Friday, 10 November 2017

Rebound hammer test

Rebound hammer test (Schmidt Hammer) is used to provide a convenient and rapid indication of the compressive strength of concrete. It consists of a spring controlled mass that slides on a plunger within a tubular housing.
The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the Spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read from a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.
What is Rebound Hammer?
Rebound hammer is an instrument or a device, which is used to assess the relative compressive strength of concrete based on the hardness at or near its exposed surface. Rebound hammer is also known as Schmidt’s Hammer or Swiss Hammer as it is invented by Ernst Schmidt, a Swiss engineer.
The following points should be observed during testing:
(a) The concrete surface should be smooth, clean and dry.
(b) Ant loose particles should be rubbed off from the concrete surface with a grinding wheel or stone, before hammer testing.
(c) Rebound hammer test should not be conducted on rough surfaces as a result of incomplete compaction, loss of grout, spalled or tooled concrete surface.
(d) The point of impact of rebound hammer on concrete surface should be at least 20mm away from edge or shape discontinuity.
Procedure to determine strength of hardened concrete by rebound hammer.
1.   Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer.
2.   Apply light pressure on the plunger – it will release it from the locked position and allow it to extend to the ready position for the test.
3.   Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound reading in that position.)
4.   Take the average of about 15 readings.

The rebound hammer test method is used for the following purposes:
1)   To find out the likely compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.
2)   To assess the uniformity of concrete.
3)   To assess the quality of concrete in relation to standard requirements.
4)   To assess the quality of one element of concrete in relation to another.
How to Check the Quality of Concrete Based on Rebound Number?
The table below shows the quality of concrete based on the average rebound number or rebound index.

Average Rebound Number
Quality of Concrete
> 40
Very Good
30 -40
Good
20-30
Fair
< 20
Poor and/or Delaminated
0
Very Poor and/or Delaminated

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