Post-Tensioning Tendons

Definition

Post-tensioning is a method of prestressing concrete in which high-strength steel strands or bars are tensioned after the concrete has reached a specified strength, then locked off against anchorages cast into the member. The tensioned steel squeezes the concrete into permanent compression, so the member can carry load and span further with less depth and less reinforcement than conventionally reinforced concrete. Tendons may be bonded — grouted into ducts after stressing — or unbonded, greased and sheathed, stressed against end anchorages only.

Why It Matters

Post-tensioning is what makes a 200 mm slab span eight metres, a transfer beam carry forty storeys, and a bridge deck cantilever over a motorway. But the energy stored in a tensioned tendon is unforgiving: a strand that lets go under 200 tonnes of force will punch through plywood barriers and, tragically, through people. The process is therefore one of the most tightly controlled operations on site — cube results before stressing, calibrated jacks, measured elongation within tolerance, and a permit-style sign-off at every stage. On the record side, every tendon location must survive into the as-built file, because coring through a live tendon twenty years later is a known killer.

The Operation, Step by Step

1. Fix ducts, anchorages and bursting reinforcement exactly to the shop drawings — profile tolerances are typically ±5 mm vertically on slabs.
2. Install strands before or after the pour depending on system; keep duct vents and grout caps accessible.
3. Pour and cure — stressing strength (often 75–80% of characteristic, e.g. 28–32 MPa on a C40 slab) is proven by cubes or temperature-matched cylinders.
4. Stress in the designed sequence with calibrated jacks, recording pressure and elongation; elongation within about ±5–7% of theoretical is the acceptance test.
5. Lock off, crop, and grout bonded systems within the specified window, using bleed-controlled grout and venting from high points.
6. Record everything — stressing records, calibration certificates, grout logs — and mark tendon positions on the as-builts before closure.

Real-World Example

On a post-tensioned transfer plate for a residential tower, the first pour stressed cleanly, but pour two showed elongations 11% over theoretical on four tendons. The young engineer wanted to average the readings and move on; the PT supervisor refused and opened an inspection window. The duct had kinked where a reinforcement chair had been moved during the concrete pour, and the strand was binding at the kink — the jack pressure was real, but the force in the middle of the span was not. The tendons were de-tensioned, re-threaded and re-stressed. That plate now carries twenty-two storeys. The lesson the supervisor drilled into the team: elongation is the truth-teller, pressure is only the story the jack tells.

Practical Lessons Learned

• Cube results gate the programme. If the stressing strength is not proven, the floor cannot be stressed, the formwork cannot drop, and the whole cycle stalls — plan cube breaks into the look-ahead.
• Jack calibration expires quietly. Check certificates before every new pour; an out-of-cal jack invalidates every record it touched.
• Exclusion zones save lives. Nobody stands in line with a live anchorage during stressing — enforce it physically, not with a sign.
• Grouting is not an afterthought. Voids in ducts are where corrosion starts; bleed water at high points is the usual culprit.
• Early loading is tempting and dangerous. Materials stacked on an unstressed PT slab behave nothing like the design assumes.

Expert Tips

• Hold a ten-minute briefing at the anchorage before every stressing operation: sequence, exclusion zone, radio protocol, stop authority. Repetition is the point.
• Photograph each anchorage and duct run before the pour with a tape in shot. It takes minutes and settles arguments years later.
• Plot measured versus theoretical elongation on a simple chart as you go — a drift across pours reveals duct friction problems before a single reading breaches tolerance.
• Stress from both ends on long tendons where the design allows; friction losses on 30 m plus tendons can leave the middle under-stressed otherwise.
• Paint permanent "PT SLAB — NO CORING" markers at stairwells and risers, and lodge the stressing records with the handover pack, not the site cabin.

Common Mistakes

• Stressing on the basis of "the concrete looks old enough" instead of proven cube strength.
• Treating an out-of-tolerance elongation as a paperwork problem and averaging it away rather than investigating duct damage.
• Letting other trades fix services or reinforcement over live ducts after the PT inspection has signed off.
• Cropping strands before the stressing record is accepted, destroying the evidence needed to release the operation.
• Failing to transfer tendon locations into the as-built drawings, leaving a latent hazard for every future fit-out contractor.

Key Takeaways

• Post-tensioning buys span and slenderness at the price of absolute procedural discipline.
• Elongation within tolerance is the real acceptance test; jack pressure alone proves nothing.
• Cube strength, calibrated jacks and an enforced exclusion zone are non-negotiable gates.
• Grout promptly and fully — voids corrode tendons in service.
• The as-built record of tendon positions is a life-safety document for the building's whole life.

Related Concepts

Pairs with Rebar Bending Schedule, Concrete Pour Card, Formwork Cycle Planning, and Method Statement.