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August 6, 1997

NC State Engineer Develops Super Strong Concrete System

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Dr. Neven Krstulovic-Opara holds the stainless steel fiber mat used in his HPFRC concrete.

Most people don't think about the tons of concrete in the buildings, bridges and overpasses around them, but Dr. Neven Krstulovic-Opara thinks about concrete every day. When he talks to people about concrete, he flashes images of the Kobe earthquake on the projection screen--gloomy slides of cars crushed beneath cement rubble. "The high performance concrete we are developing can prevent this from happening," he says. "It has a great potential to revolutionize the way concrete structures are built and used."

Krstulovic-Opara, assistant professor of civil engineering at NC State University, has been researching a special concrete system that will possibly save lives, buildings and bridges by changing the way concrete structures fail. His High Performance Fiber Reinforced Concrete (HPFRC) system has the potential to completely change construction practices and retrofitting of concrete structures. It is super strong, durable and economical.

Since obtaining his Ph.D. at Carnegie Mellon University under the guidance of Professor James Romualdi, the original inventor of fiber-reinforced concretes, Krstulovic-Opara has been developing the HPFRC system using fiber mats injected with a special concrete slurry, a mixture of concrete, aggregate and liquids. The mats are made of recycled stainless steel fibers and come in large rolls that can be cut and shaped to fit the space or use desired. The fiber mats add tensile strength and ductility--energy absorbing properties--to the concrete.

Krstulovic-Opara has successfully used this advanced concrete composite to strengthen concrete structures against earthquakes in laboratory models. Currently he is working with Dr. Shuaib Ahmad, professor of civil engineering and director of the Constructed Facilities Laboratory; Dr. Paul Zia, Distinguished University Professor Emeritus of civil engineering; Dr. John Hanson, Distinguished University Professor of civil engineering and construction; and postdoctoral research assistant, Dr. Jamal Shannag, to develop new structural systems that would best employ the advanced features of HPFRCs--high strength, durability, low cost and easy construction. The research projects are funded by the National Science Foundation with some funding for materials provided by Ribbon Technology Corp.

"This new system uses a technology called Slurry Infiltrated Mat Concrete, or SIMCON," says Krstulovic-Opara. "It will eliminate the need to use extensive wooden or steel forms that have to be constructed and then disassembled. It can also be applied to existing structures to add strength and durability."

Because of its strength, the new concrete system has tested the limits of the equipment currently used by the Department of Civil Engineering at NC State. The existing equipment could not cause the HPFRC system to fail.

"More powerful equipment is needed to fully test the capabilities of the new concrete design," says Krstulovic-Opara. "Right now, I am looking forward to moving into the new Constructed Facilities Laboratory on [NC State's] Centennial Campus. The new laboratory will have the largest testing machinery in the United States. It will have enough power to cause failure of our advanced composite materials and structures, which is what I want to do."

"We know that concrete structures will eventually fail," says Krstulovic-Opara. "What we want to do is extend the length of time it takes for the structure to fail and control how it fails."

Because failure is inevitable in all structures, engineers have to design the best and safest way for a structure to fail. In conventional concrete systems, steel reinforcing bars (rebars) give the concrete tensile strength. But steel rusts over time and causes failure. For safety and design reasons, conventional concrete is designed so that the rebars fail before the concrete fails.

Krstulovic-Opara's concrete system breaks that tradition.

One of the problems with the conventional concrete failure design is that during extreme structural stress, such as is experienced during an earthquake, the concrete breaks apart in large chunks and separates from the steel rebars. The result is that large slabs and chunks of concrete fall from the structure, hurting the inhabitants and crushing anything beneath them.

The HPFRC system is designed in a way that prevents the separation of large pieces of concrete from the structure. When it fails, the pieces remain stuck together, held in place by the stainless steel fibers. The pieces that do separate from the structure are much smaller and less likely to cause injury.

In addition to its safety features, the HPFRC system may change the way that buildings are built and repaired. It also can be used to strengthen existing structures.

Currently, to build a concrete structure workers have to bend steel rebars into frames, build wooden or metal forms around the frames, add concrete and, once the concrete has had time to cure, remove the forms. The process is labor intensive and costly.

The new concrete system removes most of the labor costs from the construction. Instead of having to construct steel frames and build and tear apart forms, the HPFRC system can be used as both frame and form for structural support of the building or bridge. Workers simply shape the fiber mat rolls and inject them with concrete slurry. The fiber mats also can be wrapped around existing columns and beams for repairing or strengthening existing structures.

In another application, the HPFRC system can be used as a "stay-in-place" form and filled with conventional concrete. The result is a support beam or column that is super strong and more durable than conventional concrete alone.

Another problem with conventional concrete is the size and pattern of the cracks that form in the concrete as the structure ages. The cracks are large and connected, allowing water to seep into the concrete and further compromise the integrity of the structure.

The HPFRC cracks are small, disconnected hairlines, some so fine that they are barely visible, reducing the possibility of water seepage. As an added benefit, the concrete slurry uses very little water to allow it to be packed tightly into the mat, causing some of the cement powder to remain inactive. Over time, as water seeps in through the small cracks, it mixes with the inactive cement and causes it to activate, essentially making the HPFRC system self-healing.

The HPFRC system is designed to use traditional concrete construction equipment with minimal modifications, adding to the already lowered construction costs. This means that contractors will not need to invest in expensive new equipment to begin using the new, stronger concrete.

"This new concrete can reduce the cost of repairing existing concrete structures and give them added strength and durability," says Krstulovic-Opara. "And it can be used to develop new concepts in high performance structural systems. But, most importantly, because of its design, it can easily be transferred from the laboratory to everyday use."

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