AASHTO pavement design methodology

The current AASHTO pavement design methodology (AASHTO 1993) is based on the methodology developed in 1972 (AASHTO 1972), as an output of the AASHTO road test (HRB 1962), which, in turn, was revised in 1981 and 1986. 

In contrast to all other design methodologies for flexible pavements, the AASHTO methodology uses a pavement performance criterion, which, apart from cracking and subgrade deformation, includes other parameters affecting the performance of the pavement. This criterion is called serviceability, expressed as present serviceability index (PSI). 

the pavement is designed in such a way that its deterioration, manifested as cracking, deformation, surface irregularities, potholes and so on, until the end of its service life, results in the pavement offering a minimum tolerable level of serviceability. 

The above constitutes a totally different design concept, somewhat more practical, provided the user is not interested in whether the pavement has been cracked or deformed, or whether it has one or two patches. He is interested in whether or not the pavement provides some tolerable serviceability level. 

The subjectivity included in the definition of tolerable serviceability level was overridden by defining various values for the PSI. 

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In a pavement design methodology such as the AASHTO methodology (AASHTO 1993), the elastic modulus, E (ASTM C 469 2010), or alternatively, the unconfined compressive strength (7 days) (ASTM D 1633 2007) of the CTA base, needs to be determined. With either value, the structural coefficient (<72) is derived and the thickness of the corresponding layer as well as of all layers of a flexible pavement is determined (see Section 13.4.4.3). For a rigid pavement design, the elastic modulus of the sub-base is used (see Section 14.11.1). 

In Canada, there is no single pavement design methodology. The provinces and the federal government use various methodologies, primarily the AASHTO (1993) methodology (C-SHRP 2002). 

The AASHTO rigid pavement design methodology (AASHTO 1993) was developed at the same time as the AASHTO flexible pavements design methodology and is described analytically in the same design guide (AASHTO 1993). 

With respect to surface cracking causing pavement uplift attributed to frost or swelling, the AASHTO methodology (AASHTO 1993) considers it indirectly in the determination and usage of loss of serviceability index in the design procedure. 

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