Introduction: The Risk Hiding Inside Every Post-Tension Slab
When a crew prepares to core-drill or cut through a post tension slab — to add a plumbing sleeve, run conduit, or install an anchor — the slab looks the same as any other concrete pour. That visual uniformity is precisely what makes unbonded PT construction dangerous for uninformed crews: the high-strength monostrand tendons running through that slab are under permanent effective prestress typically ranging from 26 kip to 33 kip per strand (PTI DC80.3), and they are invisible from the surface.
Severing a single tendon releases that stored force instantaneously. The resulting snap can be violent enough to eject the cut strand through formwork and injure anyone in the failure zone. Beyond the immediate safety hazard, the structural consequences cascade: redistributed load paths, loss of compression in the slab cross-section, and a post tension cable repair cost that routinely runs into several thousand dollars per event, before accounting for engineering re-evaluation and schedule impact.
In experience on residential foundation slabs in the Dallas-Fort Worth area and commercial floor systems, the root cause of most tendon-cut incidents is not recklessness. It is an absence of a systematic pre-drilling protocol. This checklist establishes that protocol, step by step.
Step 1: Understand How PT Tendons Are Routed Before You Pick Up a Drill
The first step in any safe drilling-into-post-tension-slab procedure is understanding how the tendons are physically laid out. In unbonded PT slab construction — which represents the dominant PT method in Texas residential and light commercial work — tendons follow a predetermined draping profile set by the structural drawings.
Banded and Distributed Layout
ACI 318 Section 8.7.5 governs the tendon layout for two-way PT flat plates. The typical arrangement places one group of tendons in a concentrated band running through column lines (the banded direction), while the remaining tendons are distributed uniformly across the perpendicular spans (Banded-Distributed). In a 20 ft x 24 ft bay, the banded tendons are typically spaced at 18 in. to 24 in. on center through the column strip, while distributed tendons may be at 48 in. to 60 in. on center.
This means that the column-line zones carry significantly higher tendon density. Drilling within 12 in. of a column centerline, without confirmed tendon clearance, carries a high probability of tendon contact. This is not a zone for guesswork.
Tendon Profile and Drape
Beyond plan position, the tendon follows a parabolic vertical profile: it sits high at the supports and low at mid-span. In a typical 6 in. slab, the tendon may sit within 1 in. of the bottom of slab at mid-span and within 1 in. of the top of slab over the support. Any drilling perpendicular to the slab soffit that does not account for this profile risks intercepting the tendon even if the plan position appears safe.
For the full procedural context on PT construction sequencing and stressing requirements, refer to our guide on stressing and grouting procedures for PT slabs.
Step 2: Locate Tendon Positions Using Verified Detection Methods
Drawing review is the starting point, not the finish line. As-built conditions on a post tension slab frequently deviate from design intent due to field adjustments during placement. We always supplement drawing review with physical detection before any core drill or saw-cut penetration.
| Method | What It Detects | Typical Cost | Accuracy |
|---|---|---|---|
| Ground-Penetrating Radar (GPR) | Tendon profile, rebar, conduit | $300–$800/day * | High (3D imaging capable) |
| As-built Drawings Review | Tendon band layout, edge distances | No direct cost | High if drawings are current |
| On-site PT Stressing Pockets | Tendon end locations at slab edge | Visual inspection only | Reliable for edge zones |
| Electromagnetic Cover Meter | Rebar and shallow metallic objects | $150–$400/day * | Moderate (depth-limited) |
* Cost ranges are indicative estimates. Consult your local market for current pricing, as rates vary by region, provider, and project scope.
Ground-Penetrating Radar (GPR): Our Go-To Method
An example of a residential slab repair project in Dallas highlights the use of GPR scanning during a post-tension cable snap repair in a kitchen floor area. In this case, the GPR operator used a 1.6 GHz antenna array with a 6 in. grid spacing across an 80 sq ft work zone. The scan successfully mapped both banded and distributed tendons to within approximately 1 in. of their plan position, detected a rebar mat near the top of the slab, and revealed a conduit run not shown in the as-built drawings.
Such a level of detail cannot be achieved with a cover meter. For inspections of post-tension slabs prior to any penetration work, GPR remains the standard approach applied on controlled sites.
Using Stressing Pockets as a Reference
Every unbonded PT slab has physical evidence at its perimeter: the stressing pockets (or dead-end anchor pockets) where tendons terminate. These pockets are typically patched flush after stressing but remain detectable by probing or saw-cutting the edge beam. Counting strand terminations and cross-referencing with drawing panel widths gives an independent check on tendon count and approximate spacing before a GPR scan is even set up.
Step 3: Execute the Pre-Drilling Safety Checklist
Once tendon positions are confirmed on the slab surface, the following protocol governs the actual drilling operation.
- Mark all confirmed tendon centerlines on the slab surface using chalk line or paint marking. Include a 3 in. exclusion zone on each side of the confirmed tendon centerline.
- Verify the planned drill location is outside all exclusion zones and is not within 12 in. of a column centerline, unless specific GPR clearance confirms safe passage.
- Confirm core diameter and depth: calculate the full penetration depth and confirm it will not reach the tendon profile elevation at that plan position.
- Brief the drill operator verbally on all marked exclusion zones before commencing. The markings must be visible from the operating position.
- Stop drilling immediately if unexpected resistance or a sudden pressure change is detected. Treat any such event as a possible tendon contact until confirmed otherwise.
- Document all drill locations on a field sketch or marked plan. This record supports any future post tension slab repair assessment and protects all parties involved.
Never proceed with saw-cutting or core drilling through a PT slab based solely on visual judgment or experience with conventional reinforced concrete. The monostrand tendon in an unbonded system carries permanent force. There is no safe way to ‘feel’ your way around it.
