Concrete scanning before anyone cores the slab.
Rebar, post-tension cables and conduit located and marked on the surface before you core, drill or cut, then written up in a report you can hand to the engineer or head contractor. We core slabs on our own fit-outs, so we know what the coring crew needs to see.
- Findings marked directly on the concrete
- Written scan report for every location
- Limits stated plainly, never hidden
- Ground-penetrating radar, not guesswork
One core in the wrong place is a structural problem, not a patch job.
What is concrete scanning? Concrete scanning uses ground-penetrating radar (GPR) to find what is inside a slab, wall or column before anyone cores, drills or cuts it: reinforcing steel, post-tension tendons, conduit, and the strands and webs in hollowcore floors. The findings are marked on the surface and written up, so the hole goes where nothing will be damaged.
Cut a post-tension cable
The tendon is under load. Cutting one can release it violently, and the slab loses part of what holds it up. Work stops until a structural engineer assesses and designs the repair.
Cut rebar
Usually survivable, but the engineer has to sign off the core, and you carry the cost of the assessment and the delay.
Cut a live conduit
Power or comms down for the floor, a trip, and an electrician called back before anyone can carry on.
What GPR concrete scanning finds
The equipment: GPR built for concrete
A proper concrete scan uses ground-penetrating radar made for concrete inspection, not a stud finder or a cover meter. GSSI's Flex system is a good example of professional kit and shows what each part does: one base unit for floor work, plus wireless antennas for walls, ceilings and deeper targets, all saving to the same file so every location ends up in one report.
Flex NX
For slabs and floors. It runs a standard and a cross-polarised antenna in the same pass, so bars running both ways show up together, and the results appear live on its touchscreen as the operator scans. A quick prospecting mode finds the tendon bands first; detailed scans then go over each core position. Every scan is saved for the report.
- 2.5 GHz antenna
- Live view on screen, or on a paired tablet
- Scans saved per location
NX25 antenna
A small wireless antenna that pairs to the Flex NX. It gets into the places the main unit can't sit flat: walls, soffits, columns, corners and tight risers. Same frequency as the Flex NX, so the detail is the same, just in a smaller head.
- 2.5 GHz antenna
- Wireless, pairs to the Flex NX
- Built for overhead and vertical work
NX15 antenna
A lower-frequency antenna for thick slabs, slab-on-grade and columns, where the standard antenna runs out of depth. Lower frequency sees further into the concrete but with less fine detail, so it is used alongside the main scanner rather than instead of it.
- 1.5 GHz antenna
- Wireless, pairs to the Flex NX
- For thick and heavily reinforced concrete
Why radar, and not X-ray or a ferro scan?
X-ray needs access to both sides of the slab, a film or plate behind it, and the area cleared of people while it runs. Radar works from one side, needs no exclusion zone, and shows results on the spot, so we can mark the slab while we're standing on it. X-ray still has a place for very congested or very thin elements; if a job needs it, we'll tell you at the site visit. A ferro scan (an electromagnetic cover meter) is quick for finding shallow reinforcing, but it only sees metal near the surface: it won't show plastic conduit or a tendon deeper in the slab.
Scanning before core drilling, step by step
This is the same order every time, whether it's one core in a house slab or forty through a post-tensioned floor.
- 1
Site visit and drawings
We look at the slab, the core positions and sizes, and whatever drawings exist: structural, and the post-tension layout if there is one. We check access, what's directly below the slab, and whether the far side can be reached. Then we quote.
- 2
Set out
The area is cleared of debris, standing water and anything sitting on the surface. Each proposed core centre is marked, with a margin around it so a core can move if it has to.
- 3
Prospect the area
A fast sweep with the main scanner across the whole area finds the tendon bands and the main bar layout before we look at any single core. This stops a clear spot being judged in isolation.
- 4
Scan each core position in two directions
Detailed scans run across each position one way, then at right angles. Larger areas get a grid scan so we see the layout, not just a line. Walls and soffits are done with a small wall-and-ceiling antenna; thick slabs and columns get a pass with a deeper-reading antenna.
- 5
Read the tendon at the core, not next to it
Post-tension cables rise and fall across a span, high over the supports and low in the middle, so a tendon's depth a metre away tells you little. We read position and depth at the actual core location.
