The corrugated duct is used for introducing the strands wich are to be stressed once the concrete element is cured. The special shape achieves total adherence to the concrete element and to the grout used for filling the duct after the strands have been stressed.
The corrugated duct is placed and tied manually to the concrete element reinforcement bars to avoid it moving during filling and vibrating. The diameter of the duct depends on the number of strands.
