CSU Engineering Professor 'Tires' of Expansive Soils

A Colorado State University professor is researching whether some of Colorado's 40 million stockpiled rubber tires—the largest batch in the nation—can be reused to bolster residential foundations and road bases and mitigate the effects of expansive soils.

Antonio Carraro, assistant professor of civil and environmental engineering, is leading an experiment with the city of Loveland to test a mixture of expansive soil and scrap tire rubber just placed just below the pavement on a 200-ft, low-volume stretch of road near Interstate 25. Lime and fly ash are the traditional materials typically used to mitigate the shrink-swell potential of roadbed soils.

“We're always looking for opportunities to improve our roads and do a better job with making our products more environmentally friendly,” says Keith Reester, director of public works for the city of Loveland. “Hopefully, we have a product that works and will allow us to take some of those tires out of the waste stream.”

Carraro also recently received a $128,913 grant from the Colorado Commission on Higher Education to test his expansive soil-rubber mixtures in residential foundations.

The road-test section in Loveland will use only about 20% rubber by weight when combined with soil. But there are lots of scrap tires stockpiled in the state—at 40 million old tires, Colorado leads the seven states that host 84% of all stockpiled tires in the United States, according to 2005 figures from the Rubber Manufacturers Association. New York ranks second with 37 million tires.

Carraro has worked with Front Range Tire Recycle in Sedalia and Jai Tire Industries in Denver to obtain the tire samples.

“Only about 2% of scrap tire rubber products are reused in civil engineering applications in EPA Region 8 (which includes Colorado and five other western states) while other regions, such as the midwestern United States, reuse more than 25%,” Carraro says. “It's a major solid waste problem, particularly in the West, where the population is spread out and there is less demand and no established civil engineering markets for reusing the tires.

“We are trying to come up with a sustainable way of mitigating the expansive soils problem in Colorado that takes into account the beneficial use of a waste material that has great recycling potential,” Carraro adds. “Soil-rubber mixtures have been studied and used since the late 1980s, but the transfer of this technology to civil engineering applications that involve expansive soil mitigation is innovative. This is a new technology for expansive soils—these projects will allow us to understand in a more fundamental way the many interesting technical aspects associated with the design, construction and performance of expansive soil-rubber mixtures.”

Carraro and his students will monitor the effects of the expansive soil-rubber mixture on the road in Loveland over the next year. They will assess cracks, ruts, permanent deformation, potholes and the overall quality of the pavement.

In Colorado, shredded tires are largely in demand for landfill construction, says Rick Welle, general manager of Front Range Tire Recycle in Sedalia. The tire “shreds” are also used as floor material for horse arenas and children's playgrounds, but tire recyclers are always looking for other markets.

The Loveland project is using about 25 tons of shredded tires equal to 2,225 passenger vehicle and light truck tires, Welle says.

“What we're hoping with this study is to show that tire shreds are beneficial for road base and that over time, it will be a cost-effective product,” he says. “It would be a huge market to really help Colorado get back with the rest of the country as far as managing scrap tires.”

Carraro is also experimenting with the amount and size of scrap tire rubber products in his projects. Larger ones, called tire chips (containing up to 2-in. diameter particles) can cost roughly $30 per ton while smaller rubber products, called granulated rubber (containing particles less than 0.5 in. in diameter) can cost up to $450 per ton.

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