- 6
Mark the slab
Findings go straight onto the concrete with a colour key: tendons, bars, conduit and anything we can't identify are marked differently, and the clear area at each core is outlined. The key is written on the slab and in the report.
- 7
Second side, where we can
If a result is doubtful and the soffit or far face is reachable, we scan from that side too. If it isn't reachable, the report says the result is one-sided.
- 8
Photograph and report
Every marked position is photographed. The saved scans are turned into a written report: each location, what was found, the recommended core position, and what we could not read clearly.
What helps us before we arrive
- Structural and post-tension drawings, if you have them
- Core positions, sizes and what each one is for
- Access to the area below, if the slab is suspended
- A clear, dry surface on the day
Post-tensioned, hollowcore and precast floors
Auckland buildings use several floor systems, and each has a different thing you must not cut. The scan is planned around the floor type.
Post-tensioned slabs
Tendons run in bands and drape through the slab depth. Cutting one releases stored load. Australian industry guidance for PT slabs (PTIA Guidance Note GN06) sorts post-cut holes into classes by their effect on the structure and recommends a permit-to-core system on site; we scan to support that process, not replace it.
Hollowcore floors
Common in New Zealand commercial buildings. Industry guidance (CCANZ Information Bulletin 95) says penetrations should stay clear of the vertical webs and must not cut the pre-stressing strands. The scan locates the strands and webs so a core can be placed in a hollow cell.
Reinforced and precast
Conventional reinforced slabs, rib-and-infill and precast tee floors each carry their critical steel in different places. We read the drawings with the scan so the core lands outside the critical zone.
Scanning doesn't replace the engineer's approval
CCANZ's guidance is clear: a penetration through a suspended floor that isn't on the structural drawings needs written approval from the person responsible for the structural design. It also flags fire rating, acoustic separation and reinforcement cover. A scan gives that engineer the information to approve a position; it isn't the approval itself.
Paint gets mopped. The report stays.
Marks on the floor get cleaned, ground off or covered by the next trade. For every scanned location you get the marked-up photo, what was detected, where we recommend coring and how confident we are. One document you can file with the engineer's sign-off or attach to the head contractor's permit to penetrate.
- Marks on the concrete for the coring crew
- Photo of every marked location
- Written findings and recommended core positions
- What we could not read, stated plainly
Sites we scan across Auckland
Commercial fit-outs
New data, power and plumbing penetrations in office and retail slabs, including relocations.
Apartments and multi-storey
Post-tensioned floors where every core needs clearing, and body corporates who want a record.
Industrial and residential
Plant bases, warehouse floors, and house slabs before plumbing or electrical penetrations.
What a scan can't tell you
- It shows that something is there, not what it is. A tendon and a conduit at a similar depth can look alike. Drawings and judgement fill that gap, and where we can't tell, the report says so.
- The top layer can hide what's below it. Closely spaced top steel can shadow deeper tendons or conduit. Some slabs need scanning from both sides, or a lower-confidence result.
- Depth has limits. They depend on the concrete, its moisture and what's in it. We tell you on the day what we could read; we don't promise a depth in advance.
- Some materials are hard to see. Small plastic conduit, empty voids and some fibres can be faint or invisible.
- No scan removes all risk. It reduces it. The engineer's drawings and sign-off still apply.
Concrete scanning FAQ
Can you find post-tension cables before we core?
How thick a slab can you scan through?
Can you scan walls and ceilings, not just floors?
What can GPR not see in concrete?
Do you mark the surface, or just give a report?
What equipment is used for the scan?
Why do you use radar instead of X-ray?
Do I need engineer approval before coring a suspended slab?
Can you scan hollowcore floors?
Is concrete scanning the same as a ferro scan?
How is a scan priced?
References
- CCANZ Information Bulletin 95: Drilling, cutting or forming holes in suspended concrete floor slabs (Cement & Concrete Association of New Zealand, with Precast New Zealand).
- PTIA Guidance Note GN06: Post-cut holes through post-tensioned slabs (Post-Tensioning Institute of Australia).
- GSSI product documentation for the Flex NX, NX25 and NX15 concrete scanning systems.
Related: underground services locating before you dig, and data network fit-outs, where most of our slab penetrations happen.
Coring next week? Get it scanned first.
Tell us where the cores go. We'll visit, check the slab and the drawings, and quote after the site visit.
Digging rather than coring? See underground services locating.