Recalibration of the Asphalt Layer Coefficient

Abstract

The Alabama Department of Transportation (ALDOT) currently uses the 1993 DARWin version of the 1986 American Association of State Highway and Transportation Officials (AASHTO) Guide for the Design of Pavement Structures when designing flexible pavements. The AASHTO design methodology is based on information obtained at the AASHO Road Test, which was performed from 1958 to 1960 near Ottowa, Illinois. This road test provided an empirical correlation between pavement thickness and traffic loadings. However, the results stemming from the road test are limited to the pavement materials utilized, applied traffic and the climate of Illinois. Using the results of the AASHO Road Test, a flexible pavement design equation was developed and introduced in the 1986 AASHTO Guide for Design of Pavement Structures that includes inputs of soil resilient modulus, traffic, structural capacity (structural number), reliability, variability, and ride quality (change in serviceability). The structural number is calculated using the layer thicknesses, material drainage properties and layer coefficients, which are used to express the relative strength contribution of each pavement layer to the overall pavement structure. In this study, these inputs were analyzed to determine the relative influence of each on the resulting hot mix asphalt (HMA) thickness. It was found that the HMA layer coefficient, resilient modulus and traffic inputs are by far the most influential parameters. Since the soil modulus and traffic are generally given parameters for a particular design, it was decided that the layer coefficient be recalibrated to provide the greatest potential savings in HMA thickness. Furthermore, the layer coefficient has not been updated to account for advancements made in construction methods, gradation requirements, and paving materials since the AASHTO Road Test, and therefore should be reanalyzed. The recalibration was performed using traffic and performance data collected for the structural sections of the 2003 and 2006 National Center for Asphalt Technology (NCAT) Test Track cycles. These data were used in conjunction with traffic equations developed from the AASHO Road Test as well as the AASHTO flexible pavement design equation to find both the calculated and predicted equivalent single axle loads (ESALs). Once these values were found for each section, a least squares regression was performed to determine new HMA layer coefficients. The resulting average layer coefficient was 0.54 for all sections, with a standard deviation of 0.08. Using this parameter instead of the AASHTO recommended coefficient of 0.44 results in an approximate HMA thickness savings of 18%. From these results, it is recommended that ALDOT adopt this value as their new layer coefficient for flexible pavement designs.