An Idea Where the Rubber <i> Is</i> the Road : Old tires are turning into better roads in Maine. With fewer potholes and less waste, the test may make tracks for other areas and climates soon.

A University of Maine civil engineer may have the answer for a nation mired in old tires: rubber roads.

And maybe rubber retaining walls as well.

The old tires piling up all over the country are eyesores. In wet climates they trap water and become breeding grounds for mosquitoes. And when mounds of tires occasionally catch fire, they spew toxic, long-burning smoke and create a difficult and expensive battle for firefighters.

But the notion of tires as an intractable solid waste disposal problem could change dramatically if Dana Humphrey’s experiments with tire chips as road insulators work as well on asphalt-topped highways as they did on gravel in Maine this past snowy winter.

On a rural road in the hamlet of Richmond, frost penetration was reduced about one-half, and the “rubber” part of Dingley Road held up much better than all-gravel sections that became virtually impassable once mud season arrived with the spring thaw.

So roads built partly of tire chips apparently could mean fewer potholes and frost buckling around these parts. Maine alone has more than 4,600 miles of gravel roads, and its residents discard an estimated 1.2 million tires a year.

“There are . . . pastures filled with tires that just sit there,” said Bob Wieluns, vice president of Pine State Recycling in Nobleboro, which donated the tire chips for the project.

In the winter-long experiment, Humphrey used 20,000 tires chopped into 2- or 3-inch chunks--steel belts and all--as a 6-inch- to 12-inch-thick underlay on a 600-foot stretch of road in the hamlet of Richmond. Gravel was then applied in layers up to two feet deep. At various depths in the roadbed, Robert A. Eaton of the U.S. Army Corps of Engineers Cold Regions Research & Engineering Laboratory placed more than 100 temperature sensors, connected by telephone lines to computers in Hanover, N.H., to study the insulating effect.

But, Humphrey says, tire chips aren’t only useful in cold climates.

This month, with two graduate students, he began using tire chips to build a 15-foot-high retaining wall on the university campus, a method he said could be particularly useful in Southern California to help prevent landslides and buckling on hillside roads.

Tire chips have three qualities that make them useful for these purposes, Humphrey said. They’re lightweight (gravel weighs 125 pounds per cubic foot; the chips weigh 40). They are good insulators because rubber doesn’t conduct heat as well as soil. And they drain well.

Besides that, they’re available. “Whatever’s in the pile, we take” Humphrey said.

Finely ground rubber is already being used in the asphalt on highways, but that process appears to use fewer tires at much greater expense. In 1991, at the urging of “crumb rubber” lobbyists, Congress passed a highway bill amendment requiring mixing ground tires with asphalt on federal road projects.

Humphrey has no problem with that concept but points out that in one mile of an interstate resurfacing program in Maine, only 6,000 tires were used--at a cost of more than $20 a tire.

The state Department of Transportation partially funds Humphrey’s research. Maine’s policy-makers were forced to push recycling after environmental court action caused the state government to be named receiver for two huge stashes of tires.

Private recyclers in the state say revenue now is largely in the disposal side of the operation. Those who have old tires must pay the licensed recyclers to get rid of them.

The product end of the business is in its infancy, but there are some customers. The state has two waste-to-energy plants that burn chips, and other tires provide fuel for a paper mill. But the paper company has to use magnets to remove as much steel as possible, which greatly increases costs, and the chips themselves must be cut into one-inch pieces.

In August, another “rubber road”--this one topped with asphalt--will be built on a section of highway in North Yarmouth. Tests have shown that after the chips are compressed with standard road-building equipment, they support heavy trucks quite well.

Humphrey will present his findings to the International Symposium on Frost in Geotechnical Engineering in Anchorage this summer. He will address a Federal Highway Administration conference in Denver in October.