REBOUND HAMMER TEST
The SCHMIDT rebound hammer, developed by a Swiss Engineer
Ernst Schmidt
in 1948 is one of the most frequently used methods worldwide
for non-destructive
testing (NDT) of concrete structural elements.
The test is based on the principle that the rebound of an
elastic mass depends
on the hardness of the surface against which the mass
impinges. The hammer consists
of a plunger connected with a spring-driven metal mass. The
plunger is held against
at 90 to the smooth concrete surface, firmly supported and
pressed. This will impart
a fixed amount of energy. Upon release, the metal mass
rebounds, the plunger being
still in contact with concrete. The distance travelled by
the metal mass or the amount
Larger is the rebound. higher is the strength of concrete.
of rebound is noted on a scale that gives an indication of
the concrete strength.
The rebound hammer test is sensitive to local variations in
the concrete: for
instance, the presence of a large piece of aggregate
immediately underneath the plunger
would result in an abnormally high rebound number.
Conversely, the presence of a
void immediately underneath the plunger would lead to a very
low result. For this
the reason, it is desirable to take 10 to 12 readings spread
over the area to be tested, and
their average value must be taken
The test can be conducted horizontally, vertically-upwards
or downwards or at
intermediate angle. At each angle the rebound number will be
different for the
same concrete and will require separate calibration or
correction chart. Fig. 1.7 shows
the typical relationship between compressive strength and
rebound number with
hammer horizontal and vertical on a dry or wet surface of
concrete.
Factors
influencing the test results:
Although rebound hammer saves effort, time and cost, it has
serious limitations
The results are significantly influenced by :
(1) Mix characteristics like cement type, cement content
(2) The smoothness of the surface under test
(3) Types of coarse aggregate
(4) Size, shape, and rigidity of the specimen
(5) Age of concrete
(6) Moisture condition of the concrete
(7) Carbonation of concrete surface
(8) Type of mould
IS: 13311 (Part-2)-1992 explains the standard procedure for
test and correlation
between concrete cube strength and rebound number.
Strength assessment
The rebound hammer test is conducted to assess the relative
strength of concrete
based on the hardness at or near its exposed surface, It is
desirable to carry out
periodic calibration rebound hammer using standard anvil In
case new concrete
construction, rebound hammer is calibrated on concrete test
cubes for a given source
of constituent materials (viz. cement. sand and aggregates),
this calibration data can
a with reasonable accuracy in arriving at equivalent in-situ
cube strength of
Is new concrete (i.e. not more than 3 months old concrete).
This calibration
exercise may be carried out in a concrete lab by casting
concrete cubes of designed
and testing these cubes under the controlled condition with
rebound hammer as well
e test to destruction in the compression testing machine.
Calibration graphs then can be
drawn.
This method may give highly erroneous results for concrete
whose surface is
exposed to the atmosphere for longer periods say more than 3
months. This is due to
hardening of the concrete surface due to carbonation, which may
cause overestimation
as much as 50% for the old structure. Hence, strength assessment
by Rebound hammer
the test should be restricted to relatively new structures only.
Survey of
weak and delaminating concrete
For conducting the rebound hammer test, the concrete surface
should be flat. This
test is generally not suitable for use on spalled concrete
surfaces of distressed
structures. The rebound number indicate the near-surface the hardness of the concrete,
however, it can be used to identify relative surface
weakness in cover concrete and
can be used to determine the relative compressive strength
of concrete
Locations possessing very low rebound numbers will be
identified as a weak surface
e and such locations will be identified for further the investigation, like corrosion
distress, fire damage or original construction defects of
concrete. This survey should
be carried out at grid points with the grid matrix 300 mm x 300
mm.
Table 1.14 gives guidelines for qualitative interpretation
of rebound hammer test
results with reference to quality.
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