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Humboldt Field Research Institute

June, 1996

Tire Chip Research Focuses on Fire and Ice

The old, tread-worn tires we leave at gas stations and tire shops often wind up in landfills, some of legendary proportions, but a University of Maine professor is finding ways of putting them back to work on, or under, America's roads. In cooperation with the Maine Department of Transportation (DOT), Dana Humphrey, associate professor of civil and environmental engineering, studies uses for chipped tires in the construction of roads and retaining walls.

Humphrey's research is already paying off. Tire chips are being used in the construction of two major new bridges in southern Maine, and sections of three Maine roads built over tire chips have shown significantly less frost heave than conventionally built sections. Humphrey has also become a troubleshooter for the federal government on three tire-chip projects gone wrong in other states.

"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."

He adds, "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."

Nathan Whetten, a senior geo-technical engineer for the consulting firm of Haley & Aldrich in Scarborough, is using the results of Humphrey's work in designs for two new bridges on a highway bypass between Brunswick and Topsham. One bridge spans a railroad line in Topsham and the other crosses the Androscoggin River between the two towns.

"I contacted Dana when we knew that using tire chips was an economical, lightweight option for this project. It's safe to say that without his work, we would have moved away from the idea. He's probably the nation's leading tire- chip researcher," Whetten says.

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, a tire industry organization in Washington D.C., Humphrey has used the results of these studies to write proposed national standards for tire-chip use. The proposal is currently working through the bureaucracy at the American Society for Testing and Materials.

"There are about 70 projects around the United States where tire chips have been used for highway applications," says Humphrey. "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."

Fire and water

Humphrey's work has focused on two problems, one that literally flared up last winter and another that results from the use of chips near groundwater. "What we're doing is helping to solve one of our environmental problems, helping to get rid of some of our waste tires. We don't want to create yet another one," he says.

In the first case, three roads built over tire chips in Colorado and Washington State began literally to burn last year. Pavement crumbled as steam rose from cracks in the asphalt. The Federal Highway Administration hired Humphrey to investigate.

"What was different about these projects was that they had very good access to oxygen, lots of steel belt and lots of fine crumb rubber, at least in some locations," he says. "They were also relatively thick fills. In order for the heating reaction to have any significance, you have to have enough tire chips to retain the heat. Finally, they all put topsoil directly on top of the tire chips."

Humphrey is convinced that all these factors contributed to the fires. Steel, for example, generates heat as it rusts.

"We don't know which factor is more important, but our strategy is to eliminate all of these conditions," he says. "We're being very cautious at this point."

To study the effects of placing tire chips below the water table, Humphrey and Lisa Downs, a graduate student, buried several tons of chips in different types of soil near the UMaine campus. They analyzed groundwater quality for heavy metals and petroleum-based compounds.

The results have shown that water moving through the chips does not accumulate heavy metals at levels that would violate health-based drinking water standards. Two metals, iron and manganese, exceeded the secondary standards for odor and taste.

"These are common constituents in Maine's groundwater, and the Department of Environmental Protection told us they are not concerned about the levels we found," Humphrey says.

The water-quality analyses also turned up low levels of a few petroleum-based compounds. The source, however, is not clear.

"We need to continue to monitor because when you start by digging a hole with a backhoe that's full of hydraulic fluid, you get some initial problems right there," says Humphrey. "Every time you take a scoop of soil, another bit of hydraulic oil goes down into the groundwater. It's not clear if what we're measuring is from the backhoe or the tire chips."

At present, Humphrey recommends keeping tire chips above the groundwater table.

Bridge projects

In projects already under way, Humphrey and another graduate student, Tricia Cosgrove, will be monitoring the use of tire chips placed against the walls of a concrete railroad bridge and used as fill under an approach road to a bridge over the Androscoggin River.

"We're placing a 3-foot wide vertical strip of tire chips against the side walls of the railroad bridge," says Humphrey. "We're measuring the pressure of the chips on the wall and how much the chip layer actually compresses.

He adds, "We're also measuring the temperature of the tire chips. That's an addition because of the problems they've had in Washington State, but the tire chips are only three feet thick, and that's not enough to retain heat. We just want to make sure."

Tire chips and temperature sensors will also be placed under an approach road in an area with relatively weak soil. The chips weigh only half as much as gravel, which could have caused soil under the road to give way. Altogether, the bridge project will use about a half million tires.

Nathan Whetten of Haley and Aldrich points out that using tire chips in construction projects could make a serious dent in the growing piles of used tires around the country.

"It gets down to a question of quality control," he says. "If the processors, the engineers and the contractors follow Professor Humphrey's guidelines, we believe we can avoid problems like the ones they had out West. That's a hurdle we need to overcome, and the new bridges should settle these questions by showing how to use tire chips correctly."

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