Post-tensioned slabs

Tensioned steel squeezes the slab thin — longer spans, less concrete, and a file you must never lose.

Post-tensioned slabs — Concrete Frame Construction

Last updated 2026-07-28 by the BuildPedia Editorial Team.

What is Post-tensioned slabs?

Post-tensioning compresses a concrete slab with high-strength steel tendons tensioned after the concrete has gained strength. The precompression cancels the tension that bending would otherwise cause, so the slab can be thinner and span further than ordinary reinforced concrete — typically saving 20–30% of slab depth, with flatter soffits, fewer columns and less concrete, steel and embodied carbon. On large floor plates — offices, car parks, big residential plates — PT is often the economic winner once the tendons' cost is weighed against the frame and height savings.

The technique is disciplined chemistry and physics. Tendons — greased-and-sheathed monostrands in flat ducts, or bonded multistrand in grouted ducts — are laid to a designed profile, high over supports and low at midspan, on the formwork with the ordinary reinforcement. After the pour, at the specified concrete strength, each tendon is stressed with a hydraulic jack to a calculated force, and the measured elongation is checked against the calculation: force without matching extension means a blocked duct or a broken strand. Anchorages are locked off, pockets made good, and bonded systems are grouted.

PT creates a permanent legacy obligation. The slab is full of tensioned steel at tonnes of force: coring or drilling without scanning and reference to the stressing records can sever a tendon with explosive release — the "do not drill" warning is literal, and the records must survive the building. On site, the watchpoints are tendon profile accuracy (the profile is the structural design made physical), anchorage zone reinforcement (where the force actually lands), and the exclusion zone during stressing — a failed anchorage is a projectile. Done by a specialist PT contractor with rigorous records, it is routine; done casually, it is a structural and safety hazard rolled into one.

How does Post-tensioned slabs work, step by step?

Step 1: Design the tendon layout into the pour plan

Design the tendon layout into the pour plan — Post-tensioned slabs, step 1

Tendon profiles, anchorage positions, stressing sequence and pour breaks are engineered and drawn before formwork. Sleeves and cast-ins are coordinated against the tendon layout — a services sleeve through a tendon path is a redesign, not a site adjustment.

Step 2: Lay the tendons to profile

Lay the tendons to profile — Post-tensioned slabs, step 2

After bottom reinforcement, tendons are unrolled and supported on chairs at the designed profile heights, checked at support and midspan points. Ducts are undamaged, anchorages fixed true to the form with their local bursting reinforcement, and stressing pockets formed. The pre-pour inspection signs the profile — after concrete, it is unverifiable.

Step 3: Pour with tendon protection

Pour with tendon protection — Post-tensioned slabs, step 3

Concrete is placed and vibrated with care around anchorages and ducts — congested anchorage zones get small pokers and close attention, because honeycomb where the force lands is a structural defect. Slump and cubes per plan, with extra cubes marked for the stressing-strength check.

Step 4: Stress at proven strength

Stress at proven strength — Post-tensioned slabs, step 4

At the specified cube strength, the specialist crew stresses each tendon in the designed sequence with a calibrated jack, recording force and elongation per tendon. Both must match the calculation within tolerance; discrepancies stop the operation for investigation. The exclusion zone is enforced — nobody stands behind or in line with a live jack.

Step 5: Lock off, grout and make good

Lock off, grout and make good — Post-tensioned slabs, step 5

Tendons are locked off at the anchorages, tails cropped, and bonded systems grouted through the ducts with tested grout. Stressing pockets are made good with shrinkage-compensated mortar. Every record — force, elongation, grout test — is filed against the tendon reference.

Step 6: Mark, record and protect for the building's life

Mark, record and protect for the building's life — Post-tensioned slabs, step 6

PT zones are marked on the as-builts and physically where the design requires; the stressing records go into the handover and H&S file as permanent documents. Trades following are briefed: scan before you drill, and the records are the law.

What are the benefits of Post-tensioned slabs?

What are the limitations of Post-tensioned slabs?

What is Post-tensioned slabs best suited for?

What plant does Post-tensioned slabs need?

How is Post-tensioned slabs quality-checked?

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