What GPR Actually Detects: Rebar, Post-Tension Cables, Conduit & Voids

What Does Concrete Scanning Detect Before You Cut?

Concrete scanning detects rebar and wire mesh, post-tension cables, electrical conduit and pipe, voids and honeycombing, plus slab thickness and depth of cover. Ground penetrating radar sends radio pulses into the slab and reads what bounces back, so CDS crews know what is embedded before the first cut. CDS has scanned and cut concrete across Ontario since 1967.

Concrete scanning is not guesswork or a metal detector waved over the floor. It is a controlled survey that maps the objects embedded in reinforced concrete. Here is what a scan actually reveals.

What Ground Penetrating Radar Sees Inside Concrete

A GPR unit sends radio pulses into the slab and records what bounces back. Different materials reflect differently, so a trained operator can identify:

• Rebar and wire mesh – the structural steel grid that holds the slab together.
• Post-tension cables – high-tension steel tendons that must never be cut.
• Electrical conduit and pipe – live power, data, gas, and water lines cast into the concrete.
• Voids and honeycombing – air pockets or poorly consolidated concrete that weaken the slab.
• Slab thickness and depth of cover – how thick the slab is and how deep each object sits.

Rebar, mesh, and structural steel

Rebar is the most common find. Knowing its spacing and depth lets us place a core hole or saw line in the clear gap between bars, so we keep the slab strong instead of slicing through its reinforcement.

Post-tension cables – the one you cannot get wrong

Post-tension slabs hold hundreds of pounds of stored tension. Cutting a cable can whip the tendon loose and cause serious injury or structural damage. Mapping cable paths first is the single biggest reason to scan before drilling, and it is a standard step on every post-tension job we touch.

Conduit, voids, and thickness

Electrical conduit and plumbing lines are expensive and dangerous to hit. Voids matter too – coring into an unseen air pocket can wreck a bit or leave an anchor with nothing to grip. A scan also confirms slab thickness, which tells us whether a proposed hole will punch through into a ceiling, duct, or occupied space below.

Where Concrete Scanning Reaches Its Limits

GPR is powerful, but it is not x-ray vision. Honest expectations keep projects safe:

• Very small or shallow targets can blur together with nearby steel.
• Signal weakens with depth, so deep, thick slabs may hide lower objects.
• GPR shows an object’s location and depth, but not its exact material – it flags “something here,” not “this is a gas line.”

When a target sits beyond radar range, or the job needs a two-sided image, we turn to other tools. Our guide on how GPR compares with X-ray and utility locating breaks down when each method wins. For buried services beyond the slab edge, private utility locating traces lines GPR alone would miss.

Why Detection Accuracy Protects Your Cut

Every cut we make – a backyard slab removal, an egress window opening, a factory-floor cut-out, or a selective demolition on an occupied site – starts with knowing what is inside the concrete. Accurate detection is what turns a risky guess into a clean, predictable job.

Skip the scan and the costs stack up fast: a severed post-tension cable, a flooded floor from a nicked water line, or a stalled project waiting on an electrician. A few minutes of scanning before we core drill protects your budget and your timeline. If you want to see how findings are documented, our post on reading a GPR scan report walks through it.

Concrete Scanning Across Hamilton and Southern Ontario

CDS runs from a single Hamilton headquarters, the same base we have worked from since 1967. Our mobile scanning crews load the GPR gear and travel out from there – serving Hamilton, reaching Brantford, and covering the Waterloo Region including Cambridge and Kitchener. You can see the full menu on our concrete scanning and GPR page.

See our full range of concrete services.

More on this topic: Why Scan Before You Core

Frequently Asked Questions

Can concrete scanning detect post-tension cables?

Yes. Post-tension cables are one of the most important targets GPR finds, because cutting one can be dangerous and costly. Our crews map every cable path before a saw or core bit goes in, so the cut stays well clear of the tendons.

What can’t GPR detect in concrete?

GPR struggles with very small or very deep targets, and it cannot tell you the exact material of what it finds – only that something is there and how deep. When an object sits below radar range or we only have access to one side of the slab, we pair scanning with utility locating or recommend X-ray for a sharper image.

Do I need a scan before every core hole?

For any hole in a slab whose contents you don’t already know, yes. A quick scan is far cheaper than repairing a cut conduit or water line, and it confirms clear drilling zones and slab thickness before we core – which keeps your project on schedule.

Scan First, Cut with Confidence

If you have a slab to cut, core, or open up, let us scan it first. Call CDS at (905) 529-6029 or request a quote, and our crew will tell you exactly what is inside before the first cut.