Shrinking the Tire Pile

Imagine a pile of 5 million worn tires teetering above you. That’s a lot of old rubber. It’s also roughly the number of tires that Arizonans dispose of each year—one tire for every man, woman, and child. How to dispose of all those used tires without causing serious environmental hazards has state officials scratching their heads. Han Zhu believes he has the answer.

Zhu is an ASU assistant professor of civil and environmental engineering. He says that adding a small amount of inexpensive crumb rubber to fresh concrete can improve the strength and durability of the final mixture. Crumb rubber is the end result of grinding used tires into one-millimeter chunks. One tire produces about 10 pounds of crumb rubber and sells commercially for less than 20-cents per pound.

Zhu is not the first researcher to experiment with adding tire bits to Portland cement concrete (PCC). However, he does own rights for the first “real world” application of the material—a section of sidewalk on the ASU main campus.

“This is my baby,” says Zhu, as he bends to get a closer look at the sidewalk between the Memorial Union and the ASU Bookstore. Black flecks of rubber sparkle in the morning sun. “I have been coming out here to examine this sidewalk for two years. Most people think I am just a guy looking for pennies,” he jokes.

Zhu began exploring uses for crumb rubber in 1998 with a grant from Arizona Department of Environmental Quality. His research came to a screeching halt soon after because he could not find a natural environment as an experimental site, a critical step in testing new materials prior to certification. He says that few people were interested in using a new material because there was no guarantee of success.

In February 1999, Zhu personally added 200 pounds of crumb rubber to the concrete mixture being prepared for the ASU site. The ratio of crumb rubber added to the mixture was equal to about 8 percent of the cement weight.

Previous lab studies were not encouraging for the waste product. Earlier research showed that adding crumb rubber to concrete would lower the compressive strength, the major criterion used in designing PCC.

Zhu's study showed that adding crumb rubber into PCC actually produced several benefits that would compensate for the loss in compressive strength, particularly for projects that are not considered load-bearing. These benefits include reductions in thermal expansion, also known in Arizona as “summer fatigue,” and reductions in drying shrinkage and brittleness. The recycled rubber also shows promise in ending the crumbling associated with freeze and thaw damage in colder climates.

Zhu says these benefits alone significantly improve the overall durability and serviceability of PCC. More recently, the ASU engineer made new advances. He boosted the compressive strength levels of crumb rubber PCC to specifications by simply adding a small amount of gypsum to the mix.

More than 12,000 cubic-yards of PCC are produced each day in the Phoenix metro area alone, according to the Arizona Cement Association. Using conservative estimates, Zhu says that if just 20 pounds of crumb rubber per cubic yard of fresh PCC were added, all 5 million scrap tires produced annually in the state could be recycled into stronger and more pliable PCC for use in sidewalks, parking lots, and concrete floors.

In May 2001, the Arizona Department of Transportation used the new PCC technology to build a 12-foot by 12-foot parking lot at its Phoenix division site. Builders used 50-pounds of crumb rubber per fresh cubic foot of PCC. Quality monitoring is still in progress, but Zhu says, so far so good.—Lynette Summerill