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Prestressing beds are used to produce a number of similar units at the same time. Tendons are threaded through a series of moulds and tensioned between anchor plates fixed to concrete abutments at opposite ends of a long stressing bed. The concrete is then cast. When it has achieved sufficient strength, the jacks tensioning the tendons are released, transferring the prestressing force to the individual units.
As an alternative to individual moulds, the concrete may be cast using a horizontal slip-forming or extrusion process, which moves along the length of the prestressing bed. Again the tendons are released once the concrete has achieved sufficient strength. The resulting continuous unit is cut into the required lengths. The approach is used for hollow core flooring units and lintels. One advantage of this form of construction is that it obviously does not require any moulds and is readily adaptable to meet varying length requirements.
To assit with the concrete strength development the beds are sometimes heated using embedded hot water pipes or an electric system. This enables a faster turn round of units.
Acknowledgement: The Concrete Society
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