Can GPR detect post-tension cables?

Yes, GPR is a widely used non-destructive method for detecting post-tension cables in concrete floors. It is also one of the most safety-critical uses 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 one of the most serious avoidable mistakes on a 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 the tendon paths detected 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.
• Proposed drilling positions marked clear of detected tendons, for the engineer to approve.

The surveyor doing PT detection work should be experienced in concrete GPR. 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 a short delay; the downside of a strike is much greater.

What GPR cannot do

GPR locates tendon position within the limits of the equipment, the site conditions and the surveyor’s skill. 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.