Improvements to IDOT's mechanistic hot-mix asphalt pavement design saves dollars, the environment

Continual improvement has been the theme for the Illinois Department of Transportation (IDOT) and the University of Illinois at Urbana-Champaign (UIUC) when it comes to hot-mix asphalt (HMA) pavement and mechanistic pavement design.

Mechanistic–empirical (M–E) design uses material properties, fatigue theory, and engineering mechanics to determine pavement response. Through variables such as traffic, soils support, climate, and selection of performance-graded (PG) asphalts, pavement thickness can be determined. Since the 1980s, when IDOT adopted M–E design, IDOT and UIUC have monitored and evaluated the resulting pavements through a series of research projects culminating with Illinois Center for Transportation (ICT) project R27-60, “Mechanistic–Empirical Design, Implementation, and Monitoring for Flexible Pavements.” Based on that research, IDOT implemented additional improvements to the design and construction of full-depth HMA.

HMA Pavement Thickness Reduction Based upon Former HMA Thickness Design

Recently, ICT had an opportunity to look back and determine the impacts of implementing those changes. The review included examination of the impacts of pavement design updates issued between April 2011 and March 2013 via IDOT’s Bureau of Design and Environment Manual (Chapter 54, Pavement Design). The basis for those changes was adoption of a new HMA fatigue algorithm and adoption of limiting strain criteria, which resulted in reduced pavement thickness for full-depth HMA and HMA overlays of rubblized pavement. To evaluate the impacts, ICT researchers compared the published pavement designs using the revised procedure with the former design to determine the reduced pavement thickness.

The results were applied to typical pavement designs constructed year to year. The new design reduced pavement thickness approximately 1.5 to 2.5 inches for the average pavement design statewide.  The average quantity of HMA pavement used annually has been just over 590,000 square yards, which translates to approximately 63,180 tons of HMA binder saved as a result of the changes. This material savings is valued at $5.1 million annually.

Environmental Benefits of Implementation

Using life-cycle assessment, researchers analyzed the materials used, along with the construction, rehabilitation, operation, and eventually the salvage phases. When a design modification results in use of less material but achieves equivalent performance, the environmental savings are directly proportional to the material reduction.

The typical Illinois mix has approximately 25% asphalt binder replacement from reclaimed asphalt pavement and recycled asphalt shingles. It is estimated that approximately 80 pounds of CO₂ are emitted for every ton of HMA produced. Applied to each ton of HMA eliminated, the design implementation is estimated to spare the environment approximately 2,527 tons of CO₂ emissions annually—equivalent to the emissions from nearly 6 million vehicle miles traveled annually by the average U.S. passenger car. The reduced carbon emissions savings in CO₂ is valued at approximately $96,000 annually.

The combined material and environmental savings associated with the research total $5.2 million annually.