CONTENTS
Modern reinforced concrete can contain varied reinforcing materials made of steel, polymers or alternate composite material in conjunction with rebar or not. Reinforced concrete may also be permanently stressed (concrete in compression, reinforcement in tension), so as to improve the behaviour of the final structure under working loads.
Below are the Different Types of Reinforced Concrete:
[1] REINFORCED CEMENT CONCRETE (RCC)
Concrete is good in resisting compression but is very weak in resisting tension. Hence reinforcement is provided in the concrete wherever tensile stress is expected. The best reinforcement is steel, since tensile strength of steel is quite high and the bond between steel and concrete is good. As the elastic modulus of steel is high, for the same extension the force resisted by steel is high compared to concrete.
However in tensile zone, hair cracks in concrete are unavoidable. Reinforcements are usually in the form of mild steel or ribbed steel bars of 6 mm to 32 mm diameter. A cage of reinforcements is prepared as per the design requirements, kept in a form work and then green concrete is poured. After the concrete hardens, the form work is removed. The composite material of steel and concrete now called R.C.C. acts as a structural member and can resist tensile as well as compressive stresses very well.
PROPERTIES AND REQUIREMENTS OF GOOD R.C.C.
[1] It should be capable of resisting expected tensile, compressive, bending and shear forces.
[2] It should not show excessive deflection and spoil serviceability requirement.
[3] There should be proper cover to the reinforcement, so that the corrosion is prevented.
[4] The hair cracks developed should be within the permissible limit.
[5] It is a good fire resistant material.
[6] When it is fresh, it can be moulded to any desired shape and size.
[2] REINFORCED BRICK CONCRETE (RBC)
It is the combination of reinforcement, brick and concrete. It is well known fact that concrete is very weak in tension. Hence in the slabs, lintels and beams the concrete in the portion below the neutral axis do not participate in resisting the load. It acts as a filler material only. Hence to achieve economy the concrete in tensile zone may be replaced by bricks or tiles. Dense cement mortar is used to embed the reinforcement. The reinforcement may be steel bars, expanded mesh etc.
[3] FIBRE REINFORCED CONCRETE (FRC)
Plain concrete possesses deficiencies like low tensile strength, limited ductility and low resistance to cracking. The cracks develop even before loading. After loading micro cracks widen and propagate, exposing concrete to atmospheric actions. If closely spaced and uniformly dispersed fibres are provided while mixing concrete, cracks are arrested and static and dynamic properties are improved. Fibre reinforced concrete can be defined as a composite material of concrete or mortar with discontinuous and uniformly distributed fibres. Commonly used fibres are of steel, nylon, asbestos, coir, glass, carbon and polypropylene. The length to lateral dimension of fibres range from 30 to 150. The diameter of fibres vary from 0.25 to 0.75 mm.
Fibre reinforced concrete is having better tensile strength, ductility and resistance to cracking.
USES OF FIBER REINFORCED CONCRETE
[1] For wearing coat of air fields, roads and refractory linings.
[2] For manufacturing precast products like pipes, stairs, wall panels, manhole covers and boats.
[3] Glass fibre reinforced concrete is used for manufacturing doors and window frames, park benches, bus shelters etc.
[4] Carbon FRC is suitable for structures like cladding and shells.
[5] Asbestos FRC sheets are commonly used as roofing materials.
[4] PRESTRESSED CONCRETE
Strength of concrete in tension is very low and hence it is ignored in R.C.C. design. Concrete in tension is acting as a cover to steel and helping to keep steel at desired distance. Thus in R.C.C. lot of concrete is not properly utilized. Prestressing the concrete is one of the method of utilizing entire concrete. The principle of prestressed concrete is to introduce calculated compressive stresses in the zones wherever tensile stresses are expected in the concrete structural elements. When such structural element is used stresses developed due to loading has to first nullify these compressive stresses before introducing tensile stress in concrete. Thus in prestressed concrete entire concrete is utilized to resist the load.
Another important advantage of PSC is hair cracks are avoided in the concrete and hence durability is high. The fatigue strength of PSC is also more. The deflections of PSC beam is much less and hence can be used for longer spans also. PSC is commonly used in the construction of bridges, large column free slabs and roofs. PSC sleepers and electric piles are commonly used. The material used in PSC is high tensile steel and high strength steel. The tensioning of wires may be by pretensioning or by post tensioning. Pretensioning consists in stretching the wires before concreting and then releasing the wires.
In case of post tensioning, the ducts are made in concrete elements. After concrete of hardens, prestressing wires are passed through ducts. After stretching wires, they are anchored to concrete elements by special anchors.
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