Structural Design of Pavements is an evolving process. Gradually, we have moved from purely empirical design relationships based largely on the results of observations at the AASHO Road Test to the current mechanistic-empirical process incorporated in FPS 21, where material stiffness is related to performance (loss of smoothness) through use of the surface curvature index (SCI, a deflection parameter).

Ultimately, the goal is to incorporate other intrinsic material characteristics, the environment, and their interaction under traffic loading, to the progression of specific forms of distress by using mathematical transfer functions. As a result, multiple performance relationships for any given structure subjected to a given regional environment and regional or site-specific axle loads are needed.

In 2001, the Flexible Pavement Design Task Force (FPDTF) studied structurally designed deep HMA pavement; a type of pavement typically associated with high assurance of long pavement life.

As a result of the 2001 study, the following guidelines were established:

Current guidelines have taken into consideration design and constructability issues experienced in the structures designed under the original guidelines. Recommended structural layer composition for facilities with a projected 20-yr. one direction cumulative loading of at least 30 million ESALs is clarified. When 30 million ESALs are exceeded, premium mixtures, such as Item 346, “Stone Matrix Asphalt (SMA),” and Item 344, “Superpave Mixtures,” should be used in lieu of conventional QC/QA dense-graded specifications (Item 341) to ensure adequate top-down crack resistance in the surface and rut resistance within lower HMA layers. For the fatigue resistance characteristic to be effective, all HMA layers must be fully bonded to allow the system to act as a composite mass. The SMA and Superpave mixes are specifically engineered to offer exceptional performance under heavy traffic loads. Dense-graded mixes under Item 341 may be used as an alternative to the stiff, rut-resistant HMA base layers with prior approval by the director, MNT, or designee.

Along with establishing an engineered foundation, using a dense, low air void HMA bottom layer (Ndes = 35 gyrations or 97.5% lab density) shall be considered when: