SFRC is a composite material having fibres characterized by a significant residual tensile strength to absorb strain energy due to fiber bridging mechanisms. It is mainly used for improving long-term behavior, enhancing strength, toughness, and stress resistance. Steel fibre reinforced concrete is a castable or sprayable composite material of hydraulic cements, fine, or fine and coarse aggregates with discrete steel fibres of rectangular cross-section randomly dispersed throughout the matrix. Steel fibres strengthen concrete by resisting tensile cracking. Fibrereinforced concrete has a higher flexural strength than that of unreinforced concrete and concrete reinforced with welded wire fabric.
Effects of steel fibres in concrete
Steel fibres are usually used in concrete to control cracking due to both plastic shrinkage and drying shrinkage. They also reduce the permeability of concrete and thus reduce bleeding of water. Some types of fibres produced greater impact, abrasion and shatter resistance in concrete. Generally fibres do not increase the flexural strength of concrete and so cannot replace moment resisting or structural steel reinforcement. Indeed, some fibres actually reduce the strength of concrete. The amount of fibres added to the concrete mix is expressed as a percentage of total volume of the composite (concrete and fibres), termed volume fraction (Vf). It ranges from 0.1 to 3%. Aspect ratio (l/d) is calculated by dividing fibre length (l) by its diameter (d). fibres with a non-circular cross section use an equivalent diameter for the calculation of aspect ratio. If the modulus of elasticity of the fibre is higher than the matrix (concrete or mortar binder), they help to carry the load by increasing the tensile strength of the material. Increase in the aspect ratio of the fibre usually segments the flexural strength and the toughness of the matrix. However, fibres which are too long tend to ball in the mix and create workability problems. Some recent research indicated that using fibres in concrete has limited effect on the impact resistance of the materials. This finding is very important since traditionally, people think that the ductility increases when concrete is reinforced with fibres.
Reinforcement mechanisms in fibre reinforced concrete (FRC)
In the hardened state, when fibres are properly bonded, they interact with the matrix at the level of micro-cracks and effectively bridge these cracks thereby providing stress transfer media that delays their coalescence and unstable growth. If the fibre volume fraction is sufficiently high, this may result in an increase in the tensile strength of the matrix. Indeed, for some high volume fraction fibre composite, a notable increase in the tensile/flexural strength over and above the plain matrix has been reported. Once the tensile capacity of the composite is reached, and coalescence and conversion of micro-cracks to macro cracks has occurred, fibres, depending on their length and bonding characteristics, continue to restrain crack opening and crack growth by effectively bridging across macro cracks. This post peak macro-crack bridging is the primary reinforcement mechanism in the majority of commercial fibre reinforced concrete composites.
Effects on workability of steel fibre
Slump tests are carried out to determine the workability and consistency of fresh concrete. The efficiency of all fibre reinforcement is dependent upon achievement of a uniform distribution of the fibres in the concrete, their interaction with the cement matrix, and the ability of the concrete to be successfully cast or sprayed. Essentially, each individual fibre needs to be coated with cement paste to provide any benefit in the concrete. Regular users of fibre reinforcement concrete will fully appreciate that adding more fibres into the concrete, particularly of a very small diameter, results in a greater negative effect on workability and the necessity for mix design changes. The slump changed due to the different type of fibre content and form. The reason for lower slump is that adding steel fibres can form a network structure in concrete, which restrain mixture from segregation and flow. Due to the high content and large surface area of fibres, fibres are sure to absorb more cement paste to wrap around and the increase of the viscosity of mixture makes the slump loss.
Effect of steel fibre on compressive, splitting tensile and modulus rupture of concrete
Presently, a number of laboratory experiments on mechanical properties of SFRC have been done. Their investigations conducted a uniaxial compression test on fibre reinforced concrete specimens. The results showed an increase in strength of 6% to 17% compressive strength, 18% to 47% split tensile strength, 22% to 63% flexural strength and 8% to 25% modulus of elasticity respectively. And the mechanical properties of concrete have been studied, these results showed an increase in strength of 6% to 17% compressive strength, 14% to 49% split tensile strength, 25% to 55% flexural strength and 13% to 27% modulus of elasticity respectively. The slabs were 2x2x0.12m, reinforced with hooked end steel fibres and mill cut steel fibres.
Effect of steel fibre on impact capacity and toughness of concrete
Toughness is a measure of the ability of the material to absorb energy during deformation estimated using the area under the stress-strain curves. According to the conducted test on the mechanical properties and resistance against impact on steel fibre reinforced high-performance concrete. Five different geometries of fibres included steel sheet-cut fibres and steel ingot milled fibres with four fibre volume fractions (4%, 6%, 8% and 10%) were applied into the mix. When the fibres were used in a hybrid form, the increase in above study parameters was about 31.42%, when compared to the plain concrete.
Advantages of SFRC:
- Fast and perfect mixable fibres and High performance and crack resistance.
- Optimize costs with lower fibre dosages.
- Steel fibres reinforced concrete against impact forces, thereby improving the toughness characteristics of hardened concrete.
- Steel fibres reduce the permeability and water migration in concrete, which ensures protection of concrete due to the ill effects of moisture.
Steel fibres are widely used as a fibre reinforced concrete all over the world. Lot of research work has been done on steel fibre reinforced concrete and a lot of researchers work prominently over it. This article tried to focus on the most significant effects of addition of steel fibres to the concrete mixes.
ER. Dinesh Biswal, Structural Consultant at Subadra Consultant