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October / November 2001

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On the Road Again

Photo by Joe Devenney

On the Road Again
UMaine researcher finds valuable uses for old tires

About the Photo: "Maine had more stockpiled tires per person than any other state. Today, with the Maine Department of Transportation, Maine Turnpike Authority, Maine Department of Environmental Protection, and UMaine working together, the state is headed toward cleaning up all its stockpiles." — Dana Humphrey

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Dana Humphrey likes it when the chips are down. It means that one more stockpile of old tires is no longer an eyesore or taking up space in a landfill.

Humphrey is the nation's leading expert on the use of tire chips in civil engineering projects. For the past decade, The University of Maine professor has studied the uses of shredded tires as a durable alternative to conventional materials — such as soil and gravel — in the construction of roads, drainage areas and retaining walls.

Humphrey's extensive research and consulting work with state transportation and environmental protection officials coast to coast has earned him a nickname — "Dr. Shred."

"Civil engineering is not just to discover something new but to put what is discovered into practice," says Humphrey. "My technology is about solving engineering and environmental problems by reusing tires and saving money."

Each year in this country, more than 270 million used tires are discarded, according to the Scrap Tire Management Council of the Rubber Manufacturers Association based in Washington, D.C. Approximately 84 percent of all scrap tires come from passenger cars.

In Maine, the state Department of Environmental Protection estimates that there were 30 million old tires in abandoned stockpiles in the woods when Humphrey began his research 11 years ago. After this year's tire shred construction projects, only one pile with greater than 1 million tires will remain.

"Maine had more stockpiled tires per person than any other state. Today, with the Maine Department of Transportation (MDOT), Maine Turnpike Authority, Maine Department of Environmental Protection (MDEP), and UMaine working together, the state is headed toward cleaning up all its stockpiles," Humphrey says.

In 1990, MDOT funded UMaine's first laboratory study of tire shreds for use as lightweight fill. The results were so promising, shredded tires were used as an insulating layer in a section of roadbed in Richmond, Maine.

"It was a success for Richmond," a small town just south of Maine's capital city, Augusta, Humphrey says. "The road had to be stabilized so that people could get home during spring melt.

"Temperature measurements showed the tire chips provided insulation that kept frost from penetrating into the ground in the winter. Twelve inches of tire chips did what 24 inches of conventional construction material — gravel — could not."

Experiments continued, including the first-ever use of tire shreds as backfill for a retaining wall. Humphrey's studies found that tire shreds are lightweight, provide good thermal insulation and drainage, and can be compressed — "four characteristics that make interesting combinations for unique projects," he says.

The next milestones in the research came in 1996 when MDOT made use of large quantities of discarded tires — a total of 500,000 — on two more road construction projects.

Late that same year, a new interchange was designed to provide better access to the Portland, Maine Jetport. When the Maine Turnpike Authority faced the major challenge of finding suitable fill that would not sink into the thick layer of soft clay at the site, tire shreds were used.

The jetport interchange project used 1.2 million old tires from an abandoned stockpile and saved $300,000 in construction costs. The innovative, cost-saving alternative earned Humphrey the 1997 Maine Governor's Special Teamwork Award. That year he also received the International Tire and Rubber Association Friend of the Industry Award.

"The tire chip industry in Maine has worked hand-in-hand with The University of Maine as we develop the technology for tire chip applications," Humphrey says. "As a result, companies in the state are way ahead of their competition. Similarly, Maine DOT now routinely uses tire shreds for lightweight fill and drainage on highways. That's very different from other states, where only a few are including tire shreds as regular alternative construction materials. But that's changing."

Today, upward of 30 million old tires are shredded each year for civil engineering projects nationwide. That compares to 1990, when virtually none of the millions of scrap tires generated annually were being reused, Humphrey says.

Some of the first uses of tire chips in civil engineering projects occurred in the mid-1970s. At that time, whole tires were included in the construction of artificial reefs. Their use in road embankments and as backfill for retaining walls was on the increase in the early 1990s. Nationwide by the end of 1995, an estimated 10 million scrap tires were being reused each year in civil engineering applications.

What appeared to be an exponentially growing market for tire shreds came to a screeching halt in December 1995 and January 1996, when two roadbed embankments in Washington state and backfill in a retaining wall in Colorado caught fire. Humphrey was one of the researchers who joined industry and Federal Highway Administration representatives on a national committee to investigate the internal heating problems caused by tire chips, which by the mid-1990s had been used in more than 70 civil engineering projects across the country.

Humphrey had already been working with the Scrap Tire Management Council to draft guidelines for use of tire shreds. He subsequently took the lead in writing the national standards for use of scrap tires in civil engineering applications, which are set by the American Society for Testing and Materials.

Among the regulations is a 10-foot limit on the depth of tire shred fill. At the three sites that burned in Washington and Colorado, fill depths were 25 feet or more, causing internal combustion.

Tire shreds can be up to 12 inches long. The larger the tire shred, the lighter the fill and lower the likelihood of heating.

In highway applications, Maine and Minnesota lead the way, says Humphrey. Other states now are experimenting with tire shreds or have recently completed their first projects.

Those states now testing tire shreds in road construction include California, where in June Humphrey spent a week consulting on a project using shreds from 700,000 tires at a site just north of San José.

"Today, the biggest hurdles are concern about water quality and people not wanting to try something new," Humphrey says. "A five-and-a-half-year study that ended last year showed tire shreds pose no threat to water quality. What's needed in these states is an advocate willing to explore alternatives. That advocacy often comes from a state environmental agency responsible for managing scrap tires, or from the scrap tire industry looking to expand markets."

Humphrey fields up to 200 telephone inquiries each year about his research, in addition to his duties in the classroom and as chair of the Department of Civil and Environmental Engineering.

In the summer, Humphrey travels coast to coast, helping other states build tire shred projects. On Monday he may be giving a how-to presentation in Indiana; by Friday, he's headed to Delaware to evaluate a tire shred drainage system in a landfill.

"The best part of being a faculty member is working with students and watching them grow into mature professionals," says Humphrey, recipient of UMaine's 1998 Presidential Public Service Achievement Award, and the University's 1994 Distinguished Professor and Carnegie Foundation Maine Professor of the Year awards. "A close second is helping the state solve some of its pressing problems, such as disposal of waste tires and improving the durability of our highways."

by Margaret Nagle
October-November, 2001

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