Non-destructive testing

Non-destructive tests (NDT) assess concrete in a structure without damaging it, or with only minor local damage. They are used to check quality of new work, investigate distress, assess old structures before retrofit and map variability. NDT gives indirect measurements, so results need calibration against cores and careful interpretation.

The rebound hammer measures surface hardness through the rebound of a spring-driven mass; it is quick but reflects only the outer few centimetres and is affected by carbonation, moisture and surface finish. Ultrasonic pulse velocity (UPV) times a sound pulse through the concrete; higher velocity indicates denser, more uniform concrete, and low readings point to cracks, honeycombing or voids. Combining the two (SonReb) improves strength estimates. Cover meters locate bars and measure cover by electromagnetic induction; ground penetrating radar maps bars, ducts and voids. Half-cell potential surveys indicate corrosion probability, and resistivity tells how easily corrosion currents can flow.

Semi-destructive tests include cores, the reference for in-situ strength; pull-off and pull-out tests; and carbonation depth checks. Load tests prove capacity directly but are expensive. An NDT survey should follow a grid, include a reference area of known good concrete, and report results with their uncertainty.

Reporting matters as much as testing. A good NDT report states the instruments used and their calibration, the grid and locations on drawings, surface preparation, raw readings, any corrections, the calibration against cores and a clear statement of the limits of the conclusions. That way an owner can compare surveys done years apart and see whether deterioration is progressing.