Assessment of the 1993 AASHTO structural number as a tool for performance evaluation of asphalt pavements using falling weight deflectometer data from LTPP

Since its first development in 1960, the AASHTO design guide for flexible pavements design has been widely used by many agencies for the design of a pavement structure. The guide was substantially modified in 1986 to incorporate nationwide requirements and climatic conditions. However, with significant changes in asphalt technology such as the addition of recycled asphalt, modifiers, and change in traffic and climatic condition, the reliability of the empirical equation to predict the performance of flexible pavements has been put into question. However, even with such concerns, it still remains the most prevalent design method used for the design of a pavement structure by many agencies. One of the most significant breakthroughs of the 1993 design guide was the introduction of the concept of structural number (SN). This study investigates the capability of 1993 AASHTO SN to monitor the changes in pavement performance as it relates to roughness (IRI), rutting, and fatigue cracking using measured field data of four sections from the long-term pavement performance (LTPP) database. The backcalculated moduli values from FWD tests corrected at mid-depth reference temperature are used to determine the layer coefficients. Then, the structural strength of the overall pavement (i.e. SN) for the course of a pavement life is compared to the change in pavement performance (i.e. IRI, rutting, and fatigue cracking). The results showed a significant relationship between the change in SN and a change in pavement performance as it relates to rutting and roughness index where with a decrease in SN, the IRI and rut depth increases. The SN assessment also showed that the LTPP database can be used as a valuable source to calibrate the 1993 AASHTO SN equation based on measured field data from LTPP.