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Plain concrete possesses very low tensile strength. It has got limited ductility and little resistance to crack propagation. Micro cracks develop in the material during its manufacture due to its inherent volumetric and micro structural changes and an essentially discontinuous heterogeneous system. This exists even before nor external load is applied. It has little resistance to tensile crack propagation and hence under load virgin micro cracks develop at a low tensile cracking strain of the order of 100 x 10 m/m corresponding to 30 to 40% of ultimate strength in compression which on further loading eventually lead to uncontrolled growth of micro cracks. This, in turn, results in a low fracture toughness, limited resistance to impact and explosive loading.
So to use concrete as load bearing structural member, it is necessary to impart tensile resistance. The concrete is reinforced mainly to increase its resistance against crack propagation. This has led to search for new materials where the weak matrix is reinforced with strong stiff fibres to produce a composite of superior properties and performance.
Romualdi and Batson in 1963 Showed that the low tensile strength of concrete can be overcome by using high strength and high elastic modulus fibres, if the spacing is reduced to less than 1.0 mm. They also showed that the fibres act as crack arresters, restricting the growth of cracks.
Shah S.P. and Vijay Rangan during their studies on smooth low carbon steel fibres concluded that the fibres have negligible effect on the load at which the crack initiates in the matrix. It may not be valid when the percentage of steel volume is high where the higher modulus of elasticity of steel becomes important. They showed that fibres considerably increase the resistance of concrete to crack propagation. The post cracking resistance of fibre largely depends on their bond length i.e. aspect ratio of orientation with respect to cracking direction. The performance of fibres can be improved by increasing the bond length of fibres. They used a composite material approach to predict the reinforcing action of steel fibres.
Jackynder and David Lankard showed that the first crack strength and ultimate strength of steel fibrous mortar were influenced by the length, diameter and quality of the steel fibre. Significant increase in the first crack flexural strength (upto threefold) and ultimate flexural strength (upto four fold) of mortar and concrete can be achieved by use of short and small diameter steel fibres. They found out linear relationship between first crack flexural strength and ultimate flexural strength as a function of fibre content. The first crack flexural strength increased significantly as a function of decreased fibre spacing below 5.7 mm when fibre length and percentage were held constant and spacing decreased through the use of small diameter fibres. Addition of coarse aggregates to a fibre containing mortar results in a decrease in the first crack and ultimate flexural strength of the material.
Wai-fah-chen and J.L. Carson have investigated the stress-strain properties of random wire reinforced concrete. They carried out indirect tensile test (split cylinder test) and uniaxial compression test and concluded that the mortar gave an optimum tensile and compressive strength at 2% of the 2.54 cm and 1.27 cm fibre wire respectively. Higher strengths and greater ductility can be obtained by using different size wires and higher percentage of reinforcement. The age of the material does not significantly affect the tensile strength ratio of either the mortar or concrete. However, it increases the compressive strength ratio for the mortar material and decreases the strength ratio for the concrete material in the order of about ten percent.
R.N. Swamy wrote in his article in Indian Concrete Journal of 1974 January, about different types of fibres. He believes that asbestos, glass and steel all have high elastic moduli and can be used at higher temperature than the low modulus fibres like nylon and polypropylene. The great improvement in impact resistance and ductility at failure provided by glass, steel and plastic fibres are not reflected by asbestos. Glass and asbestos fibres greatly improve the fire resistance, while steel wire reinforcement increases the thermal conductivity of concrete and counteracts the effect of a temperature gradient.
Shridhara, S. Kumar and M.A. Sinare investigated the ability of nylon, jute and coir fibres. They found that nylon even at low fibre contents of ½ to 1% is found to be most effective for increasing the impact strength of concrete. Coir and jute also increase impact strength although not to the same extent as nylon and of course with higher percentage content. They also confirmed the ability of nylon fibres to withstand blast effects. Far coir and jute, there is optimum percentage of fibres to be used for maximum impact strength and they are also affected by alkaline medium without reducing impact strength.
F.J. Grimer and M.A. Ali showed that successful composite action was achieved by reinforcing cement with glass fibres. The increase in modulus of rupture was 2 to 4 tines of matrix strength and impact strength 10 to 30 times of matrix strength. The composites started to decrease in strength for a period of 5 to 30 days because of alkali attack on glass.
Rajgopalan, V.S. Parmeshwaran and G.S. Ramaswamy proved that closely spaced and well bounded steel fibres increases the strength of concrete beams both at first crack and at failure. The fibre impart enormous ductility and large rotation capacity.
Batson, E. Jankins and R. Spatney investigated the ability of steel fibres as shear reinforcement in beams and found that the replacement of vertical stirrups by round, flat or crimped steel fibres provided effective reinforcement against shear failure. The shear span ratio decreased with increasing fibre content.
Thus in few decade, fibre reinforced concrete has developed from a mere laboratory experiment into a proven construction material for the future. We believe that despite the remarkable progress achieved in recent times, there is still a vast future potential for the development of fibre reinforced concrete of all kinds.