Post-Tension Cable Detection
Post-tension cable detection is the location of the tensioned steel tendons inside a post-tensioned concrete slab before anyone cores, cuts or drills into it. We scan the slab with ground penetrating radar (GPR), interpret the draped tendon profile against the surrounding reinforcement, and mark the cable lines and the safe penetration zones directly onto the concrete so the work can proceed without a strike.
The stakes are high. In a post-tensioned slab, high-tensile tendons run through the concrete under enormous load and follow a draped profile that is not obvious from the surface. Cutting or drilling into one of those tendons is one of the most serious things that can happen on a coring job: the stored energy releases suddenly, the structure can be compromised, and re-stressing or repairing a severed tendon is difficult and expensive.
This is a specialist application of concrete scanning, and it depends heavily on interpretation. Distinguishing a draped tendon from ordinary reinforcement takes an experienced operator reading the depth and profile of the reflections, not just a machine. We scan for main contractors, structural engineers, M&E and fit-out teams working in post-tensioned buildings across London and the surrounding region.
- Method
- Ground penetrating radar (GPR)
- Access needed
- Single face of the slab, from above or the soffit
- Disruption
- Non-destructive and non-ionising, no exclusion zone
- Detects
- PT tendons and draped profile, reinforcement, services, anchorage zones
- Deliverables
- On-surface markup; annotated scan drawings (PDF)
- Coverage
- London and UK-wide
What we locate
Before you break into a post-tensioned element, we locate and mark:
- Post-tension cables and tendons, including their draped profile within the slab
- Conventional reinforcement running alongside or above the tendons
- Embedded electrical conduits and services
- Anchorage zones where tendons terminate at slab edges
- Clear zones where a core or cut can be made without striking a tendon
When you need it
Post-tension detection is essential:
- Before any core, cut, chase or drilled penetration in a post-tensioned slab
- Before installing anchors or fixings into a post-tensioned floor or soffit
- When forming new openings, risers or service routes in an existing PT structure
- During fit-out and alteration where the slab type is post-tensioned or suspected to be
- Whenever drawings are unavailable or unverified and the tendon layout must be confirmed on site
Deliverables
We mark the line of the post-tension cables and the surrounding reinforcement and services directly onto the slab, and we identify the safe zones for each proposed penetration. This on-site markup is what allows coring and cutting to proceed with confidence.
Where a record is needed, we provide annotated drawings or sketches showing the detected tendon runs, reinforcement, estimated depths and the agreed clear penetration points. For larger programmes across several floors we can record findings in a consistent format so the information can be shared with the design and construction team and reused as the works progress.
What affects the price
Every job is priced to its scope. The main things that move the cost are:
- The number of penetrations to be checked and the extent of slab to be scanned
- Access to the slab, from above or from the soffit, and any work at height
- The complexity of the tendon layout and reinforcement to be interpreted
- Whether you need on-surface markup only or a drawn record of the tendon runs
- Site location and out-of-hours or occupied-building working
Send us the brief or call and we will confirm a fixed price for the work, free and with no obligation.
A survey you can plan around.
A clear, repeatable method on every job, so you know what is happening on site and what lands on your desk afterwards.
Scope
We confirm the slab is or may be post-tensioned, review any available drawings, and agree the penetration locations to be checked.
Scan on site
We run GPR across each area, building a picture of the tendon profile, reinforcement and services.
Interpret and mark up
We interpret the data to distinguish tendons from reinforcement, mark them on the surface, and identify clear zones for each penetration.
Report
Where required, we issue annotated drawings recording the tendon layout, depths and safe penetration points.
Questions we are asked about this service.
What is post-tension cable detection?
Post-tension cable detection is the process of locating the tensioned tendons within a post-tensioned concrete slab before any coring, cutting or drilling. It is usually carried out with ground penetrating radar so that penetrations can be planned to avoid the tendons, preventing a strike that could compromise the structure and endanger workers.
Why is it so important to locate post-tension cables before drilling?
The tendons in a post-tensioned slab carry very high loads. Cutting or drilling into one can release that energy suddenly, damage the structural integrity of the slab and create a serious safety hazard, and repairing or re-stressing a severed tendon is costly and disruptive. Locating and avoiding the tendons first removes that risk.
Can GPR tell a post-tension cable apart from rebar?
GPR shows where objects are, and telling a draped post-tension tendon apart from conventional reinforcement relies on skilled interpretation of the depth and profile of the reflections. Tendons typically follow a curved, draped path through the slab depth, which an experienced operator can distinguish from the more regular pattern of reinforcement. This is why interpretation is central to reliable PT detection.
Can you scan a post-tension slab from one side?
Yes. GPR only needs access to one face of the slab, so we can scan a post-tensioned floor from above or a soffit from below without needing access to both sides, and without the exclusion zones that X-ray methods require.
Do I still need to scan if I have the original drawings?
Drawings are a useful starting point, but tendon positions can differ from the design, and drawings are often unavailable, incomplete or unverified in existing buildings. Scanning confirms the actual layout on site before you commit to a penetration, which is why it is recommended even when drawings exist.
What do I get at the end of the scan?
At minimum, the tendon lines, reinforcement and services are marked directly on the slab along with the safe penetration zones. Where you need a record, we also provide annotated drawings showing the tendon layout, estimated depths and agreed clear points.
One team, from point cloud to pull test.
Concrete scanning
See the rebar, conduit and post-tension cables hidden in the slab, before the drill goes anywhere near them.
See the service BS 8539Anchor pull testing
Prove the fixing holds before you trust it, pull-tested on site for anchors and ties to BS 8539.
See the service LiDAR3D laser scanning
Capture the whole structure to the millimetre and hand your designers a model they can build from.
See the serviceSend us the brief. We will scope it.
Tell us the site, the deadline and what you need to know. We will confirm the right survey or test, the standard it meets and exactly what you will receive.
- 1Send the brief or call.
- 2We confirm scope, method and price, free and with no obligation.
- 3Booked, surveyed, reported.