It is one of the life assessment test for the concrete surface.
Rebound Hammer test is a Non-destructive testing method
of concrete which provide a convenient and rapid indication of the compressive
strength of the concrete. The rebound hammer is also called as Schmidt hammer
that consist of a spring controlled mass that slides on a plunger within a
tubular housing.
Objective:
As per the Indian code IS: 13311(2)-1992, the rebound hammer
test have the following objectives:
1. To determine the compressive strength of the
concrete by relating the rebound index and the compressive strength
2. To assess the uniformity of the concrete
3. To assess the quality of the concrete based on
the standard specifications
4. To relate one concrete element with other in
terms of quality
Rebound hammer test method can be used to differentiate the
acceptable and questionable parts of the structure or to compare two different
structures based on strength.
Principle:
Rebound hammer test method is based on the principle that the
rebound of an elastic mass depends on the hardness of the concrete surface
against which the mass strikes. The operation of the rebound hammer is shown in
figure-1. When the plunger of rebound hammer is pressed against the concrete
surface, the spring controlled mass in the hammer rebounds. The amount of
rebound of the mass depends on the hardness of concrete surface.
Thus, the hardness of concrete and rebound hammer reading can be
correlated with compressive strength of concrete. The rebound value
is read off along a graduated scale
and is designated as rebound number or
rebound index. The compressive strength can be
read directly from the graph provided on the body of the hammer.
Procedure:
When the plunger of rebound hammer is pressed against the
surface of concrete, a spring controlled mass with a constant energy is made to
hit concrete surface to rebound back. The extent of rebound, which is a measure
of surface hardness, is measured on a graduated scale. This measured value is
designated as Rebound Number (rebound index). A concrete with low strength and
low stiffness will absorb more energy to yield in a lower rebound value.
Points to Remember in
Rebound Hammer Test
1. The concrete surface should be smooth, clean
and dry.
2. Any loose particles should be rubbed off from
the concrete surface with a grinding wheel or stone, before hammer testing.
3. Rebound hammer test should not be conducted on
rough surfaces as a result of incomplete compaction, loss of grout, spalled or
tooled concrete surface.
4. The point of impact of rebound hammer on
concrete surface should be at least 20mm away from edge or shape discontinuity.
5. Six readings of rebound number is taken at
each point of testing and an average of value of the readings is taken as
rebound index for the corresponding point of observation on concrete surface.
Impact
Energy for Rebound Hammers for Different Applications As per IS: 13311(2)-1992
|
Sl. No. |
Applications |
Approximate
Impact Energy for Rebound Hammer in Nm |
|
1 |
For
Normal Weight Concrete |
2.25 |
|
2 |
For
light weight concrete / For small and impact resistive concrete parts |
0.75 |
|
3 |
For
mass concrete testing Eg: In roads, hydraulic structures and pavements |
30.00 |
Relationship Between Cube Strength and the Rebound Number
Quality of Concrete for different values of rebound number
|
|
Average Rebound Number |
Quality of Concrete |
|
1 |
>40 |
Very good hard layer |
|
2 |
30 to 40 |
Good layer |
|
3 |
20 to 30 |
Fair |
|
4 |
<20 |
Poor concrete |
|
5 |
0 |
Delaminated |
The advantages of Rebound hammer tests are:
1. Apparatus is easy to use
2. Determines uniformity properties of the
surface
3. The equipment used is inexpensive
4. Used for the rehabilitation of old monuments
The disadvantages of Rebound Hammer Test
1. The results obtained is based on a local point
2. The test results are not directly related to
the strength and the deformation property of the surface
3. The probe and spring arrangement will require
regular cleaning and maintenance
4. Flaws cannot be detected with accuracy
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