Last reviewed 30 Sept 2026 · 6 min read
Why losses occur
The force in a tendon is highest at the jack. From the moment it is applied it begins to fall. The concrete shortens, the steel relaxes and friction eats into the force along a curved duct. The effective prestress that remains after all losses is what carries service load; the initial prestress is what the concrete sees at transfer.
Total losses are typically 15–25 % of the initial prestress (higher for pre-tensioned members with large elastic shortening and relaxation, lower for post-tensioned with low-relaxation strands), so they must be estimated with care: too low an estimate over-stresses, too high wastes steel.
Classification of losses
| Loss | Pre-tensioning | Post-tensioning |
|---|---|---|
| Elastic shortening of concrete | Yes — full | Small (when tendons are stressed one by one, the earlier ones lose) |
| Friction (curvature + wobble) | No (except at deflection points) | Yes — significant |
| Anchorage slip | Small (at the bed end) | Yes — draw-in at wedge anchorage |
| Shrinkage of concrete | Yes | Yes (smaller, as some shrinkage precedes stressing) |
| Creep of concrete | Yes | Yes |
| Relaxation of steel | Yes | Yes |
Losses before transfer/anchoring (friction, slip, elastic shortening) are immediate losses; shrinkage, creep and relaxation are time-dependent losses.
Elastic shortening
When the prestress is transferred, the concrete shortens elastically under the compression, and the bonded tendon shortens with it, losing tension.
Pre-tensioned: , where (modular ratio) and is the concrete stress at the level of the steel from the prestress (and self-weight) at transfer.
Post-tensioned (tendons stressed in sequence): the loss in each tendon due to later ones; for identical tendons stressed one by one, the average loss is about ; the last tendon suffers none, the first the most. For a single tendon there is no loss (the jack compensates).
Friction loss (post-tensioning)
As the tendon is pulled through the duct, friction at the duct wall and wobble of the duct reduce the force with distance from the jack:
= force at the jack; = total angular change of the tendon between the jack and the section (radians); = coefficient of friction between the tendon and duct (of the order 0.15–0.25 for strands in corrugated metal ducts — the code or supplier gives the value); = wobble coefficient per metre (of the order 0.001–0.002/m, arising from unintended deviations of the duct); = length from the jack.
For small values the loss is approximately . Friction loss can be reduced by stressing from both ends, using lubricants, keeping ducts smooth and short, and using larger radii.
A tendon is stressed to 1000 kN at one end. Its length to the far end is 20 m, total angular change = 0.15 rad, = 0.20 and = 0.0015/m.
— a loss of 5.8 %.