scanning
Why Scan Before You Cut: GPR, Post-Tension and Hidden Utilities
Cutting concrete blind risks striking rebar, conduit and — most dangerously — post-tension cables. Ground-penetrating radar maps what's inside a slab before the blade touches it. Here's what GPR finds and why it matters in Houston.
Concrete hides things. Inside almost any structural slab there is reinforcement, and often conduit, plumbing, radiant tubing or post-tension cable running through paths you cannot see from the surface. Cutting or coring into that concrete without knowing what is there is a gamble with real stakes: a struck conduit means an outage and a repair, a struck water line means a flood, and a struck post-tension cable can injure the crew and compromise the structure. Ground-penetrating radar (GPR) removes the guesswork by imaging what is inside the concrete before the blade or bit ever touches it. This article explains how it works, what it detects, and why on many Houston slabs scanning is not optional.
Cutting blind is a gamble
Cutting blind is a gamble
It is tempting to treat a slab as a solid, uniform block. It rarely is. Reinforcing steel is placed in mats or grids at engineered depths; electrical conduit and data cable are frequently cast into floors; plumbing and drain lines cross slabs; and in Houston, post-tension cables run through a large share of residential and commercial slabs. None of that is visible from above. A cut placed by eye alone is placed by luck — and the consequences of bad luck range from a costly utility repair to a serious injury.
The cost of a strike
A single struck utility can cost more to repair — and more in downtime — than the entire cutting job. A struck post-tension tendon is a safety event, not just a cost event.
How GPR works
How GPR works
Ground-penetrating radar sends short pulses of high-frequency radio energy into the concrete from a handheld antenna rolled across the surface. When a pulse hits something with different electrical properties than the surrounding concrete — a steel bar, an air-filled conduit, a plastic pipe, a cable — part of the energy reflects back. The unit measures the timing and strength of those reflections and builds a picture of what lies below, along with an estimate of its depth. A technician interprets the returns in real time, marking the surface directly above targets so the crew can plan cuts around them.
GPR is non-destructive and one-sided: it needs access to only one face of the concrete and leaves no holes behind, unlike exploratory drilling. It works on slabs, walls, decks and columns. Its accuracy depends on the operator's skill and on conditions — very wet or very heavily congested concrete is harder to read — which is why scanning is a skilled interpretation task, not just a device that beeps.
What GPR finds
What GPR finds
A GPR scan can locate a wide range of embedded targets, including:
- Reinforcing steel — rebar mats, grids, and their approximate depth and spacing
- Post-tension cables and their routing through the slab
- Electrical conduit and cast-in data or communication lines
- Metallic and many non-metallic pipes, including plumbing and drains
- Radiant heating tubing and other embedded conduit
- Voids, honeycombing and changes in slab thickness
- The location of the slab's back face, indicating overall thickness
The practical output is a marked-up surface: lines and symbols showing where it is safe to cut and where it is not. See the concrete scanning service for how this is delivered.
The post-tension danger
The post-tension danger
Post-tension (PT) construction is common across the Houston area because our expansive clay soils move seasonally, and PT slabs resist the resulting stresses well. In a PT slab, high-strength steel tendons are tensioned after the concrete cures and then anchored, putting the whole slab into compression. That tension is exactly what makes PT dangerous to cut. A tendon that is nicked or severed can release its stored energy suddenly — the strand can whip, the anchorage can fail, and the slab loses designed capacity at that point. This is not a minor utility strike; it is a potential injury and a structural concern.
The correct sequence on any slab that may be post-tensioned is to scan first, map the tendon routing, and plan cuts and cores to avoid the cables entirely — or, where a cut must cross a tendon, to involve a structural engineer and a proper detensioning procedure. No cut should ever be placed on a suspected PT slab without scanning. Our cost guide explains why this step is standard rather than an upsell.
Non-negotiable in Houston
If a slab could be post-tensioned, scan before cutting or coring. Given how common PT construction is here, treat unknown slabs as potentially post-tensioned until scanning proves otherwise.
GPR vs. other locating methods
GPR is not the only way to see inside concrete, but it fits most cutting situations. Here is how it compares.
| Method | Detects | Access needed | Destructive? |
|---|---|---|---|
| Ground-penetrating radar (GPR) | Rebar, PT, metallic & non-metallic conduit, voids, thickness | One side only | No |
| Electromagnetic (cover meter) | Metallic reinforcement and conduit | One side | No |
| X-ray radiography | High-detail image of embedments | Both sides (source + film) | No, but requires exclusion zone |
| Exploratory drilling | Confirms a single point | One side | Yes — leaves holes |
When to scan
When to scan
Scanning earns its place whenever cutting blind carries risk. That includes essentially all structural slabs, any slab that could be post-tensioned, floors likely to contain conduit or plumbing, and any location where a strike would be costly or dangerous. It is less critical for shallow, non-structural, clearly unreinforced concrete — but even then, confirming beats assuming. When in doubt, scan.
How scanning fits your project
How scanning fits your project
In practice, scanning happens just before cutting: the technician scans the cut area, marks targets, and the crew lays out cuts and cores in the clear zones. On larger jobs, scanning may inform the design of the openings themselves. Many crews we route work to can perform scanning as part of the same visit as the cutting, which keeps the workflow tight.
To include scanning in your scope, mention it when you request pricing, or upload plans so a crew can see where reinforcement and utilities are expected.
Frequently asked questions
- Is GPR scanning always necessary before cutting?
- Not for every job, but it's strongly recommended for any structural slab, any slab that might be post-tensioned, and any cut where striking a utility would be costly or dangerous. Given how common post-tension construction is in Houston, unknown slabs should be treated as potentially post-tensioned and scanned first.
- What happens if a post-tension cable is cut by accident?
- A severed tendon can release its stored tension suddenly — the strand may whip and the anchorage can fail — creating an injury risk and reducing the slab's structural capacity at that point. This is why suspected PT slabs are always scanned before any cutting or coring.
- Can GPR find plastic pipe and conduit, not just metal?
- Yes. GPR detects contrasts in material properties, so it can locate many non-metallic pipes and conduits as well as steel — an advantage over electromagnetic locators, which only find metal.
- Does scanning leave any marks or damage?
- GPR is non-destructive and needs access to only one surface. The only marks left are the crew's own layout lines showing where reinforcement and utilities are, so cuts can be planned around them.
- How accurate is GPR?
- Accuracy depends on the operator's interpretation and on conditions — very wet or heavily congested concrete is harder to read. A skilled technician can reliably locate and depth-estimate typical embedments, which is why scanning is a skilled task, not just a device reading.
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