IMPROVED STRENGTH AND DURABILITY BY STEEL FIBER
HOw steel fiber can IMPROVe
STRENGTH AND DURABILITY of
concrete
Steel fiber reinforced concrete is a castable or sprayable composite material of hydraulic cement,fine, or fine and coarse aggregates with discrete steel fibers of rectangular cross-section randomly dispersed throughout the matrix. Steel fibers strengthen concrete by resisting tensile cracking. Fiber-reinforced concrete has a higher flexural strength than that of unreinforced concrete and concrete reinforced with a welded wire fabric. But unlike conventional reinforcement which strengthens in one or perhaps two directions–steel fibers isotropically reinforce, greatly enhancing the concretes resistance to cracking, fragmentation, spalling, and fatigue. When bending stresses an unreinforced concrete beam, its deflection increases in proportion to the load at a point where failure occurs and the beam breaks apart. Note that the unreinforced beam fails at point A and a deflection of B. A Steel fibre reinforced beam will sustain a greater load before the first crack occurs (point C). It will also undergo considerably more deflection before the beam breaks apart (point D). The increased deflection from point B to point D represents the toughness imparted by fiber reinforcement. Note that at point A and a B deflection, the unreinforced beam fails. Before the first crack (point C) occurs, a steel fiber reinforced beam will carry a greater load. Before the beam breaks apart (point D), it will also undergo significantly more deflection. The increased deflection from point B to point D is the toughness of fiber reinforcement. The increased deflection from point B to point D is the strength that fiber reinforcement imparts. The load of the first crack is called the first crack force. The first crack strength is generally proportional to the amount of fibre in the mix and the concrete mix design.
Two theories have been proposed to explain the strengthening mechanism. The first proposes that as the spacing between individual fibers become closer, the fibers are better able to arrest the propagation of micro-cracks in the matrix. The second theory holds that the strengthening mechanism of fiber reinforcement relates to the bond between the fibers and the cement. It has been shown that micro-cracking of the cement matrix occurs at very small loads. Steel fibers, then serve as small reinforcing bars extending across the cracks. So as long as the bond between the fibers and cement matrix remains intact, the Steel fibers can carry the tensile load. The surface area of the fiber is also a factor in bond
strength.
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