FIBER REINFORCED CONCRETE

Fiber reinforced concrete may be defined as concrete made with hydraulic cement, containing fine or fine and coarse aggregate and discontinuous discrete fibers. The fibers can be made from natural material like asbestos, cellulose, sisal or are a manufactured product such as glass, carbon, steel and polymer. The purpose of reinforcing the cement based matrix with fibers are to increase the tensile strength by delaying the growth of cracks and to increase the toughness by transmitting stress across a cracked section so that much larger deformation is possible beyond the peak stress than without fiber reinforcement. Fiber reinforcement improves the impact strength and fatigue strength and also reduces shrinkage.

Figure 1. Fiber Reinforced Concrete

FACTORS AFFECTING PROPERTIES OF FIBER REINFORCED CONCRETE

Fiber reinforced concrete is a composite material consisted of fibers in cement matrix either in an orderly manner or randomly distributed manner. Due to the presence of these fibers the properties of fiber reinforced concrete vary a lot. The factors affecting the properties of fiber reinforced concrete are given as under:

1. Relative fiber matrix stiffness: For effective and efficient transfer of stress, the modulus of elasticity of the matrix must be lower than that of fiber used. The fibers of low modulus of elasticity such as polypropylene and nylon impart greater degree of toughness and resistance to impact as they have the capacity of absorption of large amount of energy, but they do not contribute to the improvement of strength and stiffness of concrete.
2. Volume of fibers: The quantity of fibers used in fiber reinforced concrete is usually 1 to 5% by volume. The strength of the composite largely depends on the quantity of fibers used in it. With the increase in volume of fibers upto 1.5% tensile strength and toughness of the composite increases. The use of higher percentage of fiber is likely to cause segregation and harshness of concrete and mortar.
3. Aspect ratio of the fiber: It is the ratio of the length to mean diameter of the fiber. The stress bearing capacity of the fiber depends upon its aspect ratio. The increase in aspect ratio upto 75 increases the ultimate strength and relative toughness linearly, but beyond this value of aspect ratio both the ultimate strength and relative toughness decreases.
4. Orientation of fibers: It has been observed that the orientation of fibers influences a lot on the strength of concrete. The maximum benefit occurs when the fiber is unidirectional and parallel to the tensile stress. Experiments have shown that the fibers aligned parallel to the load offered more tensile strength and toughness than randomly or perpendicular distributed fibers.
5. Workability and Compaction of Concrete: Incorporation of steel fiber decreases the workability considerably. This situation adversely affects the consolidation of fresh mix. Even prolonged external vibration failed to compact the concrete. Another effect of low workability is non-uniform distribution of fibers. The workability of the fiber reinforced concrete is improved by increasing the water/cement ratio or the use of some water reducing admixtures like plasticizers and super plasticizers.

USES OF FIBER REINFORCED CONCRETE

The following are the important uses of fiber reinforced concrete:

1. Fiber reinforced concrete can be used for all types of works as road pavements, industrial flooring, bridge decks, canal lining, explosive resistant structures, refractory linings etc.
2. Fiber reinforced concrete can also be used for the fabrication of precast products like pipes, boats, beams, stair case steps, wall panels, roof panels, manhole covers etc.
3. Fiber reinforced concrete is also being tried for the manufacture of prefabricated formwork moulds of “U” shape for casting lintels and small beams.