The AASHTO pavement design equations have some variables that are common to both rigid and flexible pavements, including serviceability, traffic loading, reliability, overall standard deviation, and roadbed soil resilient modulus. These parameters are discussed in the following articles. Subsequently, the design procedure is presented for rigid pavements in Art. 3.6 and for flexible pavements in Art. 3.7.
The AASHTO design equations are developed around the concept of serviceability, which serves as the pavement performance parameter by which a pavement’s condition is valued. Present serviceability is defined as the momentary ability of a pavement to serve traffic. The present serviceability rating (PSR) was developed to measure serviceability. PSR is a rating of pavement ride based on a scale of 0, for impassible, to 5, for perfect. For the development of the original AASHO equation, individuals (the raters) would ride the pavements and assign a PSR value. To avoid riding and rating every pavement to determine serviceability, a relationship is usually developed between PSR and measurable pavement attributes. The value determined by this relationship is called the present serviceability index (PSI). At the AASHO Road Test, the PSI was derived to be related to slope variance, cracking, and patching for concrete pavements, and to slope variance, rutting, cracking, and patching for asphalt pavements. The relationship between pavement thickness and serviceability index is defined by the AASHTO pavement design equations.
