The AASHTO design guide's rigid pavement equation that is used for thickness design was originally developed in 1960 at the conclusion of the road test. This equation predicts the number of axle loads for a given slab thickness and loss in serviceability. During the last 30 years, the original equation has been extended to include several additional design factors and has been used by many highway agencies for rigid pavement design. Due to the limited inference space of the original road test equation and the subjective nature of the subsequent extensions, there is considerable interest in determining the adequacy of the equation. The availability of the nationwide long-term pavement performance data has finally made an overall evaluation possible. The evaluation included determining the adequacy of predicting the number of heavy axle loads required to cause a given loss of serviceability. The results indicate that the original 1960 equation generally overpredicts the number of 80-kN (18-kip) equivalent single axle loads for a given loss of serviceability. However, extensions to the original model improve predictions considerably. These results were determined at the 50-percentile (mean) level. At a higher level of reliability such as 95 percent, the 1986 AASHTO model provides a conservative design for a majority of the pavement sections. However, several deficiencies that need to be improved still remain.
Technical note: On comparing greenhouse gas emission metrics
