Cutting a post-tension cable is one of the fastest ways to turn a routine core-drilling job into a catastrophe. When a PT strand snaps, it releases 30,000+ pounds of stored energy in a fraction of a second. The recoil has killed workers, launched chunks of concrete through walls, and produced repair bills that regularly top $40,000 per cable — before counting downtime, engineering review, and stop-work orders. Even a partial nick that doesn't snap immediately can lead to a delayed failure days or weeks later, at which point it's the GC's insurance carrier problem.
None of this is necessary. Every one of these incidents was preventable with a proper GPR scan by a trained operator before the first bit ever touched the slab. Here's exactly how we do it at Earthworks Engineering, and what you as a GC or superintendent should expect to see the scanning team do on your job.
What Makes Post-Tension Slabs Different
In a conventional reinforced slab, rebar sits at a consistent depth (typically 2-3 inches from the top or bottom face) in a predictable grid pattern. You can miss the rebar by drilling between the grid lines, and even if you nick one, the failure mode is gradual — the slab spans a little further, the crack propagates slowly, and you have time to react.
Post-tension slabs work completely differently. High-strength steel strands (usually seven-wire cables) are tensioned to 30,000-40,000 pounds of force and anchored at the slab edges. The cables curve up and down through the slab thickness following a "drape profile" designed by the structural engineer — they're near the top in negative-moment zones (over columns) and near the bottom in positive-moment zones (mid-span). This means the cable depth changes continuously as you move across the slab.
A PT cable that's 2 inches deep at one location may be 6 inches deep 15 feet away. And unlike rebar, they aren't laid out on a uniform grid — they're placed in bundles or evenly-spaced strands running in one or both directions based on the specific structural design. As-built drawings help, but they're not gospel: contractors adjust cable layouts in the field for embedments, penetrations, and column blockouts. On any slab more than a few years old, the drawings are indicative, not exact.
The upshot: you cannot core a PT slab safely without real-time scanning at every proposed hole location. Not "we scanned a few spots and extrapolated." Every hole.
The 7-Step Field Procedure
Step 1: Review the structural drawings
Before we arrive on site, we ask for the original PT tendon layout drawings, any as-builts, and the location of the core points marked by the GC or MEP contractor. This gives us a hypothesis of where cables run, cable spacing, and general cable depth by zone. It's a starting point, not the final answer — but it tells us where to be extra careful.
Step 2: Set up the scanning grid
On arrival, we chalk-line or laser-mark a 2-foot by 2-foot grid centered on each planned core location, extending at least 12 inches beyond each hole in every direction. This gives us enough area to characterize the surrounding steel and identify the cable's approach and exit path, not just what's directly under the bit.
Step 3: Scan the first axis
Using a high-frequency antenna (2.6 GHz for slabs under 8 inches, 1.5 GHz for thicker sections), we run the GPR unit in parallel lines across the grid on one axis — say, north-south — spaced 2-3 inches apart. Each pass produces a B-scan (a vertical cross-section image) showing every embedment the unit crosses. We watch the display in real-time and mark the concrete surface with paint pens at every reflection.
Step 4: Scan the perpendicular axis
Repeat step 3 on the perpendicular axis (east-west). Cables and rebar running in either direction only show up clearly when the GPR unit crosses them perpendicular to their run — parallel passes produce weak or invisible returns. Scanning both axes catches everything.
Step 5: Interpret the signatures
This is where operator training matters. On a GPR B-scan, different embedments produce visually distinct signatures:
- Rebar — sharp, symmetrical hyperbolic reflections at consistent depths
- PT cable duct (grouted) — larger, wider hyperbolic reflections, often with a shadow beneath from the duct fill
- PT cable duct (unbonded, greased strand) — softer reflections with less shadow, sometimes double signatures from the strand within the sheath
- Conduit (electrical) — depends on whether it's steel (looks like rebar) or plastic PVC (softer signature, may be nearly invisible on some frequencies)
- Voids or delamination — negative-phase reflections that look inverted compared to steel
A trained operator distinguishes PT from rebar in real-time and marks the surface accordingly. Untrained operators mark everything as "steel" and force the GC to core in the widest gap, which may or may not be safe.
Step 6: Verify with a second pass and depth measurement
Once we've marked suspected cables, we run a second scan along the marked line to confirm the cable's continuous path and measure depth at multiple points. This is critical for PT because the depth changes along the cable — a hole that would clip the cable at one X-coordinate may safely miss it 6 inches away. We map the cable's depth profile, not just its lateral position.
Step 7: Mark, document, and hand off
We paint a "safe zone" perimeter around each core location — typically a red circle marking the actual proposed hole and a green boundary showing the minimum safe area we've confirmed is clear of PT cables. We photograph the marked slab from multiple angles for the project record. Then we brief the coring crew on-site: what we found, where the closest cables are, cable depth at the drill point, and any locations where we recommend the GC move the core instead of drilling.
What GCs Should Insist On
Not every scanning vendor follows this procedure. Some skip the perpendicular axis. Some only mark rebar (ignoring PT-specific signatures because their tech isn't trained to distinguish them). Some don't document. Here's what you should require:
- Two-axis scanning at every core location — not just where the GC expects PT to run
- Real-time on-site marking — not a report delivered days later
- Photo documentation — for your project record and any subsequent QA/QC review
- Written scanning report — depth at each marked location, cable signature type, any red-flag areas where the GC should consider relocating the hole
- On-site handoff to the coring crew — the scanner briefs the crew face-to-face, not via a text message
Not sure whether your job needs GPR or concrete X-ray? Read GPR vs Concrete X-Ray: Which One Do You Actually Need? for the field guide.
When to Recommend Moving the Core Location
Sometimes the honest answer is "this hole can't be drilled here safely." When we find that a proposed core location:
- Falls within 3 inches of a PT cable (any depth)
- Is directly over a bundled cable group with no obvious safe gap
- Is in a negative-moment zone near a column with dense cable convergence
...we recommend the GC move the core rather than proceed. This is a professional judgment call, and it's worth more than the scanning fee itself. A good scanning contractor tells you "no" when the answer is no, even if it means a phone call to the structural engineer to relocate the penetration.
Follow the industry procedure guidance from the Post-Tensioning Institute (PTI) and the ACI Foundation's published field practices for concrete cutting near PT cables — both of which specifically require GPR (or equivalent) scanning prior to any drilling operation.
The Cost of Skipping This
A proper GPR scan for a coring job typically costs $600-$1,400 in Southern California. A struck PT cable typically costs $30,000-$50,000 in cable repair, structural engineering review, load testing, and downtime — and that's if nobody gets hurt. If a worker is injured or killed, the liability chain reaches the GC, the coring subcontractor, and often the structural engineer of record.
There is no version of the math where skipping GPR saves money.
How to Book It Right
When you call a scanning contractor for a PT job, give them:
- Project address and slab access details
- Slab thickness (from the drawings)
- Age of the structure (older PT designs use different cable layouts)
- Number of core locations and approximate diameters
- Whether original PT tendon drawings are available
- Site restrictions — occupancy, work hours, other trades on site
Any scanning contractor who doesn't ask about these before quoting isn't taking the job seriously enough. We serve Rancho Cucamonga, Riverside, San Bernardino, Ontario, and the broader Inland Empire and LA/Orange County region with typical same-day and next-day response for PT scanning work. See our GPR Utility Scanning services page for full coverage details.