...monitor instruments buried in the pavement through an ODOT-sponsored research study. The department says it saved costs by not asking for a warranty.

One subcontractor will perform all paving. Eastbound lanes will have 10 in. of unreinforced portland cement concrete on top of 3 in. of hot-mix asphalt, over 4 in. of crushed aggregate. The concrete sections will be doweled for load transfer across joints. Outside lanes will be widened to 14 in. into the shoulder but striped at the usual 12-in. mark. This helps counteract loading of heavy truck traffic. “We think the design will be successful if we don’t have to touch it for 25 to 30 years,” says Norris.

On the westbound side, the contractor will lay 16 in. of asphalt over a 6-in. aggregate base. The mix is computer-calibrated to account for regional materials, temperatures, loading and other variables. It is designed to last indefinitely with no replacement except for occasional surface milling and topcoating. “The idea is that if we can keep the fatigue-stress level at the bottom of the pavement below the endurance limit of the whole material, it will never fail,” Frecker says.

Both mixes use some “mechanistic” rather than strictly “empirical” design procedures, O’Leary says. Other states looking for accuracy and cost-savings are in early stages of implementing mechanistic analysis, which requires data and computing power to run models. In California, mechanistic design is “part of our strategic planning,” says Tom Hoover, California Dept. of Transportation chief of materials and infrastructure research. “This is the way we should have been designing pavements for a long time,” says David Lippert, Illinois DOT engineer of physical research.

HARD SCIENCE Simulator helps University of Illinois researchers study long-life pavements. (Photo by Tudor Hampton for ENR)

IDOT is sponsoring several studies of its own. Scientists at the University of Illinois at Urbana-Champaign are evaluating new designs for rigid and flexible pavements. Two years ago, they built an 85-ft-long accelerator costing $1.2 million. It can simulate up to 14,000 equivalent single-axle loads per day, says Samuel H. Carpenter, a professor studying asphalt at the university. “Field studies have shown that we haven’t been getting structural failure in these thicker pavements,” he says.

Transportation Research Board’s National Cooperative Highway and Research Program is writing its first mechanistic design guide for review by the American Association of State Highway and Transportation Officials. The new guide “is getting a lot of interest,” Lippert says. A preliminary copy is scheduled for release next month.