The U.S. Army Corps of Engineers is being asked to determine what touched off fires last year within layers of tire shreds used as fill material under two highways in Washington state and a roadside retaining wall in Colorado. The fires, in Ilwaco and Garfield County, Wash., and Glenwood Canyon, Colo., are thought to have resulted from spontaneous combustion of the steel-impregnated rubber shreds. Their occurrence early in 1996 dealt an unexpected blow to tire recycling by drastically curtailing demand for such scrap-tire byproducts.
Proponents hope the proposed Army Corps research program will dispel concern over the possibility of future fires and revive what once was a promising market for shredded tires in civil engineering applications—such as roadfill, landfill covering or light-weight backfill behind retaining walls.
Until last spring, when the so-called ``burning roads'' attracted national attention and prompted transportation officials to postpone similar projects using shredded tires, recycling proponents were forecasting that civil engineering applications would account for as many 20 million tires in 1996.
But concern over the possibility of similar occurrences and a Federal Highway Administration (FHWA) memorandum suggesting future shredded tire installations be delayed until more is known about what caused the fires, shrank actual consumption to less than half the predicted amount.
Tire recycling proponents are hoping for at least a modest upturn in demand this year. One reason is the expected issuance of newly formulated interim guidelines intended to permit continued use of tire shreds—albeit on a limited basis—until the Army Corps concludes its research and more permanent guidelines can be established.
The interim guidelines, based on the findings of an FHWA-sponsored study by University of Maine Professor Dana Humphrey, are being prepared by an ad hoc committee made up of members of the tire recycling community, the FHWA and several technical experts, including Mr. Humphrey.
Now in draft form and awaiting the approval by committee members and FHWA officials, these preliminary guidelines cover basic construction features such as shred depth and material quality.
Their recommendations are based on Mr. Humphrey's observations of the fire sites and more than 70 similar embankment projects where tire shreds have been used without incident.
``We don't know what caused the problems in Washington, but we do know what has worked,'' explained Michael H. Blumenthal, executive director of the Washington-based Scrap Tire Management Council and co-chairman of the committee.
Assuming these guidelines meet with the approval of all concerned, they are likely to be issued jointly by the committee and the FHWA. Some members of the committee expect this to happen within a matter of weeks.
In the meantime, several members of the committee traveled to Vicksburg, Miss., in late January to talk with Army engineers about the proposed research.
The Army's Vicksburg research facility houses the most sophisticated geotechnical laboratory in the U.S., according to Mr. Blumenthal. He views issuing the interim guidelines as the first step in healing the market's woes and the proposed Army Corps research as the ultimate way to restore user confidence.
``I think eventually we will have a very complete package and we will resurrect this market segment. Because we think it is both economically and environmentally sound,'' he told TIRE BUSINESS.
As soon as the arrangements can be worked out, according to Mr. Blumenthal, researchers at the Vicksburg facility will seek to:
1) Develop a working hypothesis as to what caused the Washington and Colorado blazes, building on the findings of Mr. Humphrey's previous study; then
2) Using that hypothesis, replicate the conditions that led to the heat buildup and eventual combustion of the tire shreds in those fires.
One theory that developed out of Mr. Humphrey's previous study is that chemical ``geothermic'' reactions—possibly those resulting from oxidation of the steel reinforcing cords—created hot spots deep within these rubber-filled embankments.
Nancy Boyd, an engineering geologist with the Washington Department of Transportation and co-author of a technical paper on the state's Ilwaco fire, concurs with the theory that ``geothermic'' reactions may have precipitated the heat that eventually led to the fires.
But Ms. Boyd, who did the original geotechnical design on the Ilwaco site, is convinced it was the depth of the tire shred filling the embankment that was the most critical factor.
``I don't think we can ignore the fact that the two deepest scrap tire embankments caught fire,'' Ms. Boyd said.
Other factors possibly contributing to the fires, such the presence of ground water, organic soil or nitrate fertilizers, also have been observed in shallower tire-filled embankments where no problems have been experienced, she said.
The state has five other shredded tire embankments that continue to perform well, Ms. Boyd said. But the depth of fill was shallower in those sites—less than 15 feet.
By comparison, the fill used in the Ilwaco site was 26 feet deep. The fill depth of the Garfield County site was 45 feet deep.
In both cases, she noted, the embankments caught fire less than two months after their construction. The Ilwaco site, which had contained more than 700,000 shredded tires, cost an estimated $4 million to clean up.
Nevertheless, Ms. Boyd said she still sees potential in tire shreds and hopes someday to be able to use them again, provided the state can be persuaded to lift its present moratorium on their use—something that may take some doing.
But once the threat of fire is eliminated, she said, tire shreds could fill a widespread need in the geotechnical community for a light-weight, low-cost fill material.
On that point at least, she'll get no argument from Mr. Humphrey, who considers such use the most successful civil application of tire shreds to date.
During the past summer, according to Mr. Humphrey, two similar projects using tire shreds for backfill behind retaining walls were carried out in his state.
One reason tire shreds were chosen is that they exert only half the pressure of conventional gravel fill against the retaining wall, thereby permitting use of a much thinner, cheaper wall.
What's more, according to Mr. Humphrey, the cost of tire chips was about half that of the next cheapest alternative—extruded polystyrene, perhaps better known by the tradename, Styrofoam.