26 August 2026 Uncategorized 0

Avoid Six-Figure Repairs: Post-Tension Scanning for Contractors

Before you drill, cut, or core a post-tension slab, get a qualified ground penetrating radar (GPR) scan and, where the signal is unclear or the slab is congested, pair it with a complementary NDE method like EMI or impact echo. Route the marked-up results to your structural engineer for sign-off before anyone touches a bit to concrete. That sequence, done in order, is what keeps a routine penetration from becoming a tendon strike.

TL;DR:

GPR scans should be paired with complementary NDE methods like EMI or impact echo if signals are unclear or the slab is congested, to ensure safety.

Proper scanning and marking must precede drilling, with a sign-off from a structural engineer to prevent tendon strikes and related structural damage.

GPR accuracy locates tendons within about half an inch depth, but methods like EMI or X-ray are necessary when assessing grout voids, corrosion, or tendon integrity.

The standard workflow involves detailed pre-survey reviews, precise grid scans, markings, safety checks, and comprehensive documentation before any core drilling.

Cost and turnaround depend on scan type, coverage area, and report complexity, with same-day results possible for small-scale 2D scans and longer for extensive 3D documentation.

Why Post-Tension Scanning Matters for Structural Safety
How Ground Penetrating Radar Locates Tendons
When Do You Need More Than GPR?
What Happens Before You Drill: The Standard Scanning Workflow
On-Site Controls That Keep Crews and Schedules Safe
What Does Post-Tension Scanning Cost and How Long Does It Take?
How an Experienced Contractor Builds Scanning Into Safe Work
Requesting a Post-Tension Scan and Quote
The Real Gap in Most Post-Tension Scanning Advice
Sources

Why Post-Tension Scanning Matters for Structural Safety

A post-tension cable under load carries enormous stored energy. Cut one without warning, and the strand can release violently, sending debris across the work area and putting anyone nearby at real risk. Beyond the immediate hazard, a severed tendon compromises the slab’s load path. That can mean sagging, cracking, or a structural repair bill that dwarfs the cost of the original job, plus the liability that follows a preventable incident.

Tendons don’t sit at a fixed depth either. Because they’re stressed in a draped profile, a strand that’s 2 inches from the surface at one point can be 4 inches deep just a few feet away. That variability is exactly why scanning before drilling near post-tensioning tendons is standard practice, not an optional precaution.

As-built drawings compound the problem. Field changes during original construction rarely make it back onto the plans, so:

Tendon spacing can shift by several inches from what’s documented
Anchor zones may be relocated to accommodate other trades
Drawings often show design intent, not installed reality

You’re always scanning the slab in front of you, not the one on paper.

How Ground Penetrating Radar Locates Tendons

GPR sends electromagnetic pulses into the concrete and reads what bounces back. A tendon, being metal surrounded by concrete of a different density, produces a distinct hyperbolic signature on the radargram. An experienced operator reads that curve, calculates depth from the signal’s travel time, and marks the tendon’s path directly on the slab surface.

Antenna frequency is the main technical decision that shapes results. Higher-frequency antennas in the 1.6 to 2.0 GHz range are the standard choice for commercial slab work because they balance resolution against penetration depth, typically holding accuracy to around plus or minus half an inch in normal slab conditions.

Depth accuracy at a glance: In typical commercial concrete, a properly calibrated 1.6 to 2.0 GHz GPR scan should locate a tendon’s depth within about half an inch, tight enough to plan a core path with real confidence.

Scanning approach depends on the job:

Real-time 2D line scans give you an immediate read on tendon position for quick site decisions, same-day coordination, or spot-checking a single proposed hole.
Grid-based 3D scanning, run across a defined grid at close intervals, builds a full plan and depth map of the tendon drape across a larger area, which is what you want for as-built documentation or a slab with multiple penetrations.
Hybrid sequencing, a fast 2D pass to orient the crew followed by a targeted 3D grid only where cores are actually planned, keeps cost and downtime reasonable on active job sites.

Deliverables should include surface markings for every located tendon, annotated radargrams showing depth at each point, and, when the job calls for it, a digital 3D model your engineer can review before anyone picks up a saw.

When Do You Need More Than GPR?

GPR is the right starting point, but it isn’t the right tool for every question. It tells you where metal is and roughly how deep. It doesn’t reliably tell you whether a grout duct has a void or whether a strand has started corroding inside its sheathing, and inspection guidelines for post-tensioned structures recommend layering in other methods for those conditions.

Electromagnetic induction (EMI) confirms ferromagnetic mono-strand tendons and builds a plan-view heat map, particularly useful when heavy rebar is scrambling your GPR returns.
Impact echo and ultrasonic testing detect voids, delamination, and grout condition inside the duct, information GPR simply can’t extract.
X-ray radiography delivers the sharpest image available, resolving tendon position to a fraction of an inch, but it demands access to both faces of the slab and a radiation exclusion zone, which limits it mostly to forensic or high-stakes congested cases.

Escalate to intrusive testing or a specialist condition survey when corrosion or prestress loss is suspected. No surface method fully substitutes for a direct look at the strand.

Pro Tip: If GPR returns look muddy in a heavily reinforced slab, run an EMI pass before you jump straight to X-ray. It often resolves the ambiguity for a fraction of the cost.

