Arizona State University research finds recycling cure for used tires

September 10, 2001

Tempe, Ariz. --Imagine for a moment seeing 5 million worn tires heaped up in a pile: That is roughly the number that Arizonans produce each year-one tire for every man, woman and child. How to dispose of all those used tires without causing serious environmental hazards used to have state officials scratching their heads, but one ASU researcher believes has an answer. Civil and Environmental Engineering Assistant Professor Han Zhu says adding a small amount of the inexpensive crumb rubber to fresh concrete can improve strength and durability. 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.

While Zhu is not the first researcher to experiment with adding the tire bits to Portland cement concrete or PCC, he does own rights for the first "real world" application, a section of sidewalk on the ASU Main Campus in Tempe. "This is my baby," says Zhu of the sidewalk between the Memorial Union and the campus bookstore. He bends down to get a closer look. 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 to explore uses for crumb rubber in 1998 with a grant from Arizona Department of Environmental Quality. His research, however 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.

The researcher said few people were interested in using the new material because there was no guarantee of success. As luck would have it, Zhu found a test site in his own backyard.

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

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

Contrary to earlier research, Zhu's study showed adding the crumb rubber into PCC actually produced several benefits that would compensate for the loss in compressive strength, particularly for projects that are not considered loadbearing. These benefits include reductions in thermal expansion, also known in Arizona as Summer Fatigue, along with 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 researcher made new advances in restoring the compressive strength level of crumb rubber PCC to specifications by simply adding a small amount of gypsum to the mix. Grounds Construction Supervisor Andy Castillo said from his own observations of the ASU project, the crumb rubber PCC appeared to avoid cracking better than its original counterpart. 'I'd be willing to use it again in another campus project," he said.

According to the Arizona Cement Association, within the Phoenix metro area, some 12,000 cubic-yards of PCC are produced each day. By conservative estimates, if just 20 pounds of crumb rubber per cubic yard of fresh PCC were added, all 5 million scrap tires produced annually statewide could be recycled into stronger and more pliable PCC for use in sidewalks, parking lots and concrete floors. This past May, Arizona Department of Transportation used the new PCC technology to constructed a 12-foot by 12-foot parking lot at its Phoenix Division site using 50-pounds of crumb rubber per fresh cubic feet of PCC. Quality monitoring is still in progress, but Zhu says, so far so good.

"It appears crumb rubber deserves public attention and more crumb rubber PCC structures should be built. This way, time and nature can help determine additional benefits this technology can bring to our communities," Zhu said.

Arizona State University

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