ICT Research Provides Improved Flexible Pavement Design Procedures for IDOT

Illinois was one of the first states to institute a mechanistic-empirical
(M-E) approach to pavement thickness design. It was adopted by the Illinois Department of Transportation (IDOT) in 1989, and the department, with the assistance of the Illinois Center for Transportation (ICT), continues to support a variety of M-E design, analysis, implementation, and monitoring techniques for flexible pavements.

Conventional pavement design approaches have traditionally been empirical, based primarily on observations from road tests. They required designers to be conservative, which could lead to overdesign of pavement structures—and possibly result in inefficient use of resources. The mechanistic-empirical approach to flexible pavement design, however, uses models to predict pavement responses to loading under various conditions based on inputs such as materials characterization, load characterization, geography, and climate. As its name suggests, M-E pavement design also encompasses aspects of the empirical approach by correlating the physical causes of stresses with observed pavement performance.

Recently, as part of IDOT’s continuing use and analysis of M-E pavement design, ICT researcher Marshall Thompson assisted in the review and revision of the pavement design section of IDOT’s Bureau of Local Roads and Streets Manual, provided inputs for modification of IDOT’s asphalt overlay design procedures, monitored and evaluated new and developing technologies for M-E pavement analysis and design, and evaluated the impact on M-E design and pavement performance of using recycled materials. These efforts and findings are summarized in an ICT report, “Mechanistic-Empirical Implementation of Monitoring for Flexible Pavements,” (R27-060). Thompson is professor emeritus in the Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign.

As part of the project, Thompson considered various approaches for recycling in-place pavements. The researchers cooperated with IDOT and industry in developing policies and specifications for cold in-place recycling with asphalt products (CIRWAP). Thompson continues to evaluate and review CIRWAP research and development activities and to informally monitor CIRWAP project construction and performance.

“In-place recycling of existing pavement sections with cement has become a common process in Illinois. IDOT’s AASHTO structural number thickness design procedure and layer coefficients for high-strength stabilized base (HSSB) materials were reviewed,” says Thompson. “We considered various HSSB thickness design procedures and developed some alternative approaches that more realistically predict pavement performance for several Jasper and Edgar County projects.”

The project was conducted under the direction of a Technical Review Panel led by Charles Wienrank, Pavement Design Engineer in IDOT’s Bureau of Materials and Physical Research. “IDOT has used the findings of Professor Thompson’s research to update and refine their mechanistic-empirical design procedures for flexible pavements. An updated design procedure was issued that utilizes more accurate and comprehensive climate data, accounts for improvements in HMA materials and mixes, and uses a new, less-conservative fatigue algorithm,” says Wienrank.

He adds, “For the first time, the design procedure also includes a limiting strain criterion design that incorporates the concept of a fatigue endurance limit into the full-depth HMA pavement and HMA overlay of rubblized concrete pavement design procedures to determine a maximum pavement thickness. IDOT anticipates significant cost savings will be realized through the use of the improved design procedures.”

A project using cold in-place recycling with asphalt.

A project using cold in-place recycling with asphalt.