The National Center for Asphalt Technology (NCAT) recently released guidance on recalibrating the structural asphalt layer coefficient in the AASHTO 1993 Design Guide. The AASHTO 93 design methods are based on empirical data gathered in road tests from 1958 to 1960. Although newer mechanistic-empirical design methods have been developed, 78 percent of U.S. states predominately use empirical design methods. However, it is commonly recognized that the AASHTO 93 design method tends to result in an overdesign of pavement thicknesses for high traffic levels, which can add unnecessarily to the cost of a pavement.
With its new guidance document, NCAT presents a variety of methods to update empirically based design methods to better reflect modern pavement performance.
“Among all design variables, the asphalt structural coefficient has the strongest correlation to pavement thickness,” stated Dr. David H. Timm, P.E., principal investigator for the project. “Modern advances in pavement materials and construction techniques allow flexible pavements to carry more traffic with less thickness, which leads to the need to recalibrate the structural number.”
The report, “Recalibration Procedures for the Structural Asphalt Layer Coefficient in the 1993 AASHTO Pavement Design Guide” (NCAT Report 14-08), outlines three general methods for localizing and calibrating the structural number for pavement designs using either deflection data, historical performance data, or through matching to mechanistic-empirical design thicknesses. The report can be downloaded from www.ncat.us/info-pubs/technical-reports.html.
PaveXpress, a web-based pavement scoping tool that simplifies the design of flexible and rigid pavements, is also based on AASHTO 93. By default, the tool uses a layer coefficient (a) of 0.44; however, the coefficient can be changed in Step 4 of the design of a flexible pavement to match a recalibrated layer coefficient.
As the NCAT report notes, Alabama and Washington state departments of transportation have acted upon local studies and recalibrated their default layer coefficient to 0.54 and 0.50, respectively. These recalibrations better reflect actual performance and have led to thinner, less costly pavements that continue to perform well.
PaveXpress was developed by Pavia Systems, a leader in the development of software and technology for the transportation infrastructure sector, with funding from the National Asphalt Pavement Association and the State Asphalt Pavement Associations. Planned future updates to PaveXpress will add modules for the design of overlays, as well as tools for mechanistic-empirical pavement designs.