Urban Ecology

Shrinking the Tire Pile

by Lynette Summerill

Imagine a pile of 5 million worn tires teetering above you. That’s a lot of old rubber. It’s also about the number of tires that people throw away each year in Arizona. That is one tire for every man, woman, and child in the state. Arizona officials are struggling to figure out how to dispose of all those used tires without harming the environment. Han Zhu believes he has the answer.

Zhu is an ASU engineering professor. He says that adding a small amount of 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 for less than 20 cents per pound.

To test the usefulness of this cement-rubber mixture, Zhu devised his own experiment. He added tire bits to Portland cement concrete (PCC). He then tried the new mixture for a section of sidewalk on the ASU main campus in Tempe.

"This is my baby," says Zhu, as he bends to get a closer look at the test section of sidewalk. 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. His research came to a screeching halt soon after because he could not find a good experimental site. To test new materials you need good test sites. Not many people were interested in Zhu’s 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 crumb rubber made up about 8 percent of the cement weight.

Scientists had studied the material in the lab. But the results were not encouraging. Earlier results showed that adding crumb rubber to concrete would lower the compressive strength. That is a major factor in designing PCC.

Zhu's study showed that adding crumb rubber into PCC actually produced several benefits. The benefits would make up for the loss in compressive strength. For example, the new mixture does not expand as much in the heat. It is also less affected by drying shrinkage and brittleness. The recycled rubber also shows promise in stopping the damage caused by freezing and thawing in colder climates.

Zhu also found another way to boost the compressive strength of crumb rubber PCC. All he had to do was add a small amount of gypsum to the mix.

Builders use a lot of PCC in the Phoenix metro area alone. The Arizona Cement Association says that more than 12,000 cubic-yards of PCC are produced each day.

Now think again about that giant pile of 5 million scrap tires. Zhu says that if just 20 pounds of crumb rubber were added to each cubic yard of fresh PCC, all those tires could be recycled. They would become part of a stronger and more pliable PCC. That PCC could be poured for use in sidewalks, parking lots, and concrete floors.