The success of crumb rubber-modified asphalt in Arizona led Han Zhu, a researcher and assistant professor at Arizona State University, to experiment with crumb rubber as an additive to Portland concrete cement (PCC).
Not that cement has anything in common with asphalt, said Mr. Zhu, a civil and environmental engineer.
``My work is influenced by the progress made in rubber pavements,'' he said. ``But asphalt is very comparable to rubber in its composition, whereas rubber has almost nothing to do with PCC. You could almost say they're opposites.''
Nevertheless, Mr. Zhu believes rubber can add some important properties to concrete. ``The rubber may take sulfate out of the PCC, and that may help it for durability,'' he said. ``It may also give PCC a little more cushion against cracking, which is the No. 1 problem with concrete. PCC also has lots of porosity, and adding rubber would fill in the voids.''
Besides guarding against cracking, crumb rubber in concrete can reduce thermal expansion and contraction, drying shrinkage, ride noise, freeze-thaw damage, brittleness and weight, according to Mr. Zhu.
``When driving on a PCC highway, the car will vibrate every few seconds because there is a gap between two PCC slabs,'' he said. ``This gap is purposely installed to accommodate the thermal expansion and contraction of PCC. If the magnitude of thermal expansion-contraction can be reduced, then the gap will be smaller, and you will feel less vibration so as to increase driveability.''
The cement market, according to Arizona State, is potentially a big one for crumb rubber. Arizona generates 5 million scrap tires annually, and concrete producers in the Phoenix area alone turn out 12,000 cubic yards of PCC daily.
``If 20 pounds of crumb rubber per cubic yard of fresh PCC were added, all 5 million tires scrapped annually could be consumed,'' a university press release said.
Mr. Zhu began his experiments with a section of sidewalk on the Arizona State campus. A 1998 grant from the Arizona Department of Environmental Quality helped his research, but he ran into a brick wall when he could not find a natural environment for his experiments.
The breakthrough came in February 1999, when Mr. Zhu added 200 pounds of crumb rubber, or about 8 percent by weight, to a concrete mixture at Arizona State. While the crumb rubber, as predicted, lowered the concrete's compressive strength, the reduced cracking and other benefits compensated for this in non-load-bearing projects. Adding a small amount of gypsum to the mix brought compressive strength levels up to specifications, Mr. Zhu found.
Besides reducing cracking, shrinkage and brittleness, rubber in concrete cuts down on thermal expansion in warm weather and crumbling from freeze-and-thaw damage in cold weather, Mr. Zhu said.
In May, the Arizona Department of Transportation paved a small parking lot in Phoenix, using 50 pounds of crumb rubber per cubic foot of PCC. The department tested the pavement's performance at one week, one month and three months, and will continue the testing long term, according to Mr. Zhu. As expected, the compressive strength was lower, though still within acceptable limits, he said.
Mr. Zhu is seeking funding for a project to lay rubber-modified PCC in a cold region. ``There is even greater potential for the material in a cold climate,'' he said.