Before any saw touches a concrete slab, you need to know what is inside it. Rebar, post-tension cables, electrical conduit, plumbing lines, fiber optic runs — cutting through any of these creates problems ranging from expensive repairs to structural failure to electrocution. Concrete scanning is not optional on professional cutting projects. The question is which method to use.
The two primary technologies for non-destructive concrete imaging are Ground Penetrating Radar (GPR) and X-Ray radiography. Both work. Both have limitations. Choosing the right one depends on slab thickness, site conditions, what you are looking for, and how fast you need the results. At CTS, our sister company Concrete Imaging operates both technologies daily, and here is how we decide.
How GPR Works
Ground Penetrating Radar sends high-frequency radio waves (typically 1600-2600 MHz for concrete applications) into the slab from a handheld antenna rolled across the surface. When the radar signal hits a material boundary — the interface between concrete and steel rebar, for example — part of the energy reflects back to the receiver. The timing and strength of the reflected signal tells the technician what is down there and how deep it is.
Modern GPR units display real-time cross-sectional images on a screen as the antenna moves across the surface. A trained operator can identify rebar (both size and spacing), post-tension tendons, metallic and non-metallic conduit, voids, and slab thickness in a single pass. The entire scan is non-invasive and requires access to only one side of the slab.
Practical scanning depth in normal concrete is 12 to 18 inches with a standard antenna. Heavily reinforced or wet concrete reduces effective depth because the radio signal attenuates faster. GPR resolution is good enough to identify individual #4 bars at 12-inch depth but may struggle to distinguish closely spaced targets or overlapping elements.
How X-Ray Works
Concrete X-ray uses the same fundamental physics as medical radiography. A radiation source (typically Iridium-192 or Cobalt-60) is placed on one side of the slab, and a digital imaging plate or film is placed on the opposite side. X-ray photons pass through the concrete and are attenuated differently by steel, plastic, air voids, and the concrete matrix itself. The result is a high-resolution image that shows the internal structure of the slab with photographic clarity.
The image quality of X-ray is unmatched. You can see individual wires inside post-tension tendon sheaths, distinguish between conduit types, and measure rebar size directly from the image. For critical cuts near post-tension tendons or embedded high-voltage conduit, X-ray provides the confidence that GPR cannot always match.
The trade-off is operational. X-ray requires access to both sides of the slab — source on one side, detector on the other. It uses ionizing radiation, which requires a radiation safety perimeter (typically 25-50 feet) and clearance of all personnel from the exclusion zone during each exposure. Each shot takes several minutes including setup, exposure, and perimeter management. On a busy construction site with multiple trades working, this creates significant logistical friction.
Comparison Table
| Factor | GPR | X-Ray |
|---|---|---|
| Depth Range | 12-18" typical | Up to 24"+ depending on source |
| Slab Access | One side only | Both sides required |
| Safety Zone | None (radio waves) | 25-50 ft exclusion zone |
| Speed | Real-time, continuous scanning | Minutes per exposure |
| Image Clarity | Good (requires interpretation) | Excellent (photographic) |
| Non-Metallic Detection | Limited (PVC, voids detectable) | Excellent (all materials) |
| Cost | Lower per scan area | Higher (licensing, safety) |
When to Use GPR
GPR is the right choice for the majority of pre-cut scanning work. Use GPR when you need to map rebar patterns across large slab areas, locate embedded conduit before core drilling, verify slab thickness, or scan on an active jobsite where clearing an exclusion zone is impractical. GPR is faster, cheaper per square foot, and creates no safety disruption to adjacent work.
GPR is also the only practical option when you cannot access the opposite side of the slab — slab-on-grade, foundation walls, elevated decks with no access below. Since it needs only one-sided access, GPR works in situations where X-ray is physically impossible.
When to Use X-Ray
X-ray is the right choice when the stakes are highest. Cutting near post-tension cables, where a severed tendon can cause progressive structural collapse. Cutting near high-voltage embedded conduit. Cutting through slabs thicker than 18 inches where GPR cannot reach the full depth. Any situation where the clarity of a radiographic image materially reduces risk.
X-ray is also preferred on forensic investigations and structural assessments where detailed documentation of internal conditions is required — cracking analysis, grout fill verification in post-tension ducts, and condition assessment of embedded steel.
Concrete Imaging: Our Scanning Division
Concrete Imaging, our scanning and imaging division, operates both GPR and X-ray technology out of the same offices as our cutting and demolition crews. This integration means scanning and cutting are coordinated by the same project team — the technician who scans the slab is in direct communication with the crew that will cut it.
Our GPR and X-ray scanning services page covers the full scope of what we offer. For project-specific questions about which technology to use, reach out to our team — we will recommend the method that gives you the clearest picture at the most practical cost for your specific conditions.