AASHTO 1993 Plus: an alternative procedure for the calculation of structural asphalt layer coefficients

Many road agencies worldwide still use the empirical AASHTO 1993 pavement design method. Since the release of the AASHTO 1993 design guide, many aspects of pavement design have changed, including asphalt material properties, traffic, environmental factors, etc. However, many agencies use a single layer coefficient for asphalt mixtures and pavements are currently designed with inaccurate values, as proved by many. This paper presents a four-step procedure (called AASHTO 93 Plus method) to determine structural layer coefficients of asphalt mixtures. The AASHTO 93 Plus method considers viscoelasticity and temperature-dependency of asphalt mixtures to estimate the layer coefficient. Five-hundred-eighteen (518) flexible pavements from USA and Canada roads and 1599 HMAs from the Long-Term Pavement Performance (LTPP) database were analyzed. The results showed that the new material-specific layer coefficients were well above the typical value of 0.44. The total number of 3,726 IRI data records of these 518 pavements were converted to PSI and then compared to those obtained from the traditional AASHTO 93 and AASHTO 93 Plus methods for validation. Results confirmed that the conventional AASHTO 93 method predicts faster deterioration of pavement conditions over time, while the results of AASHTO 93 Plus method better aligned with the field data.