What Happens Before You Drill: The Standard Scanning Workflow

A professional post-tension scan isn’t a walk-around with a handheld unit. It follows a sequence, and skipping steps is where projects get into trouble.

Pre-mobilization. Collect existing drawings and as-built records, then submit proposed penetration locations to the structural engineer for review before the scanner shows up.
Grid execution. Scan a defined area around each proposed core, commonly a 24 by 24 inch grid with perpendicular passes, so tendons approaching from any direction get picked up.
Marking and safe-zone rules. Mark tendon paths with depth annotations, apply a consistent paint convention, and hold a site briefing before any tool contacts the slab.
Go/no-go check. Confirm minimum horizontal clearance from every marked tendon before drilling starts.
Closeout package. Deliver annotated radargrams, a core log, operator certifications, and an executive summary the general contractor or owner can file.

Deliverable	Purpose	Typically Provided By
Annotated radargrams	Depth and location evidence	GPR operator
As-drilled core log	Confirms actual conditions found	Drilling crew
Operator certifications	Verifies equipment competency	Scanning contractor
Engineer sign-off	Structural approval of penetration	Structural engineer

On-Site Controls That Keep Crews and Schedules Safe

Marking the slab is only half the job. What happens during the actual core drilling near tendon-marked zones determines whether the scan’s accuracy actually protects anyone.

Set up traffic and occupant controls before drilling begins, especially on occupied floors or active retail spaces.
Monitor depth in real time as the bit advances, and stop immediately if you feel unexpected resistance.
If a signature appears that wasn’t on the original scan, stop and reverify before continuing.
Keep equipment calibration and operator qualification records on hand. Clients and regulators ask for them.

Sound housekeeping matters too. Wet coring generates slurry, and a clear site-cleaning protocol keeps the work zone from becoming its own hazard.

Pro Tip: Establish a single communication chain, scanner to driller to supervising engineer, before the first hole goes in. Most on-site tendon strikes trace back to a marking that got misread three hands down the line, not a scanning error.

What Does Post-Tension Scanning Cost and How Long Does It Take?

Price and turnaround depend on scope, not just square footage.

Scan type drives most of the cost difference. A 2D real-time check for a handful of holes costs far less than a full 3D grid scan across a large floor plate.
Congestion matters. Dense rebar or wire mesh slows the operator down and can require a second pass with a different frequency or an EMI cross-check.
Report level changes price too. Markings-only service is cheaper than a full deliverable package with annotated radargrams, depth logs, and a digital model.
Specialist NDE add-ons, like impact echo or X-ray, are priced separately and scoped only when GPR results call for them.

Turnaround expectations: Same-day service is standard for real-time 2D scans on a handful of penetrations. Full 3D grid deliverables with engineer review typically run one to three business days, depending on area size and report complexity.

How an Experienced Contractor Builds Scanning Into Safe Work

At Vicsawing, scanning isn’t a separate service bolted onto drilling. It’s baked into the same workflow: pre-checks against drawings, GPR scanning coordinated with the structural engineer, precision coring guided by the marked results, and an as-drilled documentation package handed over at closeout. That sequence is the same one described throughout this playbook, applied on active sites across Melbourne and Victoria.

If you’re evaluating a contractor for a post-tension job, ask for proof, not promises. Request sample deliverables, a list of certifications held by the scanning operator, and the actual equipment model being used on your slab. A contractor who can show you demonstration videos and documented project cases is one who has done this enough times to know where the risk actually lives.

Requesting a Post-Tension Scan and Quote

Skipping the survey step to save time is the single most common mistake we see on post-tension jobs, and it’s the one that turns a $2,000 core job into a six-figure structural repair. Vicsawing runs GPR scanning as standard practice ahead of any coring or cutting in post-tension concrete, backed by qualified operators and documented deliverables your engineer can actually sign off on.

To get an accurate quote, send us site photos, your proposed hole or cut locations, any existing structural drawings, and your access windows for the crew. In return, expect a clear scope: GPR coverage of the affected area, NDE add-ons if the slab warrants them, coordination with your structural engineer, and a documented deliverable package with operator certifications attached. No guesswork on either side.

If you have a job coming up in Richmond or anywhere else across our Melbourne and Victoria service area, request a free site assessment and quote before you schedule the crew. It’s the fastest way to know exactly what’s inside that slab before anyone drills into it.

The Real Gap in Most Post-Tension Scanning Advice

Most guidance on this topic treats GPR as a checkbox: scan the slab, get a green light, drill. That framing undersells the harder problem, which is interpretation. A radargram doesn’t hand you a clean answer. It hands you a curve, and reading that curve correctly in a slab with mesh reinforcement, conduit, and multiple tendon layers takes judgment an inexperienced operator simply hasn’t built yet.

The industry’s bigger blind spot is treating scanning as a one-time event rather than a live process. Depth monitoring during the actual core, not just before it, catches the cases where field conditions drifted from what the scan showed. A slab that was accurately mapped an hour ago can still surprise a crew if the marked clearance gets misread on the ground.

Contractors who take this seriously build in redundancy: a second method when GPR is ambiguous, a stop-on-resistance rule during drilling, and a documentation trail that protects everyone if a question comes up later. That’s not overcaution. It’s the difference between a company that scans because a client asked for it and one that scans because they understand what’s actually stored inside a stressed tendon.

— MYBMC