Plain concrete possess a very low tensile strength, limited ductility and little resistance to cracking, Internal micro – cracks are inherently present in the concrete and its poor tensile strength is due to the propagation of such micro -cracks eventually leading to brittle fracture of the concrete.

In the past, attempts have been made to impart improvement in tensile properties to concrete in embers, by way if using conventional reinforced steel bars and also applying restraining techniques, although both these methods provide tensile strength to the concrete members, they however, do increase the inherent Leslie strength concrete itself.

In plain concrete and similar brittle materials, structural cracks develop even before loading, particularly due to drying shrinkage or other causes of volume changes. The width of these initial cracks seldom exceeds a few microns, but their other two dimensions may be higher magnitude

When loaded, the micro cracks propagate open up, and owing to the effect of stress concentration additional cracks form in places of motion defects. The structural cracks proceed slowly or by liny jumps, because they are retarded by various obstacles, changes of direction in bypassing the more resistant grains in matrix. The development of such micro -cracks is the main cause of ineleastic deformation in concrete.

It has been recognized that the addition of small, closely spaced and uniformly dispersed fibres to concert would act as crack arrester, and would subliminally improve its statie and dynamic properties. This type of concrete is known as Fibre Reinforced concrete. \ FRC can be defined as a composite material, consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibres. Continuous meshes woven fabrics and long wires or rods are not considered to be discrete fibres.

Fibre is a small piece of reinforcing material, possessing certain characteristic properties, they can be circular or flat. The fibre is often described by a convenient parameter called ‘aspect ratio’. The aspect ratio of the fibre is the true ratio of its diameter. Typical aspect ratio runges from

30 to 150.

Steel fibre is one of the most commonly used generally, round fibres are used. The diameter may very from 0.25 to 0.75mm.

The steel fibre is likely to get rusted and lost some of its strengths. But investigations have shown that the rusting of fibres tacks place only at the surface. Use of steel fibres makes significant improvements in flexural, impact and fatigue strength of concrete. It has been extensively used in various types of structures, particularly for overlays of roads, airfield pavements and bridge decks. Thin shells and plates have also been contracted using steel fibres.

Polypropylene and Nylon fibres are found to be suitable to increase the impact strength. They possess very high tensile strength, but their low modules of electricity and higher elongation do not contribute to the flexural strength.

Glass fibre is a recent introduction in making fibre concrete. It has very tensile strength

1020 t 4080 N/mm. Glass fibre, which is originally used in conjunction with cement was found to be affected by alkaline condition of cement. Therefore, alkali-resistant glass by tradename ‘CEM-FIL’ has been developed and used. The alkali resistant fibre reinforced concrete shows

considerable improvement in durability when compared to the conventional E -Glass fibre.

Carbon fibres, perhaps possess very high tensile strength -2110 to 2815 N/mm and Young’s Modulus. It has been reported that cement composite made with carbon fibre, as reinforcement, will have very high modulus of Elaticity and flexural strength. The limited studies have shown good durability. The use of carbon fibres for structure like cladding, panels and shells will have promising future.