Case study: scanning a post-tension slab before coring

An electrician contacted us before coring through a post-tension slab at a retirement home. The work itself was routine. The slab was not.

Post-tension slabs carry steel cables under high tension. Cutting one is not like nicking a piece of rebar: the cable releases its load, and the consequences run from an expensive structural repair to a genuine safety event. In an occupied care facility, neither the repair nor the evacuation is something anyone wants to schedule.

What we scanned

We scanned a 40 by 40 foot section of slab and marked the findings directly on the surface where the coring was to happen:

• Post-tension cables, so the crew knew precisely which lines could not be crossed.

• Rebar and mesh, so cores could be placed between the steel rather than through it.

• Conduit and other utilities in the slab, which rarely sit where the drawings suggest.

The electrician then cored on the same visit, in known clear locations, without stopping to guess or to open exploratory holes.

Why drawings are not enough

As-built drawings for a slab show the design intent. They do not show where a cable was pulled around an obstruction during the pour, where a conduit was added later, or how much the layout shifted in construction. On a building that has been occupied for years, the drawings are a starting point and nothing more.

Ground penetrating radar shows what is in the concrete now. The interpretation matters as much as the instrument: radar returns a change in material, and knowing whether that return is a post-tension cable, a rebar run or a conduit is the part that keeps the core safe.

Where this comes up

Tenant improvements, plumbing and conduit routed through existing floors, equipment anchoring, structural anchors, and any remodel in an occupied building where the slab is live. Hospitals, care facilities, schools and offices all share the same constraint: the work has to happen without disrupting the people using the building.

More on the method: how underground scanning saves existing buildings.

Scanning before you cut

Advanced Underground Utility Locating has been locating private utilities and scanning concrete since 2002. We scan slabs and walls before coring, cutting or drilling, mark the findings on the surface where the work is happening, and can capture them as survey-grade points with RTK collection where the record needs to outlast the job.

Spokane, Coeur d'Alene and the Idaho panhandle: 509-710-2621. Seattle, Tacoma and Portland: 509-954-8623. Bend, Medford and northern California: 541-678-0629. Eugene and Roseburg: 541-780-4149. Coos Bay and the Oregon Coast: 541-315-6728. Emergency service is available 24 hours.