NDT importance; Schmidt hammer test

Importance of NDT

• Tests the actual in-situ concrete, not only the control cubes, which may not represent the structure's compaction and curing.
• It causes no damage, and repeated tests at the same place are possible, so 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.

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 (if it is held horizontal, or vertically up or down, an orientation correction is needed), push the plunger until the mass is released, and read the rebound number.
3. Take at least 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; check the instrument on the test anvil.

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