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Post tension (PT) cables are used throughout the construction industry to reinforce concrete structures, providing the strength and flexibility that allows you to build bigger, build higher, and build more safely. Groupings of three to five PT cables are referred to as “banded.” Banded cables can significantly increase slab strength and are used in everything from elevator shafts to parking garages, and transfer slabs that have to bear loads over large spans.
Unfortunately, striking a single PT cable while cutting or coring can compromise structural integrity, create safety hazards, unwanted downtime for replacement and/or repair, and cost as much as $30,000 to replace. The hazards are only multiplied in a banded PT slab, where you risk cutting multiple cables.
The best way to mitigate the risk of a PT strike is to first detect all cables and bundles, conduit, and other reinforcements, and marking out all those reinforcements on the slab to ensure safe operations when you cut, core, or drill. For jobs where you need complete interior slab visualization, reality capture tools can be used to create 2D and 3D as-builts, if you have the appropriate training and technology.
Step 1: Understand Banded PT Cable Layout
Banded PT cables are grouped together in concentrated zones, typically in slabs over columns. They follow specific patterns, usually perpendicular to the uniform cables in the slab. Identifying these locations is crucial before performing any work.
What’s the Difference Between a Uniform and Banded PT Cable Layout?
PT Cables, also known as tendons, can be used as support throughout a slab in a “uniform” layout, spaced approximately 2-3 ft. apart. They are banded in three to five cable groups when more structural reinforcement is required, around pillars, elevator shafts, in transfer slabs, and in beam-supported slabs.
The photo below shows the complexity of a two-way post tension slab layout prior to the concrete pour. It shows both uniform and banded PT cables as well as other reinforcements.
Step 2: Gather Necessary Equipment
There are various methods of locating banded PT cables with advanced scanning technology:
- Ground Penetrating Radar (GPR): Detects PT cables within concrete via radio waves and displays them as a series of hyperbolas on a screen. A highly trained concrete imaging professional like a GPRS SIM-certified Project Manager can interpret those hyperbolas to locate virtually every cable and other reinforcement with 99.8% accuracy.
- Electromagnetic (EM) Locating: Identifies embedded metallic objects and is often used in concert with a GPR scanner to verify results and be certain that everything in the slab is found. Subsurface Investigation Methodology, otherwise known as SIM, requires the utilization of complementary technologies – like using both GPR and EM locating to get the most comprehensive slab imaging available. Every GPRS Project Manager is required to be SIM-certified, which is the most rigorous training and education standard available in our industry. SIM level 101 certification requires 80 hours of classroom instruction and 320 hours of mentored field work.
- Concrete X-Ray (if applicable): Can provide precise imaging. However, it is often unwieldy to operate, requires significantly more time to develop imagery, and brings another level of risk – radiation – to an already risky job site.
- Chalk or Paint Marking Tools: Depending on your needs, marking tools may include spray paint, paint markers, chalk, or other temporary marking tools like carpet tack, paper, and tape in areas where the markings must be removable and not permanently damage the concrete.
Other methods of interior slab visualization, like tomography, are often used to assess structural deterioration and faults in concrete. They are significantly more expensive than GPR and EM locating methodology because they provide a level of detail that is not required to find PT cables and reinforcements.
Step 3: Perform Initial Site Evaluation & Gather As-Builts
Before scanning, review structural drawings if available. Look for:
- PT cable layout diagrams, record drawings, or as-builts
- Core locations planned near support columns
- Any existing markings indicating prior investigations
In most cases, any existing documentation will be outdated or incomplete, so verification of as-built records will be required to ensure clearances. Verification is completed by scanning with GPR and EM locating devices, interpreting their findings and mapping/marking out the reinforcements on the slab surface.
Step 4: Scan for PT Cables Using GPR
- Calibrate the GPR based on slab thickness and material composition. This requires understanding dielectric values for concrete, which depend on the type of slab, the maturity of the slab, and its moisture exposure.
- Sweep the concrete surface in a grid pattern to detect embedded cables
- Identify banded PT cable zones: they will show as concentrated lines/hyperbolas in scans
- Verify findings with secondary methods like EM locating for accurate confirmation
- Mark detected cables with chalk or paint and label each band clearly
Step 5: Determine Safe Coring and Cutting Zones
- Measure distance between cables to find clear spaces for coring
- Mark a buffer zone: Typically a minimum of three inches away from cables
- Align cutting or coring locations with confirmed safe zones
- Cross-check scan results with structural engineers if needed
GPRS is the only concrete scanning & imaging company that offers a “Green Box Guarantee” on its work. If our Project Managers mark a green box with “CLEAR,” that means we guarantee that the area is free of obstructions. If we get it wrong, we’ll pay for the material cost of repair. You can learn more about the Green Box Guarantee, here.
Utilizing Ground Penetrating Radar (GPR) and Electromagnetic (EM) Locating with Banded PT Cables
Banded post-tension (PT) cables are strategically placed in reinforced concrete structures, typically concentrated in load-bearing zones. To locate and safely work around them, professionals rely on Ground Penetrating Radar (GPR) and electromagnetic (EM) locating.
