Last reviewed 30 Sept 2026 · 6 min read
Two ways of applying prestress
| Pre-tensioning | Post-tensioning | |
|---|---|---|
| When | Steel stressed before concrete is cast | Steel stressed after concrete has hardened |
| How force is held | Bond between steel and concrete | Anchorages at the ends (and bond if grouted) |
| Where made | Factory or casting yard (long-line beds) | Factory or site |
| Tendon path | Mostly straight (or deflected at fixed points) | Any profile — straight or curved (draped) |
| Member size | Smaller, standardised (sleepers, slabs, girders up to about 30–40 m) | Larger and heavier girders, bridges, tanks |
| Loss issues | Elastic shortening, shrinkage, creep, relaxation | Friction, anchorage slip, plus long-term losses |
Pre-tensioning
- Wires or strands are stretched between fixed abutments (stressing bulkheads) of a long-line bed using hydraulic jacks.
- Concrete is cast around the tensioned steel and cured, often with steam for quick strength gain.
- When concrete reaches the transfer strength, the steel is released slowly (by jacks, or cutting). The steel tries to shorten but is held by bond; the stress passes to the concrete along the transfer length.
Transfer length is the length at each end over which the prestress builds up from zero to the full effective value; it is about 50–100 times the strand diameter (longer for plain wire, shorter for indented wire or strand). Within it the member has less prestress, which is important for shear and flexural checks near supports.
Debonding — covering some strands with plastic sleeves near the ends — reduces end tension at the top and the stresses at transfer, an alternative to deflecting (harping) the strands.
Advantages: economical for mass production, no anchorages, quick turnover. Limits: long beds and strong abutments needed, tendon profile is limited, members must be transported.
Post-tensioning
- Ducts (corrugated metal or plastic) are placed in the formwork to the required profile.
- Concrete is cast and cured.
- Tendons are threaded through the ducts (or pushed in as strands) and stressed with a hydraulic jack against the hardened concrete. The stress is locked in with an anchorage.
- Grouting — cement grout (water–cement ratio about 0.40–0.45 with a non-shrink admixture) is pumped through the duct to protect the steel from corrosion and to bond it to the concrete.
Bonded tendons are grouted; unbonded tendons are greased and sheathed in plastic (used in slabs and buildings). External tendons lie outside the concrete section (in box girders, for strengthening) and are replaceable.
Stressing procedure: the jack pulls the tendon to the specified force; the elongation is measured and compared with the calculated extension (agreement within about 5 % is required); the tendon is anchored and the jack released. Long tendons are stressed from both ends (or the two ends in turn) to reduce friction loss.
Commercial post-tensioning systems
| System | Tendon | Principle of anchorage |
|---|---|---|
| Freyssinet | Groups of high-tensile wires (12 wires of 7 mm) | Wires held in a cone-and-cylinder wedge anchorage; the wedges grip by friction |
| Magnel–Blaton | Wires in pairs (flat cables of 4 or 8 wires) | Sandwich plates with wedges (each pair of wires in a two-grooved wedge) |
| Gifford–Udall / CCL | Wires or strands | Button-head or individual wedges in a bearing plate |
| Lee–McCall | High-tensile alloy bars, threaded at ends | Threaded nut and bearing plate |
| Dywidag / bar systems | Threaded bars | Nut on a plate; used for short tendons, stitching and soil anchors |
| Multi-strand systems (VSL, etc.) | 7-wire strands in groups | Individual strand wedges in a multi-hole anchor head — the most common modern system |
Equipment
- Hydraulic jacks (single or multi-strand) with pressure gauges calibrated against the force; pumps and, for multi-strand tendons, a jack with a stressing frame.
- Anchorage components: bearing plates, anchor heads, wedges, trumpets and transition tubes.
- Grouting equipment: mixer, pump, pressure gauge and vent tubes; vents at high points allow air to escape.
- Ducts and couplers to join tendons in segmental work.
- Measuring equipment for elongation and force.