Post-Tension Cable Detection
Locate post-tension tendons and ducts before any coring, cutting or fixing. Safe zones marked on the slab. London, South East and UK-wide.
Post-Tension Cable Detection
Post-tensioned concrete is widely used in London's commercial offices, residential towers, car parks, retail developments and hospitals. The tendons run through the slab under high tension — typically 150–200 kN per tendon. If a tendon is severed during coring, cutting or drilling, the consequences are immediate and severe: sudden slab deflection, structural damage and a serious safety incident.
GPR is the primary non-destructive method for locating post-tension tendons and ducts before any intrusive works. ACUO uses the Proceq GP8000 and GS8000 GPR systems to produce clear B-scan and 3D depth-slice images showing tendon layout, spacing and depth across the full slab area.
All surveys are carried out by Gary Stringer, a GPR NDT Civil Engineer with over 20 years of experience in concrete scanning and post-tension slab surveys. We are Constructionline Gold and SSIP accredited.
Get a post-tension scanning quote
Tell us the slab type, the area to be scanned and what works are planned. We'll provide a fixed-price quotation, typically within 24 hours.
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Why Post-Tension Slabs Require Specialist Scanning
A post-tensioned slab looks identical to a conventionally reinforced slab from the surface. Without scanning, there is no way to know whether tendons are present, where they run, or how close they are to a proposed core or drill location.
Tendons typically run in two directions — banded in one direction and distributed in the other — at spacings of 1.2 to 2.0 metres. In car parks and commercial floors, the tendon layout is rarely documented in a form that is accessible on site. As-built drawings, where they exist, are often inaccurate.
The consequences of severing a tendon include: sudden slab deflection and cracking; loss of structural integrity across a wide area; expensive and disruptive repair works; and potential injury to workers on or below the slab. Repair costs for a severed tendon in a commercial building typically run to tens of thousands of pounds.
- Sudden slab deflection and cracking on tendon severance
- Structural damage extending well beyond the cut location
- Repair costs typically £20,000–£100,000+ depending on slab type
- Programme delay and potential HSE investigation
- Injury to workers on or below the slab
How We Locate Post-Tension Cables
GPR concrete scanning uses radar pulses to image the interior of the slab. Post-tension tendons appear as strong, consistent reflectors in the scan data — typically more prominent than conventional rebar because of their larger diameter and the metallic duct or sheathing surrounding them.
We scan the slab in a systematic grid, producing B-scan cross-sections in both directions. Where the tendon layout is complex or the floor plate is large, we use the Proceq GS8000 multi-channel GPR array to capture multiple parallel scan lines simultaneously, producing 3D depth-slice images that show the complete tendon layout at each depth level.
All detected tendons are marked on the slab surface using spray paint or chalk. We identify safe zones for proposed core or drill locations, and advise on alternative positions where no safe zone can be confirmed. The survey report documents the findings with annotated scan images and depth data.
High-resolution single-channel GPR for detailed B-scan imaging of tendon layout, spacing and depth. Used for localised surveys and complex slab configurations.
Multi-channel GPR array for rapid 3D scanning of large floor plates. Produces depth-slice images showing tendon layout across the full survey area simultaneously.
Post-Tension Slabs We Survey
Commercial Office Floors
Flat-plate and banded post-tensioned slabs in commercial offices. Typically surveyed before M&E installations, drainage penetrations and structural alterations during fit-out or refurbishment.
Multi-Storey Car Parks
Thin post-tensioned slabs with tendons running in two directions. Surveyed before drainage repairs, expansion joint works, surface treatments and structural fixings.
Residential Towers
Post-tensioned flat slabs in residential developments. Surveyed before M&E penetrations, bathroom and kitchen installations, and structural alterations.
Retail and Leisure
Large-span post-tensioned floors in retail centres, leisure facilities and mixed-use developments. Surveyed before fit-out works, service penetrations and structural fixings.
Hospitals and Healthcare
Post-tensioned slabs in hospital buildings where programme disruption must be minimised. Surveyed before M&E installations and structural alterations.
What You Receive
- On-slab mark-up of all detected tendons and rebar using spray paint or chalk
- Annotated site photographs showing the marked-up tendon layout
- Written survey report with B-scan and 3D depth-slice images
- Tendon spacing and depth data for the surveyed area
- Safe zone confirmation for proposed core and drill locations
- Alternative location recommendations where safe zones cannot be confirmed
Reports issued within 24–48 hours of survey as standard. Same-day reporting available for urgent programmes — discuss when enquiring.
Frequently Asked Questions
- Can GPR always find post-tension cables?
- GPR reliably detects post-tension tendons in the vast majority of concrete slabs. Tendons appear as strong reflectors in the scan data because of their metallic duct or sheathing. In very heavily reinforced slabs, the rebar can partially mask deeper features — in these cases we use 3D scanning and multiple scan passes to build a complete picture. We will always advise if there is any uncertainty in the results.
- Do you need drawings to carry out the survey?
- No. GPR scanning is a non-destructive survey method that does not require drawings or prior knowledge of the slab construction. We scan the slab and interpret the results from the data alone. If drawings are available, we will use them to cross-check our findings — but they are not required.
- How close to a tendon can you safely core?
- The minimum safe distance from a post-tension tendon depends on the core diameter, the tendon spacing and the structural engineer's requirements. As a general guide, a clearance of at least 75–100 mm from the nearest tendon is typically required. We mark the tendon positions on the slab and confirm safe zones for your proposed core locations — if a proposed location cannot be made safe, we advise on alternatives.
- What if the slab has both rebar and post-tension cables?
- Most post-tensioned slabs also contain conventional reinforcement. GPR detects both simultaneously. We identify and mark all detected features — tendons, rebar and any conduits — so you have a complete picture of the slab interior before works begin.
- How quickly can you mobilise in London?
- For most London surveys we can mobilise within 24–48 hours of instruction. For urgent requirements, call us directly on 07340 549892.
- Do you provide a written report?
- Yes. Every post-tension cable detection survey includes a written report with annotated scan images, tendon layout data, depth measurements and safe zone recommendations. Reports are typically issued within 24–48 hours of the survey.
Other concrete scanning and GPR services
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Map reinforcement layout, cover depth and bar spacing without breaking out concrete.
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Ground-penetrating radar surveys for concrete, ground and subsurface investigation.
Learn moreConstructionline Gold
Verified supply chain accreditation
SSIP Accredited
Health & safety pre-qualification
20+ Years Experience
Structural NDT specialists since 2003
Typical 24-Hour Response
Fast turnaround on quotes and bookings
Need a survey?
Call 07340 549892 or get a quote within 24 hours.