Post-tensioned concrete design
Post-tensioning threads high-strength tendons through a concrete slab and stresses them after the concrete has cured, squeezing the member so it resists load with far less depth and cracking. We design post-tensioned concrete structures to the Eurocodes for clients across the UK.
How is post-tensioned concrete different from reinforced concrete?
Reinforced concrete lets the concrete crack so the bars take the tension. Post-tensioning stresses steel tendons after the concrete cures, squeezing the member into compression before load arrives. That closes cracks and cancels tension, so the slab can be much thinner and span further than a reinforced one.
The idea behind post-tensioning
Ordinary reinforced concrete relies on the concrete cracking in tension so the steel bars can pick the tension up, which limits how thin and how far it can span before the cracks and deflection become a problem. Post-tensioning turns that around by putting the concrete into compression before the working load ever arrives. High-strength steel tendons are run through ducts cast into the member, and once the concrete has gained strength they are tensioned against it and anchored, so the whole member is squeezed. That precompression closes cracks, cancels much of the tension from load, and lets the member work harder for its depth.
The effect is dramatic on spans and slab thickness. A post-tensioned slab can be markedly thinner than a reinforced concrete one over the same span, which saves material and weight on every floor and reduces the overall building height, and it can cross column grids that would need deep downstand beams in reinforced concrete. The tendons are also profiled, draped low at mid-span and high over supports, so they follow the shape of the bending and actively lift the load, a balancing effect that reinforced concrete cannot reproduce. This is why post-tensioning suits long-span floors and transfer structures.
Designing to BS EN 1992
Post-tensioned concrete is designed to BS EN 1992, Eurocode 2, which covers prestressed concrete alongside reinforced. The design balances the prestress force and tendon profile against the loads, checking stresses in the concrete at transfer, when the tendons are stressed and the concrete is youngest, and in service under full load, keeping both within limits so the member neither cracks unacceptably nor is overstressed. Ultimate bending and shear are checked as for any concrete member, with the tendons contributing, and deflection is controlled by the balancing effect of the drape rather than by depth alone.
Prestress losses have to be tracked because the force in the tendon is not constant. Some is lost immediately, from friction along the duct, draw-in at the anchorage as the wedges bite and elastic shortening of the concrete, and more is lost over time as the concrete creeps and shrinks and the tendon relaxes. We calculate these losses so the long-term effective prestress is the value the design relies on, not the jacking force. The tendon layout, anchorage zones, where huge forces concentrate and need bursting reinforcement, and the un-tensioned bar for robustness and crack control are all detailed to make the system work.
Systems, durability and where it suits
Post-tensioning comes in bonded and unbonded systems, and the choice affects both behaviour and durability. Bonded tendons are grouted inside their ducts after stressing, which bonds them to the concrete and protects the steel, while unbonded tendons are greased and sheathed and rely on the anchorages, which makes them quicker and common in slabs but places more importance on protecting those anchorages. Either way the tendons are high-strength steel under permanent high stress, so corrosion protection, grouting quality and anchorage detailing are safety-critical and specified carefully.
Post-tensioning suits long-span floor slabs in offices and residential towers, transfer slabs and beams that carry columns over open spaces below, car parks, and podium and foundation slabs where deflection and crack control matter. Its strengths are thin slabs, long spans, reduced building height and better control of cracking and deflection. Its limits are that it is a specialist system needing an experienced supplier and careful site control, alterations later are far from trivial because you cannot simply cut a slab full of stressed tendons, and it rarely pays on short, lightly loaded spans. We advise where post-tensioning is the right tool and coordinate the design with the specialist system.
What we check.
The points our calculations resolve for a project like this.
- Prestress force and profiled tendon drape balancing the load
- Concrete stresses checked at transfer and in service
- Prestress losses from friction, draw-in, creep, shrinkage and relaxation
- Anchorage zone bursting reinforcement where forces concentrate
- Bonded versus unbonded systems and tendon corrosion protection
- Restrictions on later cutting and alteration of stressed slabs
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
How is post-tensioned concrete different from reinforced concrete?
Reinforced concrete lets the concrete crack so the bars take the tension. Post-tensioning stresses steel tendons after the concrete cures, squeezing the member into compression before load arrives. That closes cracks and cancels tension, so the slab can be much thinner and span further than a reinforced one.
Why can post-tensioned slabs be so thin?
Because the profiled tendons actively balance the load and the precompression controls cracking and deflection, so the slab does not rely on depth alone to stay stiff and uncracked. This lets a post-tensioned slab be markedly thinner than a reinforced one over the same span, saving weight and building height.
Can I alter a post-tensioned slab later?
Only with great care. A post-tensioned slab is full of stressed tendons, and cutting or coring into one without knowing exactly where they run can release enormous force and is dangerous. Any alteration needs the tendon layout, a survey and a specific engineering assessment before work begins.
What is the difference between bonded and unbonded tendons?
Bonded tendons are grouted inside their ducts after stressing, bonding them to the concrete and protecting the steel. Unbonded tendons are greased and sheathed and rely on their anchorages. Unbonded systems are quicker and common in slabs, but put more weight on protecting the anchorages. We advise which suits the project across the UK.
Tell us what you are building.
Send the drawings or describe the project. We confirm scope, deliverables and a realistic timescale.
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