Detecting post-installed fixings and anchors with GPR

Post-installed fixings are everywhere in existing buildings — anchors holding brackets, supports, plant, balustrades, and cladding to concrete that was poured long before the fixing was needed. They are often invisible from the surface, and they matter for two reasons: you do not want to drill into one, and you do not want to drill so close to one that you compromise its hold. This post explains how GPR and ferro scanning are used to locate post-installed fixings before further drilling or coring.

What a post-installed fixing looks like in concrete

A cast-in fixing is placed before the concrete is poured and is part of the original design. A post-installed fixing is the opposite: a hole is drilled into hardened concrete and an anchor is set into it afterwards. Common types include expansion anchors, screw anchors, and chemical or resin-bonded anchors, where a steel stud or threaded bar is bonded into the drilled hole with adhesive.

From a scanning point of view the key facts are that the fixing is steel, it sits in a hole that breaks the surrounding concrete, and around a resin anchor there is a sleeve of cured adhesive. The fixing itself is usually short — an embedment of 60mm to 150mm is typical — and it may be cut flush or hidden behind the bracket it supports.

Why locating them matters

There are two distinct risks. The first is striking the fixing while drilling for something else. A masonry bit meeting a steel anchor will stall, wander, or break, and the new hole is then in the wrong place. The second risk is subtler and more important: drilling too close to an existing loaded anchor.

Anchors transfer load into the concrete through a cone of material around the embedded section. Resin anchors in particular rely on the integrity of the concrete immediately surrounding the bonded length. A new hole or core taken close to a working anchor can disturb that cone, crack the surrounding concrete, or reduce the edge distance the anchor was designed around. The anchor may still look fine while having lost a significant part of its capacity. Where an anchor is carrying a real structural load — supporting plant, a canopy, a secondary steel member — that loss matters.

How GPR detects them

GPR finds post-installed fixings the same way it finds any steel: the metal reflects the radar pulse strongly and produces a clear signature on the scan. A short steel anchor near the surface appears as a compact, high-amplitude reflection, distinct from the long, regular pattern of reinforcement bars.

There are useful secondary clues. The drilled hole around the anchor is a small disturbance in otherwise continuous concrete, and a resin sleeve has different electrical properties to the parent concrete. A skilled surveyor reads the combination — a strong point reflector with a local disturbance around it — rather than relying on the metal signature alone. Because the fixings sit shallow, a high-frequency antenna is used to give good near-surface resolution.

GPR has a limitation worth stating plainly. Where post-installed anchors sit within a dense reinforcement layer, the anchor’s reflection can be hard to separate from the surrounding bars. This is where ferro scanning earns its place.

Where ferro scanning adds confidence

Ferro scanning uses electromagnetic induction and responds only to ferrous metal. It does not see plastic, voids, or resin, but it is very good at confirming the presence and position of steel close to the surface. Used alongside GPR, it helps confirm that a reflection genuinely is a metal anchor rather than a feature in the concrete, and it can pinpoint a shallow steel stud precisely.

The two methods are complementary. GPR gives the wider picture — reinforcement, services, depth, and the disturbance around a fixing — while ferro scanning confirms the metal itself near the surface. For locating post-installed fixings reliably, using both is good practice rather than belt-and-braces.

What the survey delivers and its limits

A scan for post-installed fixings produces marked positions on the surface showing where anchors have been found, so that new drilling or coring can be set out clear of them with a sensible buffer. Where the task is to core close to a known bracket, the survey defines how much clear concrete is genuinely available.

Two limits should be understood. Scanning locates a fixing; it does not measure its remaining capacity or whether it was correctly installed — that is a question for a structural engineer, sometimes answered with a pull-out test. And very small or deeply recessed fixings can be difficult to detect with confidence. The brief should always say what is known to be in the area.

If a project involves drilling or coring in existing concrete near known brackets, plant, or supports, scan for the post-installed fixings first. Locating them protects the new work and, just as importantly, protects the load path the existing anchors were installed to carry.