Which one of the following is the working principle of concrete hammer for non-destructive test based?
- ARebound deflectionsCorrect
- BRadioactive waves
- CUltrasonic pulse
- DCreep recovery
Solution
Concept
Rebound Hammer Test
Formula
No, there is no direct mathematical formula; the rebound hammer test uses empirical calibration curves (or correlation charts) provided by manufacturers to relate the rebound number (R) to compressive strength.
Step-by-step solution
1. Identify that the concrete hammer (Schmidt hammer) tests surface hardness by striking the concrete with a spring-loaded mass. 2. Observe that the mass bounces back, creating a rebound deflection proportional to the surface hardness. 3. Select Option A (Rebound deflections).
Explanation
The concrete rebound hammer (or Schmidt hammer) works on the principle that when a spring-loaded mass strikes the concrete surface, it rebounds back. The distance or deflection of this rebound depends on the surface hardness and elastic properties of the concrete. Harder concrete absorbs less energy, resulting in a higher rebound deflection, which directly correlates to higher compressive strength.
Shortcut
Associate hammer with bouncing: striking concrete with a hammer causes it to rebound, so the concrete rebound hammer relies on rebound deflections to measure surface hardness.
Common mistake
Many students confuse the concrete rebound hammer test with the Ultrasonic Pulse Velocity (UPV) test (Option C), as both are widely used non-destructive testing methods for concrete.
Correct answer: A) Rebound deflections