Before orange construction barrels dot pot-holed streets or highways, a vital part of planning a pavement project is determining how thick the next layer of asphalt needs to be, taking into consideration the layers that already lie beneath the surface.
A team of engineers at The University of Toledo created a new procedure and design software to more accurately estimate the structural capacity of existing pavement that could save the Ohio Department of Transportation millions of dollars on road improvement projects and be adopted by states across the country.The Transportation Research Board, a unit of the National Academies of Sciences, Engineering and Medicine, selected UT’s project for developing a revised pavement overlay thickness design procedure as one of 32 High-Value Research projects nationwide to be highlighted at its annual meeting Jan. 13-17 in Washington, D.C. The meeting attracts 13,000 transportation professionals from around the world.
The new method is specifically designed for composite pavement — concrete pavement already topped with a thick layer of asphalt — which accounts for the majority of ODOT’s four-lane and interstate highways. Previously, ODOT used a design method that was originally developed for rigid, concrete pavements that tended to produce designs often deemed too thick and wasteful for today’s roadways, as pavement becomes thicker with each additional overlay.
For an update, ODOT turned to the engineer who crafted the original design 25 years ago: Dr. Eddie Chou, UT professor of civil and environmental engineering, and director of the Transportation Systems Research Lab.
“The previous procedure did not work well with thick composite pavement. With this particular type of road, it tended to underestimate the existing structure’s worth,” said Chou, who worked on the project with Dr. Liango Hu, UT associate professor of civil and environmental engineering. “Many existing pavement sections we examined now require several inches thinner than previously demanded to withstand traffic for an additional 20 to 25 years.”
The UT research team adopted a three-layer model for back-calculating the properties of the soil subgrade and pavement layers, instead of the old two-layer model that combined cement and asphalt into one.
Chou said the new design reduces on average about five inches of overlay thickness, and the reduction of each additional inch of overlay can save approximately $120,000 per mile.
“In addition to being more environmentally friendly, the potential cost savings can be substantial considering each year ODOT rehabilitates several hundred miles of existing composite pavements by laying additional asphalt on top,” Chou said.
The revised design procedure was implemented into design software that adopts the improved back-calculation model. The software also offers an optional feature that takes into consideration the effects of temperature.
The Ohio Department of Transportation and Federal Highway Administration sponsored the UT research.
“This UT research developed a revised rehabilitation design procedure for composite pavement structures in Ohio and more accurately characterizes pavement layers for this analysis,” Patrick Bierl, pavement design engineer and pavement rating coordinator in ODOT’s Office of Pavement Engineering, said. “This revised procedure allows ODOT to continue to produce efficient and cost-effective rehabilitation designs to manage our composite pavements.”