Hitting a reinforcing bar while coring is a nuisance. Cutting a post-tension tendon can be dangerous, costly and structurally serious.

What is a post-tensioned slab?

Post-tensioned slabs contain high-strength steel tendons that are stretched after the concrete cures and locked off at the slab edges, putting the slab into compression. This allows thinner slabs and longer spans, which is why post-tensioning is common in car parks, commercial buildings and residential frames.

Why cutting a tendon is so serious

Each tendon holds a very large locked-in force. Cutting one can release that energy violently, reduce the strength of the slab well beyond the cut, and lead to slow, specialist and expensive repairs.

Why you can't avoid tendons by guesswork

Tendons don't sit at one depth. They usually drape, high over supports and low mid-span, and are often bunched in one direction and spread out in the other. Their position can't be worked out from the surface, and drawings rarely show exact as-built positions.

Signs a slab may be post-tensioned include anchorage pockets at slab edges, slabs that seem thin for their span, warning notices, or drawings referring to stressing or PT. If in doubt, treat it as post-tensioned.

How scanning keeps coring safe

GPR scanning locates tendons, ducts, reinforcement and services before any cutting or drilling. Tendons have a distinctive signature and draped profile that sets them apart from ordinary bars. We mark safe zones and no-go areas directly on the slab and scan the soffit where access allows to confirm tendon positions.

Scanning is the first step, not the whole answer. Any opening in a post-tensioned slab should be approved by a structural engineer.

Post-tension scanning in London and across the UK

ACUO Structural Insights provides GPR scanning and post-tension tendon location for contractors, fit-out teams and engineers across London, Kent, Essex and the South East.

For full details on what the service covers, visit our post-tension cable detection service page. For a broader look at what GPR can and can't show in concrete, see our article on what GPR can actually see in concrete. For void detection beneath slabs, see our void detection service.

Planning to core or cut a slab? Call 07340 549892 or email [email protected].