Anchor off cables and grout up if required Fig 315 Prestressed concrete

installed the correct way up according to the anticipated loads.

the anchorages, which are subject to very high localised forces. In the bonded system, after tensioning the free space within the ducts is grouted up, which then limits the reliance on the anchorage fixing; however, in the unbonded system the tendons remain free to move independently of the concrete. Tendon ducts are typically manufactured from galvanised steel strip or high-density polythene.

Post-tensioning has the advantage over pre-tension-ing that the tendons can be curved to follow the most efficient prestress lines. In turn this enables long spans of minimum thickness to be constructed. During demolition or structural alteration work, unbonded post-tensioned structures should be de-tensioned, although experience has shown that if demolished under tension, structures do not fail explosively. In alteration work, remaining severed tendons may subsequently require re-tensioning and re-anchoring to recover the structural performance. However, the use of post-tensioning does not preclude subsequent structural modifications.

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