Coring a slab is routine work. Right up until the core barrel meets something that was holding the building up.
The reason this remains a live risk on well-run sites is that the consequences are not proportionate to the effort of avoiding them. A five minute scan and a marked slab is a small task. Severing a post-tensioning tendon is a structural event requiring engineering assessment and remediation, and the release of energy when a stressed tendon fails is a genuine hazard to whoever is holding the equipment.
What is actually inside the element
Before deciding what to scan for, it helps to be specific about what a slab, wall or beam may contain:
- Reinforcement. One or more mats, at a cover that is often not what the drawing specifies. Cutting a limited number of bars may be tolerable; cutting the wrong bars in the wrong zone is not.
- Post-tensioning tendons. Stressed strand following a draped profile, so its depth changes along the span. This is the highest-consequence target and the one least tolerant of assumption.
- Electrical conduit and services. Frequently cast into slabs and walls, often undocumented, occasionally live.
- Voids and honeycombing. Not a strike risk, but they change how the element behaves and matter if you are about to fix into it.
- The other side. What is directly behind or below the cut, which decides where the debris and the operator's drill exit.
Drawings help you anticipate these. They do not confirm them. Reinforcement gets shifted during construction to clear other elements, and the record is not always updated.
What scanning tells you
Concrete scanning uses radar to read the element from one accessible face, which is the practical requirement on site because you rarely have both. Within the scanned zone it establishes the position and approximate depth of reinforcement and tendons, the location of conduits, indications of voids, and element thickness. Because it is non-contact and non-destructive, it can be done on a live floor without shutting an area down or opening anything up.
The output that matters is not a report emailed later. It is the marking on the slab itself, showing where it is safe to cut, backed by a record of what was found and where.
How the work runs on site
A competent scan follows the same shape every time. Agree the exact cut or core positions and a margin around them, because scanning a zone rather than a spot is what allows a position to be moved if something is in the way. Scan the zone on a grid in both directions, since a single pass along one axis will not resolve a mat running the other way. Mark detected targets directly on the surface. Then confirm, with the person who will actually cut, a clear position or a recommended alternative.
Where the intended position cannot be cleared, that needs to be said plainly rather than hedged. A scan that reports congestion honestly and sends the question back to the engineer has done its job. One that offers a confident mark over a busy zone has not.
The limits, stated up front
Radar in concrete is subject to the same physics discussed in our comparison of NDT methods. Dense, closely spaced reinforcement in the top mat can mask what sits beneath it. Depth capability trades against resolution. High moisture content attenuates the signal. Very thick elements may not be readable through their full depth from one face.
These limits are manageable when they are declared. The failure mode is a scan presented as comprehensive when conditions did not allow it to be.
What a good deliverable includes
| Element | Why it matters |
|---|---|
| Marked surface | The cutting team works from the slab, not from a document |
| Plan of detected targets | A record that survives after the marks are cut away or washed off |
| Indicative depths and cover | Decides safe cut depth and whether a fixing will hit steel |
| Zones scanned and not scanned | Defines exactly where the clearance applies |
| Declared limitations | Flags where congestion or conditions reduced confidence |
| Clear go or no-go | An answer the site can act on without interpreting a trace |
When to scan, and when to do more
Scan before any core, cut, chase or significant fixing into a structural element, and treat post-tensioned slabs as scan-mandatory without exception. Where the work is extensive, or the element is already suspect, a scan is often the point at which a broader question surfaces: not only can we cut here, but is this element in the condition we assumed. That is a structural condition audit rather than a scan, and it is better to discover the need before the opening is made than after.
Delta Scan performs concrete scanning and rebar detection across South Africa, and where a scan raises a structural question the assessment is reviewed and signed by a registered Professional Engineer, Darryl Epstein, Pr Eng, ECSA 202001436.
If you have coring or cutting scheduled into a structural element, book the scan before the crew mobilises. It is the least expensive part of the job and the only part that prevents the expensive version of it.