechanistic-Empirical Pavement Design Guide, the procedure followed an 11-step process that is effective, provided sufficient field-sections are available. The problem, of course, with new materials and technologies is that there is insufficient (if any) field data available which forces more reliance on laboratory-derived data. Laboratory-calibration of transfer functions has historically focused on bottom-up fatigue cracking through the bending beam fatigue test. This approach has been used extensively since the 1960s but it has been well-recognized that lab-to-field shift factors are required to make accurate cracking performance predictions. More recent efforts have examined other forms of testing to predict top-down and reflective cracking such as Energy Ratio, Texas Overlay Test, Semi-Circular Bend Test, and the Illinois Flexibility Index Test, to name a few. Some of these approaches were developed primarily as a screening tool for use in mix design. However, there is potential for using them as a predictor of performance for structural design purposes and could be considered in this study.
The primary objective of this research is to identify and develop methods for rapidly calibrating transfer functions for M-E design. The objectives would be achieved through five broad tasks: (1) Comprehensive literature review of existing calibration methodologies; (2) Identify or develop potential transfer function rapid calibration procedure(s); (3) Establish proof-of-concept using new procedure(s) in laboratory; (4) Validate new transfer function calibration procedure(s) with field data; and (5) Develop guidance document for new calibration procedure(s). Transfer function calibration is currently seen as a major challenge in M-E design implementation due to its cost and time requirements. The development of a more rapid approach could greatly enhance the utility and widespread use of M-E design, while also supporting new and innovative materials in flexible pavement cross-sections. The main product will be a guidance document for rapid calibration of M-E transfer functions and implementation would naturally flow through AASHTO in support of MEPDG adoption by state agencies.
Status: The project has been cancelled; no work will be pursued.