A Civil Solution

Some 30 million scrap tires a year are recycled in civil engineering projects involving highways, landfills, and septic systems—and the market is definitely on a roll.

By Robert L. Reid

Robert L. Reid is managing editor of Scrap.

The recycling of scrap tires through civil engineering projects has grown steadily and, at times, dramatically over the past decade, rising from roughly 500,000 tires annually in 1990 to 30 million tires last year, reports the Scrap Tire Management Council (STMC), part of the Washington, D.C.-based Rubber Manufacturers Association. Moreover, STMC predicts that 50 million scrap tires a year could soon be used in civil engineering applications that range from highway embankments to landfills and home septic drain fields. 
   The recycling of scrap tires through civil engineering projects has grown steadily and, at times, dramatically over the past decade, rising from roughly half a million tires annually in 1990 to 30 million tires last year, reports the Scrap Tire Management Council (STMC), part of the Washington, D.C.-based Rubber Manufacturers Association. Moreover, STMC predicts that 50 million scrap tires a year could soon be used in civil engineering applications that range from highway embankments to landfills and home septic drain fields.
   Even so, civil engineering represents only the second-largest market for scrap tire reuse behind the industry’s leading application—burning scrap tires as fuel, which consumed some 125 million tires last year, notes Michael Blumenthal, STMC’s executive director. Fuel is likely to remain the primary use for scrap tires for the foreseeable future, he adds, with civil engineering staying in second place. Still, the civil engineering market is clearly on a roll, and its growing demand could change the overall percentages associated with each tire-recycling application.
   Last year, the United States generated 273 million scrap tires, with 196 million reused in some fashion, STMC notes. Of those 196 million reused tires, tired-derived fuel accounted for nearly 64 percent of the total compared with 15 percent for civil engineering. The third spot was held by ground rubber applications (18 million tires, or roughly 9 percent) followed by exports (15 million tires, or about 8 percent), STMC reports.
   The civil engineering market for scrap tires usually involves shredded pieces measuring 3 to 12 inches long that are used in two general applications: lightweight fill for highway construction projects and drainage material for landfills and septic systems, explains Dana Humphrey, chair of the civil and environmental engineering department at the University of Maine (Orono, Maine) and an authority on scrap tire applications. 
   No hard numbers exist on exactly how many tires go into each of these applications, and there are also various niche markets for civil engineering projects to use scrap tires, such as thermal insulation in roadbeds and vibration-dampening layers under railroads. 
   At the same time, some ground-rubber applications—such as those that add the material to asphalt—could be considered civil engineering uses but are not traditionally included in the civil engineering market designation, Humphrey notes.

Solving Problems
   Scrap tires appeal to civil engineers because of special properties that help solve difficult design problems, Humphrey explains. Chief among these properties is the lighter weight of scrap tires compared with conventional materials. This is especially key in projects to improve the stability of embankments built on weak soils and in building highway retaining walls.
“Tires weigh less than half of what conventional soils weigh,” Humphrey says, “so you can put a lighter-weight material in that fill—you’re kind of floating the embankment on top of the weak soil rather than using something heavy that would sink down.”
   In addition, scrap tires are often less expensive than other traditional civil engineering materials such as sand, gravel, and stone. They also produce low horizontal stress—which enables civil engineers to design thinner, cheaper retaining walls—and provide good drainage even when compressed under the weight of material such as dirt and/or the road surface.
   In the late 1990s, the state of Maine used tire shreds to help construct two 32-foot-high highway embankments approaching a bridge over the Maine Turnpike. Since the site rested on weak marine clay, embankments made of conventional soil would be too heavy while efforts to strengthen the site would be too costly, Humphrey notes. Instead, the solution was to make the embankments lighter.
   Though the project’s designers considered other materials—including expanded shale and expanded polystyrene insulation boards—they ultimately chose tire shreds because they cost $300,000 less than these alternatives, Humphrey says. On the environmental side, the project found a beneficial reuse for 1.2 million tires from an enormous stockpile.
   In another embankment project, the California Department of Transportation saved $500,000 by using around 600,000 scrap tires for a highway interchange near San Jose where, again, the soil conditions required a lightweight material.
   Such roadwork represents a huge potential market for scrap tires. “In Maine, we’ve got a population of 1.2 million people and we’re using an average of 1 million tires a year as lightweight fill,” Humphrey says. “Scale that up to states with larger populations and more construction, and it’s not unreasonable” to expect larger states such as California and others to “use several million tires a year on these types of applications.”

