Before you core, cut or drill a post-tensioned slab, scan it with ground penetrating radar (GPR) and mark every tendon on the surface. Post-tension tendons carry enough force that cutting one can damage the slab, injure workers and require specialty repairs. This guide explains what post-tension slabs are, how GPR finds tendons, and what a good scan gives you before work starts.
A post-tensioned (PT) slab is reinforced with high-strength steel strands, called tendons, that are pulled tight after the concrete cures. Most building tendons are half-inch monostrand cables in plastic sheathing. Each one is stressed to roughly 33,000 pounds and locked off with anchors at the slab edge. That tension lets the slab span farther and stay thinner than a conventionally reinforced slab, which is why PT is common in parking garages, high-rise residential, offices and hospitals.
When a tensioned strand is cut, the stored force releases at once. The strand can whip out of the slab edge or blow out the concrete around it, and the slab loses part of its capacity in that area. Repair means a specialty PT contractor splicing and re-stressing the tendon, often with the engineer of record reviewing the slab. That can take the area out of service for weeks, on top of the safety risk to anyone nearby.
A GPR unit rolls across the slab and sends radar pulses into the concrete. Steel tendons, rebar and conduit reflect those pulses and show up on screen as distinct curves, so the technician can see each target's position and approximate depth in real time. Tendons are usually told apart from rebar by their draped profile: they sit high over supports and low at midspan, while rebar stays at a constant depth. Banded tendons also run in tight groups along column lines.
GPR reads most reliably in the top 12 to 18 inches of a typical slab. Dense rebar mats, metal deck, wet concrete and very thick sections can limit what it sees, and a good technician will tell you when a reading is uncertain instead of guessing. When a location can't be cleared with confidence, the safer choice is to move the hole.
Trinity Subsurface scans PT slabs in parking garages, hospitals, labs and office buildings across Delaware, Pennsylvania, New Jersey, Maryland and Washington DC. GPR emits no radiation, so occupied buildings stay open while we work. Request a proposal or call (855) 387-4648.
