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Concrete made stronger and tougher with bone-shaped wires

DOE/Los Alamos National Laboratory : 02 June, 2007  (Technical Article)
Department of Energy's Los Alamos National Laboratory researchers have discovered that enlarging the ends of small wires mixed into concrete substantially increases the material's overall strength and toughness. The Los Alamos team found that adding the unique steel wires in amounts equal to just one percent of the concrete's volume increases its maximum strength by as much as 84 percent, and its toughness by as many as 93 times.
It has been recommended by The American Society of Civil Engineers that the United States invest $1.3 trillion in its physical infrastructure, roads, bridges, dams, water systems and public buildings. Since many of these deteriorating structures are made of concrete, finding a way to extend the life of concrete will give the nation more value for this huge investment.

The wires, shaped like a cartoon dog bone, anchor firmly into the concrete matrix at each end because of their shape, but bond weakly with the concrete along their length. Principal investigator, Theodore Zhu, and his team designed the enlarged wire ends so they don't experience the stresses that usually snap reinforcements and thereby limit the matrix's performance.

'The design of the bone-shaped wires was aided by analytical modeling done on Laboratory supercomputers,' Zhu said. 'This modeling allowed us to not only optimize the shape and length of the wires, but it also permitted us to match different materials to different applications or environmental forces.' The team found that carbon wires or fibers can be used in high corrosion environments and metal wires work better in extreme temperatures.

'Straight steel wires can pull free of a concrete since they bond weakly with the surrounding matrix at their ends. Alternatively, if steel wires happen to bond strongly with the matrix along their length, they can snap under high stress without much bending or stretching. In our stress tests the bone-shaped wire concrete got even stronger for a short time right after the first cracks as the load was increased because the load was being transferred to the wires.' This small degree of stretching and bending by the steel wires gives the concrete increased toughness.

The concrete samples developed for the experiment were subjected to loads to the point of failure and then examined microscopically. The concrete containing the bone-shaped wires significantly outperformed the straight-wire concrete for both toughness and strength. Toughness is the amount of energy required to damage the concrete and strength is the concrete's resistance to force spread over a given area.

The bone-shaped wire concrete resisted the propagation of cracks better; the wires bridge the crack and refuse to let go. Close inspection showed that even though a crack in the concrete matrix had snaked through the sample, the sample remained intact.

Civil engineers and contractors use steel, fiberglass and other similar materials to increase concrete's strength and toughness, but using those materials often requires costly construction techniques. Short-wire reinforced concrete should become a favorite technology since the process is compatible with standard construction processes and the steel used for the bone-shaped wires is relatively cheap. The Laboratory is currently seeking partners to collaborate with in the licensing and development of the bone-shaped wires.
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