Before cutting or coring concrete, you need to know what is inside it. The two common ways to find out are X-ray (radiography) and ground penetrating radar (GPR). X-ray produces a detailed image but needs access to both sides and a cleared radiation zone. GPR works from one side, gives results on the spot and has no radiation hazard, but its images need interpretation and it can miss things in congested or deep areas.
What you are looking for
Scanning is usually done to locate:
- Reinforcing steel (rebar) and wire mesh
- Post-tensioned tendons, which are dangerous to cut
- Electrical conduit and embedded cables
- Pipes, sleeves and other embedded items
- Voids, or the edges of beams and slab thickenings
The goal is a safe location for a core, cut or anchor, or information about how a slab was built.
How concrete X-ray works
A radiation source is placed on one side of the concrete and a film or digital detector on the other. Radiation passes through the concrete, and dense objects such as steel absorb more of it, producing an image. The result is a picture of what lies within the area scanned, similar in idea to a medical X-ray.
Strengths:
- Clear, detailed images that are easy to read
- Shows congested reinforcing and small objects well
- Distinguishes items like conduit and tendons by their shape
- Produces a permanent record
Limitations:
- Needs access to both sides of the slab or wall
- Ionizing radiation means a controlled area must be cleared of people, which often means after-hours work in occupied buildings
- Done by licensed operators under federal and provincial radiation safety rules
- Practical limits on the thickness of concrete that can be imaged
- Does not directly show depth, only position in plan
- Not usable on slabs on grade, since there is no access below
How GPR works
A GPR unit is rolled across the concrete surface. It sends radio pulses into the concrete and records reflections from objects and changes in material. The operator reads the results on screen and marks findings directly on the surface.
Strengths:
- Works from one side, including slabs on grade and walls with one face accessible
- No ionizing radiation, so no exclusion zone; work can proceed around occupants
- Results on site, marked on the concrete in real time
- Gives approximate depth of objects
- Covers larger areas quickly
Limitations:
- Results depend heavily on operator skill and interpretation
- Dense top reinforcing can mask objects beneath it
- Distinguishing a tendon from rebar or a non-metallic conduit can be difficult
- Moisture, certain aggregates and some surface materials reduce accuracy
- Depth range is limited, and accuracy drops with depth
Side by side
| X-ray | GPR | |
|---|---|---|
| Access needed | Both sides | One side |
| Radiation hazard | Yes, area must be cleared | No |
| Image detail | High, direct image | Lower, needs interpretation |
| Depth information | Limited | Approximate depth |
| Speed and coverage | Slower, small areas | Faster, larger areas |
| Slabs on grade | Not possible | Yes |
Choosing a method
Ask these questions:
- Can you get to both sides? If not, GPR is the main option.
- What is the consequence of a hit? Near post-tensioned tendons or critical conduit, the detail of X-ray may justify the extra effort.
- How congested is the reinforcing? Heavy reinforcing favours X-ray.
- Can the area be cleared now? X-ray in occupied buildings needs planning, often at night or on weekends.
- How large is the area? For mapping large areas, GPR is more practical.
Many projects use both: GPR to map a wide area quickly and X-ray to confirm critical locations.
Preparing for a scan
- Mark each proposed core, cut or anchor location on the concrete and on a plan, with its size
- Share structural drawings, especially anything showing post-tensioning, beams or slab thickenings
- Find out what services run through the area from the electrical and mechanical trades
- For X-ray, arrange access below and a plan for clearing both floors during exposures
- Clear the surface of materials, debris and standing water
- Allow some flexibility in each location, so a core can move to a clear zone if needed
What the results should include
Whichever method is used, the deliverable should let the cutting crew act without guesswork:
- Markings on the concrete showing detected items and the clear zones for each location
- A short report listing each location, the method used and what was found
- Images or scan captures with a reference to where they were taken
- For GPR, approximate depths where they matter
- Clear notes on limitations, such as areas where heavy top steel may hide items below
Marks on the surface wear off and get covered. Photograph them before work starts.
A typical case
A building owner is adding a new elevator machine room drain and several floor penetrations for piping in a mid-rise residential building. The upper floors are post-tensioned, and the ground floor is a slab on grade. For the ground floor locations, GPR is the only option and is used to find reinforcing and any buried conduit. For the post-tensioned floors, GPR first maps the wider areas around each penetration to find tendon bands, then X-ray confirms the exact positions at the final core locations. Because residents live on the floors above and below, the X-ray exposures are booked for a weekday morning with the affected units cleared by arrangement. Two cores are shifted slightly to stay clear of tendons, and the trades proceed.
Scanning reduces risk; it doesn't remove it. No scan is perfect. Combine scanning with drawings, careful core placement away from detected items, and safe cutting practices. See cutting and coring concrete safely.
How ehZee can help
We review drawings, recommend the right scanning method for each location, and coordinate X-ray or GPR scanning around your schedule and occupants, then help interpret results so cuts and cores are placed safely. See our concrete inspections and X-ray, or call 1-833-888-0091.
Common questions
Is concrete X-ray safe for people in the building?
When done properly by licensed operators with a controlled exclusion zone, yes. The zone is cleared during exposure, which is why X-ray is often scheduled outside working hours.
Can GPR find plastic pipes?
Sometimes. Non-metallic items give weaker reflections, and their visibility depends on depth, contents and surrounding conditions. Don't assume a clear GPR scan means no plastic conduit.
Do I still need drawings if I scan?
Yes. Drawings tell the scanner what to expect, such as post-tensioned slabs or embedded services, and help interpret the results.
How far should a core be from a detected item?
There is no single distance that suits every case. It depends on the method's accuracy at that depth, the type of item and the consequences of a hit. Agree the clearance with the scanning operator and, near tendons or structural members, with the structural engineer before anyone drills.