Fiber reinforced concrete (FRC) is a new structural material which is gaining increasing importance. Addition of fiber reinforcement in discrete form improves many engineering properties of concrete. Currently, very little research work is being conducted within the King-dom using this new material. The use of fibers eliminate the sudden failure characteristic of plain concrete beams. It increases stiffness, torsional strength, ductility, rotational capacity, and the number of cracks with less crack width. In conventionally reinforced concrete beams, fiber addition increases stiffness, and reduces deflection.
Fibers are produced from different materials in various shapes and sizes. Typical fiber materials and types of fibers are following.
It may be Straight, crimped, twisted, hooked, ringed, and paddled ends. Diameter range from 0.25 to 0.76mm
Straight, Diameter ranges from 0.005 to 0.015mm (may be bonded together to form elements with diameters of 0.13 to 1.3mm).
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Wood, asbestos, cotton, bamboo, and rockwool. They come in wide range of sizes
Plain, twisted, fibrillated, and with buttoned ends.
Kevlar, nylon, and polyester. Diameter ranges from 0.02 to 0.38mm. A convenient parameter describing a fiber is its aspect ratio (LID), defined as the fiber length divided by an equivalent fiber diameter. Typical aspect ratio ranges from about 30 to 150 for length of 6 to 75mm.
Mixing of FRC can be accomplished by many methods. The mix should have a uniform dispersion of the fibers in order to prevent segregation or balling of the fibers during mixing. Most balling occurs during the fiber addition process. Increase of aspect ratio, volume percentage of fiber, and size and quantity of coarse aggregate will intensify the balling tendencies and decrease the workability. To coat the large surface area of the fibers with paste, experience indicated that a water cement ratio between 0.4 and 0.6, and minimum cement content of 400 kg/m are required. Compared to conventional concrete, fiber reinforced concrete mixes are generally characterized by higher cement factor, higher fine aggregate content, and smaller size coarse aggregate. A fiber mix generally requires more vibration to consolidate the mix. External vibration is preferable to prevent fiber segregation. Metal trowels, tube floats, and rotating power floats can be used to finish the surface.
Fibers combined with reinforcing bars in structural members will be widely used in the future. The following are some of the structural behaviour.
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The use of fibers in reinforced concrete flexural members increases ductility, tensile strength, moment capacity, and stiffness. The fibers improve crack control and preserve post cracking structural integrity of members.
The use of fibers eliminate the sudden failure characteristic of plain concrete beams. It increases stiffness, torsional strength, ductility, rotational capacity, and the number of cracks with less crack width
Addition of fibers increases shear capacity of reinforced concrete beams up to 100 percent. Addition of randomly distributed fibers increases shear-friction strength, the first crack strength, and ultimate strength
The increase of fiber content slightly increases the ductility of axially loaded specimen. The use of fibers helps in reducing the explosive type failure for columns
Fibers increases the ductility of high strength concrete. The use of high strength concrete and steel produces slender members. Fiber addition will help in controlling cracks and deflections.
Addition of fibers to concrete influences its mechanical properties which significantly depend on the type and percentage of fiber.
The presence of fibers may alter the failure mode of cylinders, but the fiber effect will be minor on the improvement of compressive strength values (0 to 15 percent).
Modulus of elasticity of FRC increases slIghtly with an increase in the fibers content. It was found that for each 1 percent increase in fiber content by volume there is an increase of 3 percent in the modulus of elasticity
The flexural strength was reported[ to be increased by 2.5 times using 4 percent fibers. Toughness For FRC, toughness is about 10 to 40 times that of plain concrete.
The presence of 3 percent fiber by volume was reported to increase the splitting tensile strength of mortar about 2.5 times that of the unreinforced one
The total energy absorbed in fiber as measured by the area under the load-deflection curve is at least 10 to 40 times higher for fiber reinforced concrete than that of plain concrete. Addition of fiber to conventionally reinforced beams increased the fatigue life and decreased the crack width under fatigue loading. At elevated temperature it has more strength both in tension and compression. Cost saving og 10-30% over conventional concrete system.
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