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Precast of the future?
By Floyd Allen
You may already have enjoyed the benefits of riding on asphalt that has had rubber mixed with it, but have you considered the viability of utilizing rubberized concrete in various precast concrete applications? While this may seem a bit preposterous, the host of possible applications is extensive, and the advantages may make it well worth the consideration.
Before discussing rubberized concrete, a brief history of the product should prove beneficial. In the late 1990s, Dr. Han Zhu, an engineering professor at Arizona State University, was amazed at the number of recyclable tires he saw while visiting a shredding plant in the greater Phoenix area. His immediate thought was to develop a way to convert them for use in concrete. Utilizing facilities at the university and the ready labor force his students provided, Zhu started putting his ideas to work.
As he experimented with this concept, Zhu drafted a few other people to help him with his project. Two individuals who quickly became major players were George Way and Doug Carlson. Way is currently an engineering consultant but at the time was the chief pavement design engineer for the Arizona Department of Transportation. Carlson is a board member of the Recycle Tire Engineering and Research Foundation, a group created to explore material properties and potential engineering applications of recycled rubber. Later on, Mark Belshe, a vice president of FNF Construction, was also drawn into the mix. In 2003, Zhu accepted a professorship in China, and the crumb rubber concrete project was turned over to ASU’s Dr. Kamil Kaloush.
To date, the project has dealt mostly with poured concrete, but the group is excited about the possibilities for precast applications as well.
“The vast majority of the characteristics we discovered in regard to crumb rubber would be just as valid for precast as for poured-in-place,” Kaloush said. “For example, it produces a lightweight panel that is beneficial in noise control as it is well-insulated. Also, in an environment with a number of computers and other electrical equipment, this insulation would tend to assist with static reduction in walls,” he said.
“I was impressed with the fact that crumb rubber concrete has excellent freeze-thaw characteristics,” Way added. In fact, the team found that expansion/contraction was cut in half.
Yet another advantage is that shrinkage, and thus cracking, is reduced. “We took special note that cracking was reduced to the point that 45 degree cracking was virtually lost completely,” said Carlson.
Obviously these are all excellent advantages. But are there any characteristics that might be considered disadvantageous? “The most obvious is strength reduction. The greater the rubber content, the more reduced the strength is,” explained Kaloush.
“I don’t know if this is a disadvantage or not,” Carlson added. “But the crumb rubber takes the place of aggregate, and as a result, more cement is needed in the formula.”
Other than that short list, apparently there are no other disadvantages, said Way.
Possible applications of precast concrete utilizing crumb rubber concrete abound. “Having formally been with A-DOT, the first thing that comes to mind for me are the Jersey Barriers you see on highways during construction,” said Way. “The fact that they would be somewhat lighter would mean that they could be transported quicker and easier.”
Easier transport isn’t the only advantage. “The virtual elimination of the 45 degree cracking would particularly come into play here,” said Carlson.
Kaloush added that barriers made with crumb rubber would be much more forgiving should they be run into, which would help protect the barrier itself as well as the vehicle that hits it. “It is my hope to be doing some extensive experimentation in this area in the very near future,” he said.
Another possible application for concrete cast with recycled rubber is in sidewalks. Kaloush has found that the crumb rubber produces an end product this is nearly non-slip in nature. “What I envision is a situation where all your utility pipes – water, sewer, electrical, phone and TV cables – would be run under the sidewalk rather than the streets,” he said. “With precast sidewalk panels, you could simply lift them up when you need to get to the pipes to work on them.”
In the long run, Kaloush said that this would be far less expensive than having to constantly tear up streets to work on the pipes. “You wouldn’t need new materials every time – all you would have to do is pick them up and then replace them,” he said. “And look at how much more convenient it would be to close down a sidewalk rather than a street. This should be a tremendous selling point for precast proponents in trying to sell it to various cities and other municipalities.”
This isn’t as farfetched as it may first seem, according to Way. “In Europe, they are already utilizing a variation on this theme,” he said. “While it might be difficult to retrofit a city with this program, there is so much new construction going on that this concept could really be a major innovation. All it would take is a very proactive marketing plan on the part of the precast community,” added Way.
“Crumb rubber panels would be ideal for nearly all non-load bearing walls in an office building,” said Kaloush. “The fact that it is lighter and a nonconductor of both noise and electricity makes it an ideal candidate for use in that capacity.”
Residential housing could also benefit. “Here in the Southwest we have a lot of homes with clay tile roofs,” said Carlson, explaining that strength, for the most part, is not a major concern because of the lack of snow and ice. “You could create a precast roof with a high percentage of crumb rubber that would be lightweight, help control noise going both ways and, from experiments thus far, should prove to be a good deal cooler.”
Carlson was referring to the fact that Kaloush, and Zhu before him, have been running tests that seem to indicate that the crumb rubber concrete is cooler than its regular counterpart.
“These studies are inconclusive at this time, but to this point they seem to support the theory that it is cooler,” said Kaloush. “I was able to take some photos with an infrared camera, and from those, at least, the crumb rubber is cooler.”
The idea of a “cooler” concrete leads to a possible piece in the puzzle for battling the Urban Heat Island Affect, suggested Belshe. “That’s one of the things that caught FNF Construction’s attention. Any time we can make a major stride in controlling a problem like that, we become very interested.”
In fact, the use of crumb rubber concrete could very well open another major area for the precast industry: “green” buildings. “If enough applications can be found and used, it might be possible to land contracts with the government and companies who feel strongly enough about the Green Building program that they make adherence to those rules a prerequisite for doing business with them,” said Carlson.
Like every other innovation, crumb rubber concrete will, undoubtedly, face its share of skeptics. The experts believe, however, that end-users especially will be impressed by the fact that the process is so environmentally sound. While real evidence will need to be gathered to verify that the advantages outweigh the possible cost increase of producing crumb rubber panels, this is one area that may turn out to be a “cushy” investment.