Post-tension cables — the hidden danger in concrete floors

Post-tension construction is one of the most useful structural techniques in modern building. It produces longer spans, thinner slabs, and lighter buildings — and it dominates a significant fraction of UK commercial and residential floor construction. It is also the source of the most expensive avoidable mistake any drilling team can make. Here is what post-tension floors are, why they are dangerous if treated as conventional reinforced concrete, and how to work safely on them.

How post-tension construction works

A post-tension floor contains high-tensile steel tendons that are stressed after the concrete has cured. The tendons run inside ducts, are anchored at one end, stressed using a hydraulic jack at the other, and then locked off. The result is a slab that is held in compression by the cable’s tension load.

Compression is exactly what concrete is good at. By pre-loading the slab in compression, post-tension construction allows much thinner slabs and much longer spans than conventional reinforced concrete would. A typical post-tension floor in a modern commercial building might be 200 mm thick where a conventional reinforced equivalent would need 300 mm.

There are two common types: bonded post-tension, where the tendon is grouted into the duct after stressing, and unbonded, where the tendon remains free to move inside its duct. Both are widely used in the UK; unbonded is more common in residential floors, bonded is more common in heavier commercial work.

Why they are dangerous to drill

A stressed post-tension tendon can carry a load of several hundred kilonewtons. Damaging a tendon — even partially — can cause:

• An immediate structural redistribution as the tendon’s load is shed onto the rest of the slab.
• Unzipping of the slab if the tendon’s anchorage is compromised.
• Costly remedial works including replacement tendon installation, reanalysis, or large-scale repair.
• In the worst cases, partial collapse of slab sections.

In an unbonded system, the tendon can also fly out at speed when stressed and severed, with serious safety consequences for the operator.

These are not theoretical risks. Every year there are well-documented incidents of post-tension strikes in the UK, almost all of them on jobs where pre-drill scanning was either not done or done inadequately.

How to identify a PT floor

A clear indicator is the project documentation: design drawings will identify the floor as post-tension, the structural specification will reference PT design codes, and the floor edges may have visible anchorage stressing pockets.

Where documentation is missing — common on refurbishment projects — physical clues include:

• Slab thickness that is too thin for the span in conventional RC.
• Stressing anchorage points visible at slab edges, often as small recessed pockets.
• Edge distress patterns that suggest tendon load.
• A slab with light conventional reinforcement but capable of long spans.

If there is any doubt, the only safe assumption is that the floor may be post-tension and that pre-drill scanning is mandatory before any penetration.

How GPR finds tendons

Post-tension tendons appear in GPR data as long, often slightly curved reflectors. Their depth typically follows a parabolic profile across the span — drooping to the bottom of the slab in the middle and rising to the top near the supports. A trained GPR surveyor recognises the profile and can map the tendon path with confidence.

The surveyor will also identify the position of the conventional reinforcement that almost always sits alongside the tendons in a PT floor. The combined map gives the drilling team a clear picture of which positions are safe and which are not.

What to expect on a PT scan

A defensible PT scan deliverable includes:

• Identification of every tendon path within the surveyed area.
• A clear distinction between tendons and conventional reinforcement.
• Depth profile information at the surveyed positions.
• An explicit statement of any areas where the surveyor recommends additional intrusive verification.
• A signed-off drilling plan with safe positions only.

Surveyors with EuroGPR Certified Surveyor competencies are the appropriate level of qualification for PT work. On any non-trivial PT job, that level of qualification should be the minimum standard required.

The contractor’s role

Working safely on a PT floor requires:

• A documented method statement that explicitly addresses PT detection.
• Pre-drill scanning of every drilling position by a qualified surveyor.
• A drilling plan signed off against the survey.
• Drilling supervised by someone empowered to stop work if any unexpected resistance is felt during drilling.
• Awareness across the trades on site that the floor is PT — not just the drilling team.

The cost of these mitigations is small compared with the cost of an incident. Insurers and main contractors increasingly mandate them as a condition of access to PT floors.

The single biggest piece of advice

If there is any doubt whatsoever about whether a slab is post-tension, treat it as if it is until evidence proves otherwise. The downside of a precautionary scan is half a day on programme. The downside of a strike on an actual PT tendon is orders of magnitude worse. There is no situation in which “we’ll just go ahead and drill” is the right call on a slab that might be post-tension.