Non-destructive testing and cover surveys
Find out what is inside the concrete before the drill does - and never trust a single method on its own.
Last updated 2026-08-25
What is Non-destructive testing and cover surveys?
Non-destructive testing is a family of methods for working out what is inside an element without breaking it open. An electromagnetic covermeter locates reinforcement and measures cover, reliably through roughly the first 100-150 mm and to within a few millimetres on a clear first layer. Ground penetrating radar builds a fuller picture - deeper bars, ducts, post-tensioning, voids and slab thickness - using antennas that commonly run from around 400 MHz to 2 GHz, where the low frequencies buy depth and the high frequencies buy resolution, and useful depth in dense concrete typically sits somewhere in the 300-600 mm band. Alongside those sit rebound hammer testing for surface hardness, ultrasonic pulse velocity for uniformity and cracking, half-cell potential and resistivity for corrosion risk, thermography for damp and detached finishes, and an endoscope for looking into a cavity through a very small hole.
On site it does two distinct jobs. The first is permitting a hole: before anybody drills, cores or cuts into an existing slab, wall or soffit, the element is scanned, the safe zone is marked on the concrete itself and photographed, and the permit to drill is tied to that mark and to a date. That single routine prevents the failures that cost real money - a severed main bar, a cut post-tensioning tendon, a drill through a buried conduit. The second job is assessment: mapping as-built cover across an element, gridding half-cell readings at spacings commonly around 0.5-1 m to map corrosion risk, checking a slab thickness, or building a picture of a structure whose drawings were lost decades ago. That is most refurbishment work, and NDT is usually the only way in.
The honest limitation is that none of these methods sees anything. Each one infers a property from a returned signal, and the interpretation is the survey. Congested top steel screens whatever lies beneath it; a mesh acts very much like a mirror; wet, very green or chloride-laden concrete cuts radar penetration sharply; and a rebound hammer measures the hardness of the outer few tens of millimetres, which is an index correlated to strength on that structure by cores, not a strength test in itself. Which is why the working discipline is corroboration - two methods that fail differently, plus a small number of confirmatory openings, drilled inspection holes or cores that tie the inference to something physical. The engineer sets the acceptance criteria and the depth of investigation; the testing supplies the evidence against them.
How does Non-destructive testing and cover surveys work, step by step?
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Step 1: Define the question and the acceptance criteria
Decide precisely what has to be known, because the method follows from it: can I drill here, how much cover is there, how thick is this slab, is the steel corroding, is there a tendon in this zone. The engineer states the acceptance criteria and how deep the investigation has to go. Gather whatever records exist first - original drawings, previous repair reports, the reinforcement schedule - because a scan read against a drawing is interpretation, and a scan read against nothing is pattern-spotting.
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Step 2: Choose and combine the methods
Covermeter for cover and first-layer bar positions, radar for depth, thickness, ducts and anything below the top mat, half-cell and resistivity for corrosion risk, hammer and ultrasonics for uniformity and comparison between elements. No single method carries a critical decision on its own - each has a failure mode, and the point of pairing them is that the failure modes are different. Where the consequence of being wrong is a tendon or a main bar, the pairing is not optional.
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Step 3: Set out a grid and calibrate on the actual element
Mark a grid on the element and reference it to something permanent - a column line, a corner, a level - so anyone can find the same spot next month. Calibrate the instruments on the day: the covermeter against a known bar or a test block, the radar velocity against a measured thickness or a core, because depth from radar depends on an assumed signal velocity and an uncalibrated assumption gives confidently wrong depths. Prepare the surface too - loose material, standing water and thick coatings distort every one of these methods.
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Step 4: Scan systematically, and in two directions
Run parallel lines at a spacing fine enough to catch the smallest thing that has to be found - a coarse grid will step straight over a single narrow duct and report a clear element. Then run a second pass at right angles, because a scan parallel to a bar sees very little of it and a scan across it sees it clearly. Log position continuously rather than taking spot readings and remembering where they were. Watch for edge effects near corners, openings and the ends of elements, where every method degrades.
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Step 5: Interpret against the drawings and mark the element
Plot the results, compare them against whatever the drawings say should be there, and flag the anomalies rather than smoothing them out - the thing that does not match the drawing is usually the thing that matters. Paint the safe zone directly on the concrete, photograph it with the grid reference in shot, and issue a permit that is specific to that marked zone and carries an expiry. Marks get painted over, elements get confused with the one next door, and a scan is valid for a place and a date, not for the element forever.
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Step 6: Corroborate, then report what was and was not investigated
Validate the interpretation with a small number of confirmatory openings, small-diameter inspection holes or cores at points chosen to test the conclusion, not to confirm it. The report then states the methods used, the area actually covered, the depth of investigation achieved, the confidence in each finding, and - the section everyone skips writing and everyone later needs - what was not investigated and why. An NDT report that reads as though the whole structure was proven sound is the one that causes the next incident.
What are the benefits of Non-destructive testing and cover surveys?
- Answers what is inside an element without damaging it, so a structure in service keeps working through the survey
- Fast area coverage - a floor plate can be scanned in a day where breaking out would take a week and a licence to make that much noise
- Prevents the genuinely expensive failures: a cut tendon, a severed main bar, a drill through a live service
- Maps cover and corrosion risk across whole elements, so repair is targeted instead of blanket
- Provides evidence about structures with missing or unreliable records, which describes most refurbishment
- Repeatable on the same grid, so deterioration can be tracked over years rather than judged from memory
What are the limitations of Non-destructive testing and cover surveys?
- Every method infers from a signal - none of them sees, and the result is only as good as the interpretation
- Congested or closely spaced reinforcement screens whatever lies beneath the first layer
- Wet, very green or chloride-contaminated concrete cuts radar penetration sharply, sometimes to almost nothing
- Depth from radar rests on an assumed signal velocity that must be calibrated against the actual element
- Surface hardness and pulse velocity give indices, not strengths, until correlated against cores from that structure
- Marks and permits go stale, and a scan record is valid for a specific place on a specific date only
What is Non-destructive testing and cover surveys best suited for?
What plant does Non-destructive testing and cover surveys need?
- Electromagnetic covermeter with its calibration block
- Ground penetrating radar with a choice of antenna frequencies, plus a survey wheel or grid pad
- Half-cell potential and resistivity equipment, with a means of connecting to the reinforcement
- Rebound hammer and ultrasonic pulse velocity equipment for comparative testing
- Core drill, small-diameter drill and endoscope for confirmatory openings
- Grid tape, marker paint, camera and a dated permit book
How is Non-destructive testing and cover surveys quality-checked?
- Acceptance criteria and depth of investigation stated by the engineer before any scanning starts
- Instruments calibrated on site on the day, against a known bar, a test block or a measured thickness
- Scan line spacing recorded, and fine enough for the smallest feature that has to be found
- Second pass at right angles wherever the direction of bars, ducts or tendons is uncertain
- A sample of findings corroborated by confirmatory opening, inspection hole or core
- Report states methods, coverage, confidence and explicitly what was not investigated; drill permits marked, dated and time-limited