Difference Between RCC and Prestressed ConcreteArticles > Difference Between RCC and Prestressed Concrete
In ordinary reinforced cement concrete, compressive stresses are taken up by concrete and tensile stresses by steel alone. The concrete below the neutral axis is ignored since it is weak in tension. Although steel takes up the tensile stresses, the concrete in the tensile zone develops minute cracks. The load carrying capacity of such concrete sections can be increased if steel and concrete both are stressed before the applications of external loads. This is the concept of prestressed concrete. As per ACI committee prestresssed concrete is that concrete in which internal stresses of suitable magnitude are introduced so that the stresses resulting from the external loadings can be counteracted to a desired degree.
In R.C.C. members, prestress induced is of compressive nature so that it balances the tensile stresses produced due to external load. It makes the whole section effective (the concrete area in the tension zone also) in resisting loads.
In reinforced concrete, prestress is commonly introduced by tensioning the reinforcement. So, compression is induced in the zones where external loads would normally cause tensile stresses. If the prestressed concrete beam of rectangular section subjected to a prestressing force P, at the centroidal axis. Due to this force, a uniform compressive stress of ‘P/A’ will be induced in concrete. Under the action of loads, the stress at any points will be ‘My/I’.
Figure showing load on the beam
Materials used in Prestressed concrete
Ordinary mild steel and deformed bars are used in RCC are not used in PSC (prestressed concrete) because their yield strength is not very high. In the prestressed concrete, loss of prestress (about 20 %) occurs due to many factors. If mild steel bars or HYSD bars are used then very little prestress will be left after the losses and will be of no use. Therefore, high tensile strength steel is used for prestressing. In addition to the high strength, the steel used in prestressing must have a higher ultimate elongation. Various forms of steel used for prestressing as follows:
Tendons are high strength tensile wires available in various diameters from 1.5 mm to 8 mm. the following table gives the ultimate tensile strength of steel wires used for prestressing.
Diameter of wire (mm)
Ultimate tensile strength (MPa)
Wires strands or cables:
A strand or cable is made of a bundle of wires spun together. The overall diameter of a cable or strand is from 7 to 17 mm. They are used for post-tensioning systems.
High tensile steel bars of diameter 10 mm or more are also used in prestressing.
Since high tensile steel is used in prestressed concrete, the concrete used should also be of good quality and high strength. Therefore, the code recommends a minimum mix of M 40 for pre-tensioned system and M 30 for post tensioned system. These mixes have high strength and a high value of modulus of elasticity of concrete which results in less deflection.
The concrete used in prestressed concrete should be well compacted. High strength concrete is used in prestressed concrete for following reasons:
- Use of high strength concrete results in smaller sections.
- High strength concrete offers high resistance in tension, shear, bond and bearing.
- Less loss of prestress occurs with high strength concrete.
When repeated loading tests are performed on the different prestressed concrete beams with the varying proportions of steel contained in grouted post tensioned cables. Similar tests when performed on the reinforced concrete beams which are reinforced with plain steel bars or cold worked deformed mild steel bars. Under the repetitions of working load it is observed that the deformations on prestressed concrete beams are slightly increased. Under the similar loads deformations were recorded and it was found that the cracks developed in the reinforced concrete beams.
Advantages of prestressed concrete:
- Prestressed concrete sections are thinner and lighter than RCC sections, since high strength concrete and steel are used prestressed concrete.
- In prestressed concrete, whole concrete area is effective in resisting loads, unlike RCC where concrete below the neutral axis is neglected.
- Thinner sections in prestressed concrete results in less self weight and hence overall economy.
- Long span bridges and flyovers are made of prestressed concrete because of lesser self weight and thinner section. So, prestressed concrete is used for heavily loaded structures.
- Prestressed concrete members show less deflection.
- Since the concrete does not crack in prestressesd concrete, rusting of steel is minimized.
- Prestressed concrete is used in the structures where tension develops or the structure is subjected to vibrations, impact and shock like girders, bridges, railway sleepers, electric poles, gravity dams, etc.
- Precast members like electric poles and railway sleepers are produced in factories using simple pre-stressing methods.
Disadvantages of prestressed concrete:
- Prestressed concrete construction requires very good quality control and supervisions
- Cost of materials used in prestressed is very high (high tensile steel is about three times costlier than mild steel).
- Prestressed concrete requires specialized tensioning equipment and devices which are very costly
- Prestressed concrete sections are more brittle because of use of high tension steel.
The comparisons between the RCC and prestressed concrete
- In RCC beam the concrete in the compression side of the neutral side of the axis alone is effective. The concrete in the tension side of the neutral axis is ineffective. But in the prestressed concrete beam, the entire section is effective.
- Reinforced concrete beams are generally heavy. They always need shear reinforcements besides the longitudinal reinforcement for flexure. Prestressed concrete beams are lighter. By providing the curved tendons and the pre-compression, a considerable part of the shear is resisted.
- In reinforced concrete beams, high strength concrete is not needed. But in prestressed concrete beams, high strength concrete and high strength steel are necessary. High strength concrete is needed to resist high stresses at the anchorages. High strength steel is needed to transfer large prestressing force.
- Reinforced concrete beams being massive and heavy are more suitable in situations where the weight is more desired than strength. Prestressed concrete beams are very suitable for heavy loads and longer spans. They are slender and artistic treatments can be easily provided. Cracks do not occur under working loads. Even if a minute crack occurs when overloaded, such crack gets closed when the overload is removed. The deflections of the prestressed concrete beams are small.
- In reinforced concrete beams, there is no way testing the steel and the concrete. In prestressed concrete beams, testing of steel and concrete can be made while prestressing.
Figure showing difference between RCC and prestress concrete
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