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Humboldt Field Research Institute

September, 17, 1997

Maine's Glut of Used Tires Used to Reduce Construction Costs

Every two hours, day and night, engineers at the University of Maine in Orono get a message via phone line from the new railroad bridge in Brunswick. The call is placed by an automatic device which constantly monitors a layer of tire chips placed against the walls of a bridge spanning the Maine Central Railroad tracks.

Dana Humphrey, winner of a 1997 Governor's Special Teamwork Award and professor of civil and environmental engineering, and Tricia Cosgrove, a graduate student from Akron, Ohio now living in Old Town, are pleased with the information they have received from these calls.

"So far, our data show that the chips have reduced the pressure on the wall and that they aren't getting hot," says Humphrey. "They're working as well as we had thought they would."

Those conclusions translate into lower construction costs and a badly needed new use for Maine's glut of used tires.

Humphrey studies uses for chipped tires in the construction of roads and retaining walls and has worked as a consultant for both the Federal Highway Administration and state highway departments. Closer to home, he has advised the Maine Department of Transportation, the Maine Turnpike Authority and the Tri-Community Landfill in Aroostook County.

"Tire chips have properties that road builders need," says Humphrey. "They're lightweight, so they're good for building over soft compressible ground or weak soil. If you use them as backfill behind retaining walls, they reduce the pressure on the wall, so you can have a thinner, cheaper wall."

"They're also a good insulator, eight times better than soil. In cold climates like Maine, they limit the depth of frost penetration and reduce problems from frost heave. They can even limit heat loss from the basements of homes."

Maine has no lack of used tires. A survey by the Scrap Tire Management Council recently concluded that Maine has more used tires per capita than any other state. With 34 old tires per person, Maine just edges out Rhode Island which has about 31. Ohio is a distant third at nine tires per person.

Construction projects have begun to make a small dent in the state's tire piles. More than half a million tires were used in the Brunswick railroad bridge and under an approach road to the new Androscoggin River bridge between Topsham and Brunswick. Most recently, almost three times as many, 1.4 million tires, were used in a project on the Maine Turnpike at the Congress Street exit in Portland. In Aroostook County, 300,000 tires are being chipped and used as a drainage layer in the new Tri-Community Landfill near Fort Fairfield.

"In Portland, we saved about $300,000 by using tire chips over the next cheapest lightweight fill alternative," says Humphrey. "If we had chipped the same tires to smaller sizes needed for burning in power plants, it would have cost another $300,000, so altogether we saved the state about $600,000."

Humphrey's enthusiasm for this plentiful source of building material stems from research he and his students have done on the properties of the chips themselves and on their use in actual practice. With funding from the Maine DOT and the New England Transportation Consortium, he has run laboratory tests to determine how strong they are, how much they compress under pressure, how quickly water passes through them and how they affect water quality.

In cooperation with the Scrap Tire Management Council in Washington DC, a tire industry organization, Humphrey has used the results of these studies to write proposed national standards for tire chip use. The proposal is currently working its way through the bureaucracy at the American Society for Testing and Materials.

"There are about 80 projects around the United States where tire chips have been used for highway applications," he says. "The majority of those are in Minnesota. Most of the projects are small. They unload them out of a dump truck and spread them out, but they work. They've been successful in other states too, including Wyoming, Virginia and Maine."

Humphrey's recent work has focused on two problems, the potential for tire chips to pollute groundwater and the possibility that they can heat up and burn. His results show that there is no significant impact from chips placed above the water table. Work on chips below the water table is ongoing, and for the present, he does not recommend placing tire chips below the water table.

The fire problem can be addressed, he says, by good quality control in the chipping process and installation. Tire chips should be free of steel belt, dirt and the tiny pieces of rubber known as "crumb rubber." On the ground, they should also be capped by a layer of clay to keep air out of the pile. These recommendations have been incorporated into national guidelines approved by the Federal Highway Administration.

Humphrey can be contacted at 207-581-2176 or via the Internet at Dana_Humphrey@umit.maine.edu.

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