Gaining by Draining
An even larger market for scrap tires could exist in landfills and septic drain fields, notes STMC’s Blumenthal. These applications could grow at a greater and faster rate than road projects, he says, because they are less affected by weather than roadwork and they don’t suffer from the bureaucratic delays that can stretch a year or more between the design of a new road embankment and the start of construction.
   Blumenthal lists five leading landfill applications for tire shreds: as alternative daily cover, in cap closures, in gas venting systems, in leachate liners, and as operational liner material.
   Landfills, like road projects, could consume an enormous number of scrap tires. Dana Humphrey recently helped Delaware use approximately 1 million tires—chopped to 3-inch pieces—in a leachate-drainage system.
When you combine such large volumes with the fact that there are more than 3,000 landfills in the country, “the opportunities are huge,” notes Blumenthal.
   Equally large—albeit more on a cumulative basis than in any individual proj-ect—is the scrap tire market for septic systems, which would also use pieces about 3 inches long as drainage material. Blumenthal points to a recent success story in Horry County, S.C. Though the contractor at first had doubts about using tire shreds, he ultimately found that they were easier to handle, cost less, and worked better than competing stone, Blumenthal notes. In fact, the tires were so easy to use that contractors discovered they could complete two jobs in the same time it used to take them to do one.
   “Soon, Horry County was using a million tires a year, and today in South Carolina you can’t get a drain-pad job without using tire shreds,” Blumenthal says. “Well, the folks in North Carolina have seen this, Georgia has seen this, Florida has seen it, and Mississippi now sees it.”
   Jokingly comparing the use of tire shreds in septic fields to kudzu—the ubiquitous and unstoppable southern vine—Blumenthal says, “It’s starting to spread. It’s taking over down there.”

Bad Weather, Bad Vibrations
In addition to the lightweight fill and landfill/drainage applications, tire shreds can be used in specialty civil engineering projects such as roadway thermal insulation. Though definitely a niche market—such insulation is only needed in regions that face incredibly cold winters such as Maine, Vermont, and Québec—tire shreds can help reduce expensive roadway maintenance that results from annual freezings and thawings that can heave one part of road surface as much as 5 inches higher than another, Humphrey notes.
   During the winter of 1993-94, a test in eastern Maine compared the frost penetration in various thicknesses of tire shreds and soil versus a conventional gravel subbase. In the two road sections using 12-inch layers of 100-percent tire shreds, frost was unable to penetrate below the scrap tire layers to reach the subgrade soil. But frost reached a depth of 54 inches beneath the conventional gravel road. 
   Humphrey concluded that tire shreds provide seven times more insulation than gravel while also being more permeable.
   Another civil engineering niche for tire shreds is as a vibration-damping layer under rail lines. The idea here is to use the tire shreds under the stone “ballast” beneath railroad ties to absorb the otherwise annoying vibrations that hit nearby homes or businesses whenever a train passes, explains Humphrey. One such project is under development in San Jose, Calif., where a light-rail commuter line is being extended and the tracks come quite close to residences and businesses, he notes.
   Humphrey expects this application to appeal mostly to a relatively small market of similar commuter lines as they expand operations rather than to the nation’s extensive freight railroad system whose neighbors have long-since learned to live with the vibration. Nonetheless, he predicts that light-rail operators will find that tire shreds save “substantial amounts of money.”