For the full stressing safety protocols your PT crew must follow before and after any slab penetration near active tendons, see our article on post-tension slab stressing safety protocols.
What It Costs When the Protocol Is Skipped: Post-Tension Cable Repair Costs
Examples of post-tension cable snap repair projects in the Dallas area illustrate a recurring issue: in several cases, slab owners were not aware that their slab was post-tensioned. Typically, a contractor had proceeded with drilling or cutting for plumbing or electrical penetrations without following a pre-drill protocol, resulting in tendon damage.
The table below presents typical repair scopes along with estimated cost ranges observed in such scenarios. These values are provided for reference only and should not be considered fixed quotes, as post-tension cable repair costs vary depending on tendon accessibility, slab thickness, anchor configuration, and local labor rates.
| Damage Scenario | Typical Repair Scope | Estimated Cost Range * |
|---|---|---|
| Single unbonded PT cable severed | Tendon replacement + anchor repair | $1,500–$4,500 |
| Multiple tendons cut (cluster drill) | Full panel re-tension + grouting | $6,000–$18,000+ |
| Secondary cracking at cut zone | Epoxy injection + surface restoration | $800–$2,500 |
| Structural re-evaluation required | Licensed PE review + revised drawings | $2,000–$8,000 |
* Cost ranges are indicative estimates based on observed projects. Consult your local market for current pricing, as actual costs vary by region, slab conditions, and contractor rates.
These figures do not include consequential damages: schedule delays, tenant disruption, structural monitoring during the repair period, or the cost of a licensed PE review to confirm the remaining slab capacity. When we add those layers, a single inadvertent tendon cut on a commercial floor can realistically exceed $25,000 in total project impact.
If you are evaluating whether a PT slab can accommodate a span modification or additional penetration zone, see our guide on maximum recommended spans for residential post-tension slabs.
Field Observations: What Worked and What Did Not
What Worked on Site
- Pre-marking tendon lines with spray paint at 12 in. intervals before the GPR crew left the site gave the drill crew a permanent reference that survived foot traffic and dust.
- Running GPR at two perpendicular orientations (longitudinal and transverse) caught a tendon deviation that a single-pass scan had missed on a pour with mid-span splice adjustments.
- Reviewing the original PT shop drawings alongside the GPR output resolved a 4 in. discrepancy between as-built and design positions in the distributed tendon band.
What Did Not Work
- Relying on rebar detector apps or low-cost stud finders near PT zones. These tools cannot distinguish a tendon from rebar and provide no depth information.
- Assuming tendons follow perfect grid lines in slabs with irregular column spacing. Field-adjusted routing around MEP penetrations and re-entrant slab corners routinely diverges from the orthogonal grid assumed in the drawings.
- Deferring GPR to ‘just this one small hole.’ The one case where the protocol was skipped is consistently the one where the tendon was hit. The economics of a GPR scan, typically $300 to $800 per day, are negligible relative to the cost of a post tension slab repair.
Frequently Asked Questions
Can I drill anywhere on a post-tension slab as long as I avoid the column strips?
No. While column-line zones carry the highest tendon density, distributed tendons run throughout the mid-span field of the slab. There is no zone in a two-way PT flat plate that can be considered unconditionally safe for drilling without first verifying tendon positions using GPR or confirmed as-built drawings.
What is the minimum safe distance from a confirmed tendon centerline for a core drill?
ACI 318 does not prescribe a universal minimum clearance for post-drill activities, as this is an operational protocol rather than a design code requirement. In our practice, we maintain a minimum 3 in. clear distance from a confirmed tendon centerline for cores up to 2 in. diameter. Larger cores or saw cuts require individual assessment based on the tendon spacing and slab geometry.
How do I find out if my slab is post-tensioned before starting work?
Inspect the slab perimeter at the edge beam for patched pockets (stressing pockets) spaced at regular intervals, typically 18 in. to 60 in. on center. If pockets are present, the slab is likely post-tensioned. Contact the structural engineer of record, the original contractor, or the local building department for permit drawings before proceeding.
Is a post-tension cable snap repair covered by standard contractor insurance?
Insurance coverage for PT tendon damage depends entirely on the specific policy and the circumstances of the incident. In experience, coverage disputes arise frequently because the policy language does not explicitly address post-tensioned concrete. We recommend that any general contractor working on PT slabs confirm their policy terms with their insurer before beginning penetration work.
Who is qualified to perform an inspection of post-tension slabs prior to drilling?
A licensed GPR scanning technician can locate tendon positions. However, the interpretation of that data and the determination of safe drill zones requires engineering judgment. For complex layouts or slabs with suspected damage or prior repair history, the review should involve a structural engineer with specific PT design experience. The Post-Tensioning Institute (PTI) maintains resources on qualified PT engineering practices.
Need PT Slab Drawings or Calculations?
Need PT Slab Drawings or Calculations?
At TensionOne, we prepare complete PT slab layout drawings and calculation notes as a freelance assignment. This includes tendon profiles, band/distributed layout, stressing sequences, and serviceability checks per ACI 318.
Submit Your Project ScopeTensionOne provides structural engineering support services. All deliverables are prepared for review and use by a licensed Professional Engineer. TensionOne does not provide PE-stamped documents directly.
References: ACI 318-19: Building Code Requirements for Structural Concrete, American Concrete Institute. PTI DC80.3: Specification for Unbonded Single Strand Tendons, Post-Tensioning Institute.
About the Author: Joseph is a Civil Engineer and Founder of TensionOne LLC, specializing in post-tensioned slab design, structural calculations, and tendon layout for projects across Texas and internationally. All content is based on field experience and published engineering standards. No PE-stamped structural guarantees are expressed or implied by this article.
