Techshore QA/QC : Civil Engineering [Schmidt Rebound Hammer]

Schmidt Rebound Hammer Testing

What is Schmidt rebound hammer ?

A Schmidt hammer/Swiss hammer/rebound hammer invented by Ernst Schmidt in 1948, a Swiss engineer, is a mechanical device used to measure surface hardness and penetration resistance strength of concrete or rock.

Working Principle

Schmidt rebound hammer works on the principle that rebound of an elastic mass depend on the surface against which it strikes. 

The hammer measures the rebound of a spring-loaded and the mass impacting against the surface of the sample.The device is allowed to hit the concrete surface with a defined energy. The rebounding of the device depends on the surface hardness and penetration resistance strength of concrete and is measured by the test equipment.The conversion chart could be then referenced to calculate the rebound value which further can be used to calculate the compressive strength of the concrete or test specimen.

Instrument description :

1. Schmidt rebound hammer consists of
2. An outer body
3. A pluncher rod
4. A hammer mass
5. A main spring
6. A latching mechanism that holds the hammer mass and pluncher road together .
7. An arbitrary scale (marked from 10 to 100) with a sliding rider to indicate the rebound number
8. A locking mechanism to lock the sliding rider after the rebound of the hammer
9. Calibration chart provided by the manufacturer affixed on the outer  body of the hammer

  

Inspection Procedure :

1. Ideally hammer should be held at right angles to the surface. Rebound reading of the hammer has a direct dependency on the orientation of the hammer , when used in vertical positions the force of gravity increases the rebound    distance of the hammer mass and vice versa .
2. Test procedure begins with pushing the rebound hammer hard against the surface to be inspected . This cause the outer body to move away from the surface
3. The latching mechanism latches the puncher rod and hammer mass with in the device together.
4. Hammer body is again pushed harder to the surface which causes the spring holding the mass to extend.when the maximum extension of the spring reaches , the latch releases and hammer mass is pulled towards the pluncher road by the spring this movement forces the hammer mass to rebound because the plunger road is pushed hard against the surface
5. During the rebound movement of pluncher rod the sliding rider also moves along with hammer mass and the sliding rider stops at maximum distance of rebound of hammer mass.
   That particular point of sliding rider denotes the rebound number which could be locked using the locking mechanism
6. The rebound number hence obtained is used to calculate the compressive strength and quality of the concrete referring the calibration chart provided by the manufacturer.
7. The calibration has its x - axis graduated with rebound no and y - axis with compressive strength
8. Although Hammer is ideally used in horizontal position it could also be could be used in vertical position . while using the hammer in vertical position we will have to factor the force of gravity(positive and negative g value) during calculation.
9. When using the hammer in upward position g value is factored as positive and vice versa during calculation procedure.