Last reviewed 30 Sept 2026 · 7 min read
The role of NDT
Non-destructive testing (NDT) examines a structure without damaging it, to measure strength indicators, find hidden defects, locate reinforcement and assess corrosion. It answers questions that visual inspection cannot: Is the concrete sound inside? Is the steel corroding? How much cover is there? Are there cracks in a weld? Where are the tendons? NDT results are indirect: they measure a property (rebound, wave speed, potential) that correlates with the quantity of interest. They are therefore used together and calibrated against cores or coupons.
NDT of concrete
| Test | What it measures | Detects / assesses |
|---|---|---|
| Rebound hammer (Schmidt) | Surface hardness by the rebound of a spring-driven mass | Uniformity of concrete; rough estimate of strength; identifies weak areas |
| Ultrasonic pulse velocity (UPV) | The time for an ultrasonic pulse to travel through the concrete | Quality, voids, cracks, honeycombing, depth of surface cracks |
| Cover meter (profometer) | Electromagnetic response of embedded steel | Location, spacing, depth of cover and approximate bar size |
| Half-cell potential | Electrical potential of embedded steel against a reference electrode | Probability of active corrosion |
| Concrete resistivity | Electrical resistance of the concrete | Corrosion risk and rate; moisture and permeability |
| Ground-penetrating radar (GPR) | Reflection of radar pulses | Position of bars, tendons and ducts; voids; slab thickness; delamination |
| Impact-echo | Stress wave reflected by internal defects and boundaries | Thickness, delamination, voids in grouted ducts |
| Infrared thermography | Surface temperature patterns | Delamination, moisture |
| Acoustic emission | Sound bursts from active cracking | Damage progression, wire breaks |
| Pull-out / pull-off test | The force to pull out an embedded insert or a bonded disc | Strength indication; bond strength of a repair |
| Carbonation test | Phenolphthalein on a freshly broken surface | Depth of carbonation (colourless = carbonated, pink = alkaline) |
| Chloride profile | Powder samples at several depths analysed chemically | Chloride content — whether it exceeds the corrosion-initiation threshold |
| Cores | A 75–100 mm core drilled from the structure | The direct strength, density, carbonation, chloride and petrography; the calibration of the other tests |
Rebound hammer
A spring-loaded mass strikes a plunger against the surface, and the rebound number (R) is read. A higher R indicates a harder surface. It is affected by the surface condition (smooth and dry), carbonation (increases R), moisture, aggregate type and the direction of the hammer; corrections are applied. It is used to compare different parts of a structure and, with cores for calibration, to estimate strength.
Ultrasonic pulse velocity
The velocity where is the path length between the transmitting and receiving transducers and the transit time. Sound travels faster through dense, uniform concrete and slower through cracks and voids. A commonly used classification (from the Indian Standard on the test):
| Pulse velocity | Quality |
|---|---|
| Above 4.5 km/s | Excellent |
| 3.5–4.5 km/s | Good |
| 3.0–3.5 km/s | Medium |
| Below 3.0 km/s | Doubtful |
Readings depend on moisture, reinforcement along the path and the aggregate; a combined method (SonReb) using both rebound and UPV gives a better strength correlation.
Transducers are placed on opposite faces of a 300 mm thick wall, and the transit time is 70 µs.
→ "good" quality.
A reading of 2.6 km/s at another point, across a line of hollow-sounding concrete, falls below 3.0 km/s and is "doubtful" — this area is cored and inspected.
Half-cell potential
The potential of the embedded steel is measured against a copper/copper-sulphate reference electrode (CSE) at a grid of points. The interpretation (a widely used standard practice):
| Potential (vs CSE) | Probability of corrosion |
|---|---|
| Less negative than −200 mV | About 90 % probability of no corrosion |
| −200 to −350 mV | Uncertain |
| More negative than −350 mV | About 90 % probability of active corrosion |
The test shows where corrosion is likely, not how fast; results are affected by moisture, carbonation and coatings, and are combined with resistivity and the chloride and carbonation measurements.
Cover meter and GPR
Cover to the steel is the main defence against corrosion: a cover meter maps the bars and their cover; GPR reaches deeper and locates tendon ducts and voids. These are also used to choose where to core or to drill without hitting the steel.
Other assessments
- Carbonation depth — approaches or exceeds the cover, corrosion begins.
- Chloride content — commonly compared with a threshold of about 0.4 % by weight of cement (or 0.05–0.1 % by weight of concrete, depending on the standard and structure).
- Cores — compressive strength (corrected for length/diameter ratio, moisture and the presence of steel) and petrographic examination (alkali–silica reaction, sulphate attack).