Non-destructive tests in buildings

Non-destructive testing (NDT) assesses the strength, quality and condition of a building's materials and members without damaging them, so the structure can stay in service. It is used for quality control, for assessing damage and ageing, and before retrofitting.

Common NDT methods

1. Rebound (Schmidt) hammer test (IS 13311 Part 2): a spring-loaded plunger hits the concrete surface; the rebound number relates to surface hardness and approximate compressive strength. It is quick and cheap, but it is affected by surface moisture, carbonation and smoothness, and it gives only the surface strength.
2. Ultrasonic pulse velocity (UPV, IS 13311 Part 1): measures the travel time of an ultrasonic pulse through the concrete. Concrete quality by velocity: above 4.5 km/s excellent, 3.5-4.5 good, 3.0-3.5 medium, below 3.0 doubtful. It detects voids, cracks and honeycombing.
3. Cover meter (profometer): an electromagnetic device that locates rebars and measures cover and bar spacing.
4. Half-cell potential test: shows the probability of rebar corrosion by measuring electrical potential.
5. Carbonation test: a phenolphthalein spray shows the depth of carbonated concrete (pink = alkaline, colourless = carbonated).
6. Penetration (Windsor probe) and pull-out tests: partially destructive tests that estimate strength.
7. Radiography, ground penetrating radar and infrared thermography: detect voids, delaminations, moisture and rebar layout.
8. Core test (partially destructive) is used to calibrate the above methods.

The results of different methods are combined for a reliable assessment.