NDT: importance and methods

Importance

• Tests the actual in-situ concrete, not only the control cubes, which may not represent the structure's compaction and curing.
• No damage and repeated tests at the same place are possible, so the changes with time can be tracked.
• Quick and cheap; large areas can be surveyed to find weak, honeycombed or cracked zones.
• Assesses old structures for repair, change of use, fire or earthquake damage, and long-term deterioration (corrosion, carbonation).
• Investigates doubts when cube results fail the acceptance criteria (IS 456, Cl. 16), before costly coring or load tests (Cl. 17).
• Checks uniformity, thickness, cover and the position of rebars, and gives a basis for deciding on repair.

Methods

Rebound hammer (Schmidt hammer) test (IS 13311 Part 2)

Principle: a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the rebound number $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength.

Procedure

1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture.
2. Hold the hammer at right angles to the surface (a horizontal shot is the reference; shots vertically up or down need an orientation correction), push the plunger until the mass is released, and read the rebound number.
3. Take about 9 to 12 readings at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest.
4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete.

Precautions: avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil.

Limits: it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)