Understanding PT cable placement and utilizing multiple detection methods ensures accuracy in locating and marking these critical reinforcement elements before cutting or coring.
How to use Ground Penetrating Radar (GPR) for Banded PT Cable Detection
GPR is the most effective tool for mapping PT cables within concrete. It sends high-frequency radio waves into the slab and detects variations in material density, revealing embedded structures.
How GPR Identifies Banded PT Cables:
- Scanning in a Grid Pattern – Operators systematically move the GPR unit across the concrete surface to ensure full coverage
- Signal Reflections and Data Interpretation – PT cables appear as hyperbolic curves in the radar scan due to their cylindrical shape. Banded cables will show as multiple strong, parallel curves clustered together
- Adjusting Depth Settings – GPR settings are optimized based on slab thickness (typically between 4” and 12”) to accurately detect cables and distinguish them from other embedded materials
- Marking Cable Locations – Once verified, cable positions are marked on the slab using chalk or paint for reference before drilling or cutting
Electromagnetic (EM) Locating for Additional Verification
While GPR provides precise visual mapping, EM locating helps detect live electrical conductors or metallic objects within the slab. This tool is particularly useful when:
- The structure contains significant rebar interference, potentially affecting GPR readings
- Metallic PT cable sheathing needs further confirmation
- Locating high-voltage conduits embedded within the slab alongside PT cables
Other Locations Where Banded PT Cables Might Be Found
Although banded PT cables are primarily concentrated near columns and load-bearing walls, they may also be present in:
- Transfer Slabs – Large spanning slabs distributing loads between multiple support points
- Parking Garage Ramps – Areas with increased stress due to traffic load require additional reinforcement
- Cantilevered Balconies – PT cables are often grouped near support edges to resist bending forces
- Beam-Supported Slabs – Where PT reinforcement aligns with primary structural beams
- Elevator Shaft Surroundings – Structural areas requiring additional reinforcement around mechanical openings
You’ve Scanned the Slab, Verified the Records, and Located Your Clear “Safe Zones.” Now, It Is (Finally!) Time to Cut
Once you have cross-checked and verified your clearances, you are ready to pierce the slab surface. There are specific, safe, “best practices” that are recommended for cutting and coring concrete. One of the most important safety practices has nothing to do with cables or reinforcements, but in keeping your team safe from respirable crystalline silica (RCS) and the incurable disease of silicosis, which breathing RCS can cause. You can learn more about the dangers of pinch points, kickback and RCS exposure during Concrete Sawing & Drilling Safety Week (CSDSW), which GPRS sponsors every winter. Learn more about CSDSW, here.
GPRS does not cut or core concrete as a service, but we work to keep concrete workers and their jobsites safe, each and every day. The steps below are those commonly taken when cutting & coring concrete.
Step 6: Execute Cutting or Coring Safely
- Use proper PPE as outlined by OSHA
- Use equipment with water delivery, aka “wet” systems to help tamp down on silica dust exposure
- Use a low-vibration coring rig to reduce stress on surrounding concrete
- Start with a pilot hole to verify no immediate cable interference
- Monitor core depth to ensure cables remain undisturbed
- Proceed with full core drilling or cutting in marked safe zones
Step 7: Post-Cut Inspection and Documentation
- Inspect the core site for signs of cable proximity or tension shifts
- Verify slab integrity post-cut to confirm no structural compromise
- Update site records with marked cable locations for future reference
When you utilize a project or facility management software solution like SiteMap® (patent pending), powered by GPRS, uploading, versioning, and aggregating your concrete records becomes simple.
We can provide tailor-made data solutions for even the most densely banded PT slabs, including capturing our mark-outs via 3D laser scanning or photogrammetry, and using that 2-6mm accurate data to create a full 3D BIM model of everything that is inside your slabs. Learn more about how SiteMap can help keep your concrete jobs safer, here.
Final Considerations
- It is advisable to consult structural engineers before cutting PT slabs if you are not confident of what’s inside your concrete
- Avoid assumptions: scan every location regardless of existing markings & documentation
- Use multiple detection methods for accuracy in identifying banded PT cables
Safely working around post-tension cables ensures structure longevity and worker safety. Proper detection, marking, and cutting techniques prevent costly errors and hazards. Hiring a professional concrete scanning company like GPRS can help. It’s our mission to Intelligently Visualize The Built World® for our customers. What can we help you visualize?
Frequently Asked Questions
What is Subsurface Investigation Methodology?
Subsurface Investigation Methodology (SIM) is the most rigorous subsurface certification in the industry. SIM allows GPRS to ensure a standardized approach for locating underground utilities and scanning concrete structures. It combines expert training, advanced technology like ground penetrating radar (GPR), and proven field methods to ensure accurate, non-destructive investigations. SIM enhances site safety, prevents damage, and improves accountability in construction projects. Learn more here.
How can reality capture be used to create BIM models of concrete slab interiors?
GPRS can use reality capture technologies like 3D laser scanning and photogrammetry to create precise BIM models of concrete slab interiors. The technology is used to accurately capture GPRS’ 99.8% accurate concrete mark-outs, which can be converted into digital 3D models for design, analysis, and construction planning. This process minimizes errors and improves efficiency. Explore further here.
