Ground penetrating radar is the reason a slab can be cut or cored without guessing. It is also oversold. Some people expect an x-ray picture of the floor; others assume it is a formality.
It is neither. This is what a scan really tells you, where it runs out, and why we still do one before every cut.
How a scan works
The scanner sends a radar pulse down into the concrete and listens for what bounces back. Anything that differs from the concrete around it returns a signal: steel, a pipe, a conduit, an air gap. The operator rolls the unit across the surface in a grid and reads the reflections as they come in.
What comes back is not a photograph. It is a pattern on a screen that a trained person interprets. The skill is in the reading, which is why the same person who scans should be the one who marks the floor.
What a scan finds
Metal shows up best. In ordinary conditions a scan locates:
- Reinforcing steel, and the pattern it is laid in.
- Post-tension cables.
- Metal conduit and pipe.
- Changes in the slab, such as a thickened edge or a void underneath.
For each target the scan gives a position on the surface and an approximate depth. That is enough to move a core a hand's width to miss a bar, or to shift a saw line off a conduit run.
What it can miss
Radar is honest about some things and quiet about others. The common blind spots:
- Plastic pipe and empty plastic conduit return a weaker signal than metal and are easier to miss.
- Tightly spaced reinforcing near the surface can mask whatever sits below it.
- Anything directly under another target tends to hide in its shadow.
- Very small or very deep items may not show clearly at all.
A scan also cannot tell you what a line is carrying. It can show a conduit; it cannot say whether the circuit inside is live. Treat every marked line as live until someone has proven otherwise.
Why new or wet concrete is harder
Water soaks up radar energy. Concrete that was placed recently still holds a lot of moisture, and so does a slab that has been flooded or sits on wet ground. The signal does not travel as far and the picture is less clear.
That does not make a scan useless on a young slab. It means the operator should say plainly what the conditions allowed, and the cut plan should leave more margin.
What the marks on the floor mean
We mark what we find directly on the concrete, so the layout and the hazards are on the same surface. Then we walk you through the marks before anything is cut.
Ask two questions about any set of marks: what is each line, and how sure is the reading? A clear answer to both is the difference between a scan and a guess. If a reading was uncertain, we will tell you what the scan could not settle.
Where the scan shows post-tension cable in the way, the plan stops there. Whether a cable can be cut or must be avoided is a decision for the building's engineer, not for the saw operator.
Why we still scan every time
Because the alternative is finding out with the blade. A struck conduit can take a building's power down. A cut cable or a severed drain is far worse. Set against that, a scan is a small step and moving a mark costs nothing.
GPR scanning is the first step on every NoCo job, whether the work that follows is core drilling, wall sawing or flat sawing. It lowers the risk a long way. It does not remove the need for care with the saw.
General information, not engineering advice — every slab is different.