Burning Roads 
Though Humphrey and Blumenthal currently see a bright future for scrap tires in civil engineering projects, there was a time when they feared the whole market might disappear.
   The scrap tire industry itself dates back only to 1985, Blumenthal notes, and the civil engineering segment was practically nonexistent until the early 1990s. That was when several universities began researching the field, government agencies such as the Federal Highway Administration (FHWA) began holding regional workshops on scrap tire management, and a wealth of information started to accumulate. By 1995, somewhere between 5 million and 7 million scrap tires were being used annually, mostly in road embankment projects and as alternate daily cover in landfills.
   “Then we had the burning-road episodes that nearly killed this segment of the industry,” Blumenthal says.
   He’s referring to the incidents in late 1995 and early 1996 in which two tire-shred highway embankments in Washington state and one tire-shred retaining wall in Colorado began to steam and then caught fire. As a result, the FHWA put a moratorium on using tire shreds in these applications, and the number of scrap tires used in civil engineering projects plummeted—from several million a year to only about 500,000, and then only in landfill applications in a few states, Blumenthal says.
   Working with state and federal agencies, STMC quickly sent Dana Humphrey and another scientist, Joseph Zelibor, to study the fires and perform various environmental leachate and toxicity tests.
   What Humphrey and Zelibor found was that compressing layers of scrap tires beyond a certain thickness could lead to “internal heating reactions” that reached “combustion temperatures.”
   In other words: Build it too thick and it will burn.
   Some of the burning-road projects had stacked up a single layer of tire shreds more than 45 feet thick and another was roughly 70 feet thick, Humphrey explains.
   “That’s very different from what we do now,” he says, noting that today’s civil engineering projects restrict each individual tire shred layer to no more than 10 feet, with at least 3 feet of soil between the next 10-foot level of shreds. 
   Such layering seems to have solved the threat of road fires, Humphrey says, noting, “I have temperature measurements from inside embankments that we built properly, and they’re cool as a cucumber.”
   In addition to sending out scientists to study the situation, STMC helped develop design and construction guidelines in 1997 for using tire shreds correctly and worked with the American Society for Testing and Materials (ASTM) to produce an engineering standard for such projects titled Standard Practice for Use of Scrap Tires in Civil Engineering Applications. STMC also sponsored some 17 technical seminars on the issue between 1996 and 2000.
   “Back in the first half of 1996, we were facing a critical point where, if we had done nothing, it would have taken decades to get back to where we had been,” Blumenthal notes. “But we did what was necessary to address the concerns, got the right information into the right hands—to the state and federal agencies and the appropriate potential users—and certainly helped get this market segment back on its feet.”

Obstacles and Opportunities
Today, civil engineering is a growing, dynamic market for tire shreds, Humphrey and Blumenthal agree. Still, it’s clearly not for everyone.
   In Maine, for instance, scrap tires are a mainstream application, especially for roadwork. “If they need lightweight fill, they use tires—there’s no special instrumentation, no monitoring,” Humphrey says. But other states—including some of the nation’s largest—have shown little or no interest in the idea or don’t offer the right market conditions.
   Michigan and Wisconsin simply “don’t want tire shreds in the ground,” explains Blumenthal. And while New York will permit civil engineering applications, the approval process is slow and cumbersome, with the state asking for more extensive leachate tests—which prove that tire shreds do not cause environmental problems—than have already been performed elsewhere, Blumenthal notes.
   Such opposition can stem from a basic reluctance to try new solutions to old problems, says Humphrey, who notes that even in California, where the San Jose embankment project saved $500,000, the state decided to put in special instrumentation to monitor for heating problems since this was the first time it had used scrap tires in such a project.
   Potentially greater obstacles could exist in the most basic principles of supply and demand, as well as the business axiom about location, location, location.
   For instance, transportation costs and contractors’ lack of familiarity with using tire shreds can create problems.
That’s what the Texas Department of Transportation (TxDOT) found in an El Paso embankment project where tire shreds cost more than twice as much as conventional fill material—$11.62 per cubic meter for tire shreds versus $5 per cubic meter for soil. But better planning and a greater understanding of the material “may reduce these costs in future projects,” TxDOT noted.
   In some regions, though, such as Pennsylvania, Ohio, West Virginia, Kentucky, and Indiana, tire shreds might never compete against the abundance of high-quality, low-cost gravel and dirt that those states enjoy. Instead, Blumenthal predicts, the best markets are likely to be in areas without such natural resources, especially from Virginia down the Atlantic coast and through much of the Gulf coast and parts of the Southwest. 
   “No one’s going to pay more for tire shreds if all other factors are equal,” Blumenthal concedes. But where tire shreds’ unique qualities and benefits do find traction, they’ll likely also produce some satisfied customers. Blumenthal compares the civil engineering potential of tire shreds to the advertising slogan of a particular brand of Canadian beer.
   “Those who like it like it a lot,” he quotes, adding, “And those who like it are going to use a lot of tires.” •