The accelerated testing done at the AASHO Road Test (2-year period) can be extended to a longer design period.Material characterizations may be applied to other surfaces, bases, and subbases by assigning appropriate layer coefficients.Loading can be applied to mixed traffic by use of ESALs.The characterization of subgrade support may be extended to other subgrade soils by an abstract soil support scale.In order to apply the equations developed as a result of the AASHO Road Test, some basic assumptions are needed: The loads used to develop the equations were operating vehicles with identical axle loads and configurations, as opposed to mixed traffic.Therefore, environmental factors were difficult if not impossible to extrapolate out to a longer period. The equations are based on an accelerated two-year testing period rather than a longer, more typical 20+ year pavement life.The equations were developed based on the environment at the AASHO Road Test only.The equations were developed based on the specific pavement materials and roadbed soil present at the AASHO Road Test.Because they were developed for the specific conditions of the AASHO Road Test, these equations have some significant limitations: The following subsections discuss:įrom the AASHO Road Test, equations were developed which related loss in serviceability, traffic, and pavement thickness. The rest of this section will discuss the specific assumptions, inputs and outputs associated with the 1993 AASHTO Guide flexible pavement empirical design equation. This equation is not the only empirical equation available but it does give a good sense of what an empirical equation looks like, what factors it considers and how empirical observations are incorporated into an empirical equation. Structural Number (an index that is indicative of the total pavement thickness required)Ī 1D 1 + a 2D 2m 2 + a 3D 3m 3+…a i = ithlayer coefficientD i = i thlayer thickness (inches)m i = i th layer drainage coefficientĭifference between the initial design serviceability index, po, and the design terminal serviceability index, pt Predicted number of 80 kN (18,000 lb.) ESALsĬombined standard error of the traffic prediction and performance prediction

Flexible pavement design aashto example