What happens if you cut a post-tension cable

Most of the concrete in Dubai's high-rise stock is post-tensioned. Steel tendons are run through the slab in ducts, tensioned to enormous force once the concrete cures, and anchored at the edges. That tension is what allows the slab to span further and sit thinner than reinforced concrete would allow. It is also what makes an accidental cut so serious.

Contractors sometimes treat a strike as a bad day that ends with a patch. It does not. Understanding why changes how seriously the scan before the cut gets taken.

The immediate moment

A post-tension tendon holds stored elastic energy along its whole length. When a core barrel severs it, that energy releases at once. The tendon whips inside its duct and the anchorage at the slab edge can fail violently, sometimes ejecting the anchor head and the concrete around it. People have been seriously injured by this, and the danger is not confined to the person holding the drill — the anchor may be many metres away at the edge of the building.

There is usually a loud report, and often visible damage at the anchorage rather than at the cut. On an occupied floor that alone triggers an evacuation and an incident investigation.

What it does to the structure

The slab was designed on the assumption that every tendon is carrying its share of the load. Remove one and the load redistributes to the tendons either side, which were not designed for it. A single severed tendon in a lightly loaded area may be tolerable once an engineer has assessed it. Several, or one in a critical location, may not be.

The consequence is that the decision leaves your hands entirely. The slab has to be assessed by a structural engineer, and the outcome is theirs to determine. Options range from accepting the loss, to installing external post-tensioning or carbon fibre strengthening, to structural propping while a permanent solution is designed.

A cut tendon cannot be repaired by filling the hole. The tension is gone from the entire length of that tendon, not just at the point of damage.

What it actually costs

The core hole is the cheapest part. The real cost sits in everything the strike sets in motion:

• Immediate stop-work on the affected area, often the whole floor
• A structural engineer's assessment, which has to be commissioned and waited for
• Remedial design and works — external PT, carbon fibre wrap or a structural repair
• Programme delay for every trade sequenced behind that penetration
• An incident report, and on many projects a formal investigation
• Damage to your standing with the main contractor and the consultant

On a live fit-out with a fixed handover date, the delay routinely costs more than the physical repair. That is the calculation worth keeping in mind when scanning looks like an expense to trim.

Why drawings are not enough

The obvious objection is that the tendon layout is on the drawings. Sometimes it is, and sometimes the drawings are accurate. But tendons are laid by hand into a profile that curves through the slab depth, and the as-built position routinely deviates from the design. Ducts get shifted to clear penetrations, service routes and reinforcement congestion during the pour.

Then the building ages. Fit-outs come and go, penetrations get made, drawings get superseded, and the set that reaches you on site is often not the set that reflects the slab. In an older building the drawings may not exist at all.

A drawing gives you an expectation. A scan gives you the position. On a post-tensioned slab, the difference between those two is the whole risk.

What scanning actually involves

Ground penetrating radar images the inside of the slab non-destructively. We use the Proceq GP8000, which reads up to 1 m into concrete and produces real-time 3D visualisation of what is embedded — reinforcement, post-tension ducts, conduits and voids.

In practice, scanning a penetration takes minutes, not hours. We scan the area around the proposed position, mark the tendons and reinforcement directly on the slab surface, and mark the safe zone where the core can go. If the intended position cannot be cleared, we say so before anything is cut and mark the nearest workable alternative, which is usually only a short distance away.

Unlike X-ray, GPR needs no exclusion zone and no access to both faces of the slab, so it can be done in an occupied building with people working nearby. That is why it has largely displaced X-ray for this work in the UAE.

The rule worth adopting

Scan every penetration in a post-tensioned slab. Not a sample, not the ones that look risky. The tendon you did not scan for is the one that costs you the programme, and the marginal cost of scanning the next hole once the operator is already on the floor is small.