Prestressed Concrete

Since concrete is weak in tension in normal reinforced concrete construction cracks develop in the tension zone at working loads and therefore all concrete in tension is ignored in design.
Prestressing involves inducing compressive stresses in the zone which will tend to become tensile under external loads. This compressive stress neutralizes the tensile stress so that no resultant tension exists, (or only very small values, within the tensile strength of the concrete). Cracking is therefore eliminated under working load and all of the concrete may be assumed effective in carrying load. Therefore lighter sections may be used to carry a given bending moment, and prestressed concrete may be used over much longer spans than reinforced concrete.
The prestressing force also reduces the magnitude of the principal tensile stress in the web so that thin-webbed I - sections may be used without the risk of diagonal tension failures and with further savings in self-weight.
The prestressing force has to be produced by a high tensile steel, and it is necessary to use high quality concrete to resist the higher compressive stresses that are developed.

There are two methods of prestressing concrete :
1) Pre-cast Pre-tensioned
2) Pre-cast Post-tensioned
Both methods involve tensioning cables inside a concrete beam and then anchoring the stressed cables to the concrete.

1) Pre-tensioned Beams

2) Post-tensioned Beams

Loss of Prestress
When the tensioning force is released and the tendons are anchored to the concrete a series of effects result in a loss of stress in the tendons. The effects are :

Total losses in prestress can amount to about 30% of the initial tensioning stress.

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Last Updated : 12/01/02
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Email: david@childs-ceng.demon.co.uk