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Types Of Prestressing

Depending Of the feature of design construction prestressing construction may be classified into number of groups.

a) External Prestressing:- It is prestress a concrete by adjusting its external reactions. For a simply supported beam external prestressing can be done by jacking against abutments . Jacking can be done at proper places to produce compression in the bottom fibres and tension in the top fibres. Such an arrangement, however cannot be easily accomplished in practice as even if abutments favourable for such a layout are obtainable, shrinkage and creep in concrete may completely offset the strains unless they can be readjusted. This type of prestressing is probably suited for arch bridges having rigid abutments.

b) Internal Prestressing:- in such cases prestress is provided by high tensile wire against concrete.Various variations of internal prestressings are discussed below.

1) Linear and circular prestressing:- linear prestressing is applied is in case of straight member like beams, slabs,piles etc. and circular prestressing is applied to circular structures such as circular tanks, silos and pipes.The prestressing tendons in linearly prestressed structures are not necessarily straight but can be either bent or curved. However, they do not go round and round in circular as in case of circular prestressing.

2) Pretensioning & Post Tensioning :-Pretensioning is the method of prestressing in which the tensioning in done before the concret is placed.

Post tensioning is the other hand is a method of prestressing in which the tendon is tensioned after the concrete has hardned.

3) End Anchored and non end Anchored tendons:- In case of post tensioning, the tendons are anchored at their ends by mechanical devices capable of transmitting the prestress to the concrete.

In case of pretensioning the tendons are not anchored at ends and prestress is transmitted to the concrete by bond action near ends of members.

4)Bonded and unbonded tendons:-Bonded tendons tendon denote those bonded throughout their length to the surrounding concrete. Non end anchored tendons are always bonded ones whereas end a anchored tendons may be either bonded or unbonded to the concrete.

5) Precast, Cast in situ and composite construction:-In case of precasting, concreting is done usually in permanent place or somewhere near the site of the structure and eventually erected at the final location.Precasting ensures better control in mass production and is often economical.

Cast in situ construction requires more formwork and scaffolding , but saves the cost of transport and erection. For large and heavy member casting in situ is preferred.

In between these two methods of construction there are certain structures in which part is precast and erected in place , the remaining part being cast in situ.This procedure is called the composite construction.

6) Full and Partial Prestressing:-When  a member is designed so that under working load there is no tensile stress in it,the concrete is said to be fully prestressed. If some tensile stressess are produced in the member under working load,the concrete is consedered to be partially preteressed.

In such a case, additional mild steel bars may have to be provided as reinforcement in the portion of the member under tension.