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
|
great work sir rebound hammer test calculation
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