Sep. 14, 2001 Most people don't think about the tons of concrete in the buildings and structures around them until disasters strike, such as Tuesday's terrorist attacks on the World Trade Center and Pentagon. But Dr. Neven Krstulovic-Opara thinks about concrete every day.
Krstulovic-Opara, assistant professor of civil engineering at North Carolina State University, has been researching a special system for making concrete that may save lives, buildings and bridges by changing the way concrete structures fail. The system is called high-performance fiber-reinforced concrete (HPFRC), and can be used in new construction or renovations.
It's 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.
"We know that concrete structures will eventually fail," said 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."
One of the problems with conventional concrete is that during extreme structural stress, such as is experienced during explosions or an earthquake, it 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.
Since obtaining his Ph.D. at Carnegie Mellon University under the guidance of 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 used this advanced concrete composite to strengthen structures against earthquakes in laboratory models. He's working with a team of colleagues to develop new structural systems that would best employ the advanced features of HPFRCs – high strength, durability, low cost and easy construction – to increase impact resistance to explosive blasts as well. The research projects are funded by the National Science Foundation with some funding for materials provided by Ribbon Technology Corp.
In addition to its safety features, the HPFRC system may change the way that buildings are built, strengthened or repaired.
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. Krstulovic's system eliminates most of these labor costs, because it can be used as both frame and form for structural support of the building. Workers simply shape the fiber mat rolls and inject them with concrete slurry. For renovations or repairs, they can wrap the fiber mats around existing columns and beams or use the fiber mats as forms and fill them with conventional concrete. The result is a support beam or column that is super strong and more durable than conventional concrete alone.
The HPFRC system is designed to use traditional concrete construction equipment with minimal modifications.
"This new concrete can reduce the cost of repairing existing concrete structures and give them added strength and durability," said Krstulovic-Opara. "But, most importantly, because of its design, it can easily be transferred from the laboratory to everyday use."
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The above story is reprinted from materials provided by North Carolina State University.
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