Pavement Design

Since the completion of the AASHO Road Test Project, the Illinois Department of Transportation has developed many new highway materials and procedures to improve pavement construction. This effort has resulted in improved material usage, construction procedures, and pavement designs which, although common practice today, were neither envisioned nor included in design procedures at the time of the AASHO Road Test. Therefore, to supplement the AASHO Road Test Project and better address modern pavement design, mechanistically based structural pavement design procedures were developed using structural mechanical analysis, computer modeling, and actual performance and response of existing pavement sections.

Mechanistic pavement design procedures are applicable to  full-depth HMA designs with HMA surface and binder; and local agency composite and conventional flexible designs.  The procedures use the actual stresses, strains, and deflections experienced by the pavement to determine its expected fatigue life. Factors that are considered in mechanistic designs include:

  • design HMA strain,
  • design pavement HMA mixture temperature,
  • design HMA mixture modulus (EHMA),
  • subgrade support ratio (SSR),
  • design reliability of 95%.

An Excel spreadsheet which will perform the full-depth mechanistic pavement design calculations is available from the IDOT website (click link on left side of this page). The Mechanistic Design procedures are contained in the BDE Manual Chapter 54 for state projects and the BLRS Policy Manual Chapter 44 for local public agency projects.