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 is common in UK commercial and residential floors. It is also the source of one of the most expensive avoidable mistakes a 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. For the same span, a post-tension floor can be noticeably thinner than a conventional reinforced slab.

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 used in the UK.

Why they are dangerous to drill

A stressed post-tension tendon carries a very large locked-in force. 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. Tendon strikes do happen, and pre-drill scanning is the main control against them.

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 essential 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 likely tendon path.

The surveyor will also identify the position of the conventional reinforcement that usually sits alongside the tendons in a PT floor. The combined map shows the drilling team where tendons and bars were located, so holes can be positioned away from them.

What to expect on a PT scan

A defensible PT scan deliverable includes:

• Located tendon paths within the surveyed area.
• Where the data allows, a 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 checks.
• Marked-up positions clear of located tendons and bars, for the drilling plan.

EuroGPR Certified Surveyor is one recognised GPR competence scheme. Whatever the scheme, ask who will scan and what experience they have of PT floors.

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 approved 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. Main contractors and insurers may require them before work on 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 shows otherwise. The downside of a precautionary scan is a short delay to the programme. The downside of a strike on an actual PT tendon is far 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.

What this means for your job

If the slab might be post-tensioned, scan it before anyone drills. See post-tension cable location.