Concrete Thinking

ASU engineers are devising more intelligent methods for using cement and concrete. The result could be better housing that is more durable and longer lasting.

A 6.6 magnitude earthquake hit the Iranian city of Bam on December 26, 2003. More than 26,000 people died in the destruction. In 1994, a 6.7 magnitude earthquake hit Northridge, Calif., just north of Los Angeles. It caused only 57 deaths.

Barzin Mobasher was already planning a trip to his native Iran when he heard of the Bam quake. When he visited the city one week after the disaster, he could see that much of the loss of life was due to construction failures. Some structural damage was unavoidable. However, Mobasher says that fatalities due to the quake could have been minimized by appropriate design considerations. If the residents of Bam had lived in better housing, much of the loss of life would have been prevented.

“The construction industry uses an average of one ton of concrete per person per year,” says Mobasher, a professor of civil and environmental engineering at Arizona State University. “But a lot of people don't have decent housing in the Third World. We need to use materials more wisely.”

Technology has enabled conservation in many disciplines. There are cars that get better gas mileage. And there are faster, more powerful computers with much smaller power needs as compared to similar systems available a decade ago. However, similar strides have not occurred in the construction industry.

For example, wood houses are still susceptible to fire and termites, require more energy to build, and contribute to the destruction of forests. Areas of the world outside the United States do not have large forests to supply lumber. Why not replace the wood used to build houses with concrete? The switch makes sense from an economic and environmental perspective, Mobasher says, yet it hasn't happened. The ASU researcher hopes to change that while at the same time providing better housing for all people.

Concrete is a composite material made of multiple other materials. Mobasher’s idea is to make concrete even better by reinforcing it with fabric. Cement is a powder of limestone and clay. Concrete is a mixture of cement and sand, gravel, pebbles, broken stone or slag. Adding fabric to concrete strengthens it and makes it energy absorbent. It also reduces the amount needed in a building project.

“The synergy between the different components in composite materials makes them better than any one of them alone,” Mobasher adds.

Fabrics are made by spinning long, thin fibers of a material into yarn. This increases the strength of the material. Consider, for example, a cotton ball and a cotton yarn. Pull on a cotton ball, and it comes apart easily. Pull on the ends of the yarn, however, and much more force is needed to break it. Weaving yarn into fabrics converts a one-dimensional strong material into a two-dimensional structure that can carry loads in both directions. Weaving is an 8,000-year-old art form. But the materials Mobasher makes into fabric aren't the ones you generally find in clothing. He uses fabrics made of polypropylene and polyvinyl alcohol. Polypropylene is often used in indoor/outdoor carpet, plastic bottles and automobile parts. Polyvinyl alcohol is found in adhesives, detergents, and paints.

Using these plastics in fabric concrete provides an end point for recycling. Mobasher also has experimented with fabrics made of carbon, glass, steel, and Kevlar, the material used in bulletproof vests.

A material called flyash is often added to the concrete to make it stronger. Flyash is a byproduct of burning coal. Two-thirds of the flyash produced in the United States goes directly to landfills. However, it can replace part of the cement in a concrete mixture.

ASU engineers have developed fabric reinforced concrete that uses flyash. In their mixture, the ratio of flyash to cement is 40/60 or 30/70. The amount of fabric in the concrete is three or four percent by volume.

Fabric reinforced concrete is made by a process called pultrusion. A machine pulls the fabric off a roll similar to a tape dispenser and dips it into a mixture of cement and flyash. This mixture has the consistency of maple syrup. The resulting fabric reinforced concrete can then be shaped into panels, tubes, or pipes before it sets and becomes hard.

Construction materials need to be strong but preferably not brittle if they are to withstand an earthquake. The best materials are ductile—able to bend—and energy absorbing. If one uses a strong but energy absorbing material in a building, then less of it can be used to carry the same load as a more brittle material. A one-inch thickness of fabric reinforced concrete is ductile and up to 1,000 times as energy absorbing as regular concrete.

Adding fabric to concrete also makes it more able to withstand tensile forces. One or two large, catastrophic cracks often form in regular concrete. But in fabric reinforced concrete, energy is better distributed through the material such that millions of smaller, interconnected and interacting microcracks form. The fabric acts like a Band-Aid. It holds the concrete together like an adhesive bandage holds together the edges of a cut. This translates to a longer lifetime for structures made with the material.

“I’ve learned that we can make building materials stronger and stiffer, but that may not translate into longer life,” Mobasher says. “This does.”

Mobasher began his research using short fibers in the concrete. He then expanded to longer fibers and eventually to using woven fabric. He found that the nature of the weaving interlocks the fabric in the concrete microstructure. This makes a very strong mechanical bond between the two materials similar to that of a screw in wood.

The ASU engineer believes he has worked out most of the technical challenges of creating fabric concrete. He's now looking to commercialize the process. This likely will be difficult because of the nature of the construction industry.

“If a company like Microsoft comes out with a new, better version of some software, then everyone gets it in a short period of time. The construction industry isn’t like that,” Mobasher explains. Sometimes bringing a fresh idea from the laboratory takes decades because of the industry’s fragmentation and resistance to change. Mobasher is hoping for better with the fabric concrete concept.

Construction companies bid on projects. The lowest bidder usually gets the commission. As such, the focus is on the lowest initial cost of a project. It is not on the life cycle cost, what the project will cost in maintenance down the road, or how long the building will last.

Mobasher says that using fabric concrete may be more expensive initially, but its strength and durability should make its life cycle cost lower than that of regular concrete.

“We are a society that builds. We will continue to build. But we never seem to consider using alternative materials,” Mobasher says. “If we can learn to use cement and concrete more intelligently in the future, we will be able to provide better housing that is more durable and longer lasting.” —Linley Erin Hall