Post-tension, prestressing strands and ducts

Pre-tensioning and post-tensioning, two ways to prestress

Prestressing puts the concrete into compression before it carries load, so that it performs better in bending. Two processes exist, and the difference lies in when the steel is tensioned.

In pre-tensioning, strands are tensioned before the pour, on a casting bed, then released once the concrete has hardened. The element therefore arrives on site already prestressed. In post-tensioning, strands are threaded through ducts placed before the pour, then tensioned once the concrete has hardened, directly on the structure, and locked off by anchorages at the edges.

The distinction matters for a survey: a post-tensioned slab carries visible edge anchorages and ducts whose position varies along the span, whereas a pre-tensioned element is generally precast and more regular in geometry.

What a duct is and why it matters

The duct is the sheath housing the strand in a post-tensioned slab. It is placed before the pour and follows the profile set by the design, which is why the depth of the cable changes from one point of the span to another.

Two cases arise. In a bonded system, the duct is grouted after stressing, which ties the strand to the concrete along its length. In an unbonded system, the strand stays free within its sheath, protected by grease. The consequence of cutting into one is not the same, so the question belongs to site preparation.

A metal duct and a plastic one do not return the same signal during a survey, which affects the reading. The terms used in reports are defined in the technical glossary.

Telling a strand from reinforcement during a survey

Passive reinforcement generally follows a regular grid at roughly constant depth, in orthogonal mats. A post-tension strand instead follows a curved profile along the span, high near the supports and low at midspan.

That curvature is what rules out any conclusion from a single point: two readings a few steps apart can give very different depths for the same cable. Locating on site therefore remains necessary even where drawings exist.

• Passive reinforcement: regular grid, roughly constant depth
• Post-tension strand: curved profile, depth varying along the span
• Duct: the sheath around the strand, metal or plastic depending on the system
• Service conduit: electrical or plumbing route, with no structural role

What locating can and cannot establish

A survey locates elements and their apparent depth, within a margin that depends on the material and the density of reinforcement. It does not say whether a system is bonded, how much tension remains in a cable, or whether an earlier repair changed the profile.

A dense upper mat reflects much of the signal and leaves little energy for whatever lies beneath. An honest report then states what was seen, and equally what the survey could not establish. The method itself is set out on the concrete scanning page.

When the question arises

It arises as soon as an operation cuts into the material in a structure that may be prestressed: core drilling, sawing an opening, setting an anchor, underpinning. It applies to shallow holes too, since a strand rises within the thickness near the supports.

The reasoning to follow before drilling, and what changes when a cable sits at the planned point, are developed in the article on locating post-tension cables before drilling. For a live project, describe the area and the planned opening on the contact page.

General documentation on post-tensioning is published by the Post-Tensioning Institute.

Frequently asked questions