Slab thickness measurement with GPR
Knowing the thickness of a concrete slab is fundamental to structural assessment, drainage design, and reinstatement work. Here is how GPR measures it and how accurate it is.
The thickness of a concrete slab seems like it should be a known quantity. Often it is not. Drawings go missing, as-builts never match the pour, slabs get topped or overlaid, and ground-bearing slabs in particular are poured to a tolerance rather than a precise figure. Knowing the real thickness matters for structural assessment, for drainage and reinstatement design, and for any work that cuts into or loads the slab. GPR measures slab thickness without breaking it open. Here is how it works and how far the result can be trusted.
Why slab thickness matters
Slab thickness drives a surprising number of decisions. A structural engineer assessing whether a floor can take a heavier load needs the thickness to do the calculation at all. A drainage designer cutting channels into a slab needs to know how much concrete there is to cut through and how much will be left. A contractor reinstating a section of slab needs to match the existing depth. And anyone investigating a slab defect needs the thickness as a basic parameter of the assessment.
Guessing is not good enough for any of these. A slab nominally 200mm thick might be 170mm in places and 230mm in others, and the difference changes the answer. Coring gives an exact figure but only at the points cored, and it is destructive. GPR gives a thickness profile across the whole slab without cutting it.
How GPR measures thickness
GPR sends a radar pulse into the concrete and records the reflections that come back. The slab produces a reflection at its underside — the boundary between the concrete and whatever sits beneath it, whether that is sub-base, void, or a different material. The instrument measures the time the pulse takes to travel down to that boundary and back.
Converting that travel time into a depth requires knowing how fast the radar signal moves through the concrete. That speed depends on the concrete’s properties, principally its moisture content. This is the single most important factor in the accuracy of a thickness measurement, and it is where calibration comes in.
The role of calibration
GPR measures time; it infers depth. The conversion is only as good as the signal velocity used. An uncalibrated measurement, relying on a typical velocity for concrete, gives a reasonable estimate — but “reasonable estimate” is not the same as “measured”.
Calibration sharpens this considerably. The standard approach is to take one or a small number of cores, or to use a known feature of established depth, and adjust the assumed velocity until the GPR reading matches the verified thickness at that point. That calibrated velocity is then applied across the rest of the survey. The result is a thickness profile that combines the exactness of a core with the coverage of GPR — verified at the calibration points and reliably interpolated everywhere else.
This is the honest way to commission slab thickness work: a GPR survey across the whole slab, calibrated against a small number of cores rather than relying on cores alone. It is more accurate than uncalibrated GPR and far more representative than a handful of isolated core measurements.
How accurate is it
With proper calibration, GPR slab thickness measurement is accurate to within a small percentage of the true thickness across most ordinary slabs — good enough for structural assessment, reinstatement, and design work. Without calibration, the figure is an estimate that should be treated as indicative.
A few conditions affect accuracy:
- A clear underside reflection. Where the slab sits on a sub-base of similar electrical properties, the boundary reflection can be weak and harder to pick out. A strong contrast — concrete over a void, or over a markedly different material — gives a clean, easy reading.
- Variable moisture. If the moisture content varies across a large slab, a single calibration velocity becomes less representative. On big areas, more than one calibration point helps.
- Heavy reinforcement. Dense reinforcement scatters the signal and can make the underside reflection harder to resolve, particularly in thicker slabs.
- Toppings and overlays. Where a slab has been overlaid, GPR can often distinguish the layers, but the surveyor needs to know an overlay may be present so it is interpreted correctly rather than read as a single pour.
What the survey delivers
A slab thickness survey is normally delivered as a thickness profile or a colour-shaded plan showing how the depth varies across the slab, together with a note of the calibration method and the velocity used. Where thickness varies significantly, the plan makes the variation visible — which is often as useful as the headline figure, because it shows the engineer where the slab is thinnest.
The survey is non-destructive apart from any calibration cores, fast to carry out, and covers far more of the slab than coring alone ever could.
When to commission it
Slab thickness measurement with GPR is worth commissioning before any structural reassessment of a floor, before cutting drainage or services into a slab, before reinstatement work that needs to match existing depth, when original drawings are missing or unreliable, and as part of a broader condition survey where thickness is one of several parameters being established.
Practical advice
GPR is the right tool for measuring slab thickness across an area, and with calibration it gives an engineer a result they can rely on. Brief the surveyor on what the slab sits on and whether it may have been overlaid, agree on calibration against at least one core rather than accepting an uncalibrated estimate, and ask for a profile across the slab rather than a single average figure. Done that way, a thickness survey gives full-coverage data at near-core accuracy without taking the slab apart.