If you’re planning to drill, core, or anchor into a post-tension slab in Phoenix, the question isn’t whether you should locate the tendons first — it’s how. Ground-penetrating radar (GPR) is how professionals find post-tension cables before drilling, mapping every tendon, rebar run, and conduit path so a coring bit never gets near a live cable. It’s the same scan-before-you-cut standard the crews fulfilled by Concrete Repairman® follow on every post-tension job across the Valley, because guessing at tendon locations in Arizona’s slab-on-grade homes isn’t a risk anyone should take.
Why you can’t guess where the tendons run
Post-tension slabs are everywhere in Phoenix and the surrounding Valley — most homes built from the 1990s forward, and nearly every pool deck, patio extension, and remodel poured over an existing slab. Inside that slab, steel tendons run under real tension, anchored at the slab edges. That tension is what’s holding the concrete together. Nick one with a hammer drill or a coring bit, and you’re not just damaging a cable — you’re releasing stored energy that can crack the slab, drop the tendon’s structural contribution, or in a worst-case scenario send the cable end whipping loose with real force behind it. That’s why an inspection that includes a proper scan matters before any drilling starts, not after something goes wrong.
The drape/profile of a tendon (it isn’t a straight line)
Homeowners often assume tendons run in flat, predictable lines from one edge of the slab to the other, evenly spaced like rebar in a sidewalk. They don’t. Tendons are draped — engineered to rise and fall in a profile through the depth of the slab, typically higher near the middle of a span and lower near the supports, because that profile is what counteracts deflection and controls cracking. A tendon that reads as 2 inches deep at one point in the slab can be 4 inches deep two feet away. That drape is exactly why “eyeballing it” or measuring from a spacing pattern on one side of the room fails — you’re not looking for a straight line, you’re tracing a curve, and the only way to trace it accurately is to scan it.
How ground-penetrating radar (GPR) works
GPR sends radar pulses into the concrete and reads what bounces back. Different materials — steel tendon, rebar, PVC conduit, an air void — reflect the signal differently, and a trained technician reads those reflections in real time on the scanner’s display as it’s pulled across the slab. The result is a live picture of what’s inside the concrete, tendon paths and depth of cover included, marked directly onto the surface before a single hole is drilled.
GPR replaced the older approach of X-raying a slab, and for good reason. X-ray concrete scanning requires evacuating the work area, radiation safety protocols, and access to both sides of the slab to place the film — none of which is practical in an occupied home or a live jobsite. GPR needs access to only one side, produces results instantly instead of requiring film processing, and doesn’t require clearing the building. For a Phoenix homeowner adding a patio slab, mounting a post, or coring for plumbing, that difference is the reason GPR is now the standard tool for this work.
The locate-and-mark process, step by step
A professional scan isn’t a single pass with a wand. It follows a repeatable process:
- Grid the area. The technician marks a scan grid over the section of slab where work is planned, so every square inch of the drill zone gets covered — not just the exact spot of the proposed hole.
- Scan in both directions. Because tendons, rebar, and conduit run at different orientations, the scanner passes over the grid horizontally and vertically to catch everything, not just lines running one way.
- Read and mark in real time. As tendons and rebar show up on the display, their paths are marked directly on the concrete with paint or chalk, including where the drape shifts.
- Identify depth of cover. The scan also reports how deep each tendon sits, which tells the crew how far they can safely drill or core in that spot without ever getting close.
- Flag safe zones. With tendons and rebar mapped, the technician marks the actual safe zones for coring, anchoring, or drilling — the areas clear of everything that matters.
Only once that map exists does drilling start. It’s a short process, usually well under an hour for a typical residential scope, and it’s the difference between a controlled cut and an expensive mistake.
GPR vs. hardware-store cable finders (why the cheap tool isn’t enough)
Handheld stud and rebar finders sold at hardware stores are built for wood-frame walls, not post-tension concrete. Most consumer models can only detect metal within an inch or two of the surface, can’t distinguish a tendon from rebar or conduit, and can’t read depth of cover at all. On a slab where tendon depth changes by the drape profile, that’s not a small gap — it’s the exact information you need and the exact information the tool can’t give you.
“I’ve seen homeowners run a stud finder over a slab, get a clean read, and drill right through a tendon anyway — because the tool never saw it in the first place. GPR is the only way to actually know what’s under that concrete before you commit to a hole,” says James Belville of Concrete Repairman®, whose crews handle post-tension work across the Phoenix area.
A cable finder can tell you “something metal might be here.” Professional GPR concrete scanning tells you what it is, where it runs, and how deep — the level of detail post-tension work actually requires.
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→ Request a Written EstimateWhat happens when a tendon is in the way of your project
Sometimes the scan comes back and the only viable spot for an anchor, post, or plumbing penetration sits right where a tendon runs. That doesn’t mean the project stops — it means the plan shifts. Options typically include relocating the anchor point a few inches to a clear zone identified in the scan, or, when a cable genuinely has to be cut, de-tensioning it in a controlled way and installing a coupler repair to restore the tendon’s load path before the slab is patched. This is specialized work that goes well beyond patching a hole — it’s structural, and it’s exactly the kind of job that should be planned around a scan rather than discovered mid-drill. If a scan turns up tendons, cracking, or settlement concerns in your slab, that’s the point to loop in post-tension slab repair specialists rather than proceeding on assumptions, and if the slab shows broader signs of movement it’s worth reading about how foundation crack repair is evaluated as well.
Frequently Asked Questions
How do you find post-tension cables in a slab?
Professionals use ground-penetrating radar (GPR) to scan the slab in a grid pattern, marking tendon paths, rebar, and conduit directly on the concrete surface before any drilling begins. This is the standard method used across the industry and the one followed on every post-tension slab scope handled by Concrete Repairman®.
Can ground-penetrating radar detect post-tension cables?
Yes. GPR is specifically effective at detecting the dense steel used in post-tension tendons, along with rebar and conduit, and can distinguish between them based on how each reflects the radar signal.
How accurate is GPR concrete scanning?
In the hands of a trained technician, GPR reliably locates tendon paths and depth of cover to within a small margin — enough precision to mark a genuinely safe drill zone. Accuracy depends on scanning the full grid in both directions and reading the drape profile correctly, not just a single quick pass.
How much does it cost to scan a post-tension slab?
Cost depends on the size of the area being scanned and the scope of the project it supports — a single anchor point costs far less to scan than a full patio or addition footprint. A written estimate that spells out the scan scope and any follow-up repair work is the right way to get an exact number for your project.
Can I use a stud/rebar finder to locate post-tension cables?
Not reliably. Consumer stud and rebar finders can’t read depth of cover and often can’t distinguish a tendon from rebar or conduit, and their detection range is too shallow to trace a tendon’s drape through the slab. They’re a wood-frame tool being asked to do a concrete-scanning job.
Do you have to scan before every cut into a post-tension slab?
Yes. Every cut, core, or anchor point into a post-tension slab should be preceded by a scan, no matter how small the hole or how confident anyone is about what’s underneath. Tendon drape means a location that looks clear can shift a few inches down the line, and the cost of a scan is always lower than the cost of cutting a live cable.
Planning any core, anchor, or plumbing penetration through a Phoenix post-tension slab? A scan comes first. Contact Concrete Repairman® to schedule a GPR locate and a written estimate before anyone drills — and if you’ve already found cracking or want to understand what’s under your slab in the first place, see our guide to what a post-tension slab foundation actually is and why you should never cut a post-tension cable without a scan first.