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fiber reinforcement cement and concrete

fiber reinforcement cement and concrete - CBD-223...

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CBD-223. Fibre-Reinforced Concrete Originally published April 1982. J.J. Beaudoin Concrete lends itself to a variety of innovative designs as a result of its many desirable properties. Not only can it be cast in diverse shapes; but it also possesses high compressive strength, stiffness, low thermal and electrical conductivity and low combustibility and toxicity. Two characteristics, however, have limited its use: it is brittle and weak in tension. Recently, however the development of fibre-reinforced composites in the plastics and aerospace fields has provided a technical basis for improving these deficiencies. This Digest describes the general properties and application of fibre-reinforced concrete used in construction. The promise of thinner and stronger elements, reduced weight and controlled cracking by simply adding a small amount of fibres is an attractive feature of fibre-reinforced concrete. Role of Fibres When the loads imposed on concrete approach that for failure, cracks will propagate, sometimes rapidly; fibres in concrete provide a means of arresting the crack growth. Reinforcing steel bars in concrete have: the same beneficial effect because they act as long continuous fibres. Short discontinuous fibres have the.advantage, however, of being uniformly mixed and dispersed throughout the concrete. Fibres are added to a concrete mix which normally contains cement, water and fine and coarse aggregate. Among the more common fibres used are steel, glass, asbestos and polypropylene (Table 1). Table 1. Physical and Mechanical Properties of Selected Fibres. Fibre Diameter µm Specific Gravity Failure Srain, % Modulus of Elasticity, GPa Tensile Strength, GPa Steel 5-500 7.8 3-4 200 1-3
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Glass 9-15 2.6 2-3.5 80 2-3 Polypropylene 7.5 0.9 20.0 5 0.5 Mica Flakes 0.01-200 2.9 -- 170 0.25 Asbestos 0.02-202.5-3.4 2.3 200 3 Carbon 7.5 1.7-2.0 0.5-1.0 300-400 2-3 If the modulus of elasticity of the fibre is high with respect to the modulus of elasticity of the concrete or mortar binder, the fibres help carry the load, thereby increasing the tensile strength of the material. Increases in the length:diameter ratio of the fibres usually augment the flexural strength and toughness of the concrete. The values of this ratio are usually restricted to between 100 and 200 since fibres which are too long tend to "ball" in the mix and create workability problems. As a rule, fibres are generally randomly distributed in the concrete; however, processing the concrete so that the fibres become aligned in the direction of applied stress will result in even greater tensile or flexural strengths. Fabrication Before mixing the concrete, the fibre length, amount and design mix variables are adjusted to prevent the fibres from balling. Satisfactory reinforced mixes usually contain a mortar volume of about 70 per cent compared with a mortar volume of about 50 percent for typical unreinforced concrete mixes.
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