Can GPR detect post-tension cables?

Yes — GPR is the standard non-destructive method for detecting post-tension cables in concrete floors. It is also the most safety-critical use of GPR on UK construction work.

How GPR sees PT cables

A post-tension tendon sits inside a duct, anchored at one end and stressed at the other. In GPR data, tendons appear as long, often slightly curved reflectors. Their depth typically follows a parabolic profile across the span — drooping to the bottom of the slab in the middle and rising to the top near the supports.

A trained surveyor recognises the profile and maps the tendon path with confidence. The surveyor also identifies the conventional reinforcement that almost always sits alongside the tendons, distinguishing the two on the deliverable.

Why it matters

A struck post-tension tendon is the most expensive avoidable mistake on any modern slab. Damage can cause:

• Immediate structural redistribution.
• Costly remediation including replacement-tendon installation.
• In severe cases, partial collapse.
• Operator safety risks, particularly with unbonded systems where a severed tendon can fly.

For any drilling, coring, or cutting on a slab that may be post-tension, GPR pre-drill scanning is the standard mitigation — and on confirmed PT floors, it is non-negotiable.

What good practice looks like

A defensible PT scan deliverable includes:

• Identification of every tendon path within the surveyed area.
• A clear distinction between tendons and conventional reinforcement.
• Depth profile information at the surveyed positions.
• Explicit statement of any areas where additional intrusive verification is recommended.
• A signed-off drilling plan with safe positions only.

The surveyor doing PT detection work should hold EuroGPR Certified Surveyor competence on concrete and structural work. PT interpretation is one of the most demanding GPR applications; experience matters significantly.

What to provide the surveyor

Before attendance, supply:

• The structural design drawings, if available.
• Any indication that the slab is post-tension (typically clear in the design).
• The proposed drilling positions, marked on the slab if possible.
• Site induction and access details.

If the design is unclear and PT construction is in any doubt, treat the slab as PT and brief the surveyor accordingly. The downside of unnecessary caution is half a day; the downside of a strike is much greater.

What GPR cannot do

GPR maps tendon position with the accuracy of the equipment and the surveyor’s skill — typically within 10 mm in plan at typical cover depths. For drilling positions that fall within the geophysical uncertainty of a tendon, the safe answer is to reposition the drill rather than test the limits of the survey.

For more on PT scanning, see our article Post-tension cables — the hidden danger in concrete floors.