Recalibration of the Asphalt Layer Coefficient

While there is an increasing interest in mechanistic-empirical flexible pavement design, this methodology will likely not be implemented by the majority of transportation agencies for several more years. Therefore, there is a need for optimization of the current design methodology, which for many agencies, is the 1993 AASHTO design guide. This approach contains a flexible pavement design equation with inputs of soil modulus, traffic, structural number, reliability, variability, and change in serviceability. A sensitivity analysis conducted on the inputs to this equation showed that the HMA layer coefficient $\left(a_1\right)$ is the most influential on the resulting HMA thickness. A recalibration of $a_1$ was performed using data from the National Center for Asphalt Technology Test Track. A least-squares regression was performed to minimize the error between predicted and actual equivalent single axle loadings applied to the test sections. The resulting average $a_1$ was 0.54 with a standard deviation of 0.08. Using 0.54 instead of the AASHTO recommended coefficient (0.44) results in a HMA thickness savings of approximately 18%.