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AASHTO Mechanistic/Empirical Design Procedures
Dan A. Dawood, P.E., Chief, Pavement Design and Analysis Section, Bureau of Maintenance & Operations, PENNDOT

In the 40 years since the empirical based pavement design procedures of AASHO Road Test were developed, very little change has come about on how we design pavement structures. Traffic volumes and pavement design requirements have outgrown the empirical procedures. Furthermore, the AASHO Road Test was based on one environmental region, geological formation and one specific vehicle type to formulate a system of pavement design for use nationally.

Pavements today are being designed well off the charts and graphs provided by the AASHO Road Test. Traffic volumes and truck axle weights are so dramatically different today that it becomes quite questionable to many if the designs seen today on major interstates and expressways are realistic. It is for this and many other reasons such as the wealth of accumulated knowledge since the Road Test, that the AASHTO Joint task Force on Pavements initiated the effort to develop an improved Guide by 2002.

National Cooperative Highway Research Project (NCHRP) 1-37A was the initial step to develop the 2002 Guide. NCHRP awarded the project to ERES Consultants, a division of Applied Research Associates, Inc., to develop the new guide using existing models and practical and realistic design procedures for the design of new and rehabilitated pavement structures. The consultant was encouraged to use large national databases, such as the Long Term Pavement Performance (LTPP) database, to validate and calibrate their models and designs.

The new guide will use a hierarchical approach for determining design inputs, which means that the level of input should be consistent with importance of the project. There will be three levels of input, with Level One being the highest quality data. Of the three levels, most state highway agencies (SHA) will be using Level Two inputs, which are more readily obtainable than Level One. Level One requires specific traffic data such as weigh-in-motion data for the specific site, resilient modulus for subgrade testing, advance materials and Dynamic Modulus Testing for asphalt materials. Level Two allows the use of regional factors and available test procedures with correlation equations. This is not to say that Level Two is easy, since SHA’s will still be required to do a considerable amount of testing in order to have good and reliable data to develop state wide or regional factors. It should also be noted that the analysis process will remain consistent and will be independent of the level of input.

Although the NCHRP 1-37A project was scheduled to be completed in 2002, unforeseen problems have delayed the completion of the project to later this year. The final product of the NCHRP 1-37A effort will be a “Paper Guide” and rudimentary software, which will allow users to use the new guide. The new design procedure will be significantly more complex than the current AASHTO 93 Guide and therefore cannot be done manually. The software being provided at the completion of the NCHRP 1-37A project will be sufficient to perform most pavement designs for new and rehabilitated pavements. However, it will only provide users with models calibrated for regional areas from national data.

It is anticipated that SHA’s will receive the NCHRP 1-37A product late this year or early next year after the AASHTO Joint Task Force on Pavements has approved it. Furthermore, to become an AASHTO product the NCHRP 1-37A product must undergo a balloting procedure with all the states and not just the states represented on the Joint Task Force on Pavements.

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