FIND THE COMPRESSIVE STRENGTH USING REBOUND HAMMER
To find out the compressive strength of concrete using
rebound hammer.
EQUIPMENT'S:
Rebound hammer.
PRINCIPLE:
The Rebound hammer is principally a surface hardness tester.
It works on the principle that the rebound of an elastic mass depends on the
hardness of the surface against which the mass impinges. There is little
apparent theoretical relationship between the strength of concrete and the
rebound number of the hammer. However, within limits, empirical correlations
have been established between strength properties and the rebound number.
Further, Kolek has attempted to establish a correlation between the hammer rebound
number and the hardness as measured by the Brinell method.
PROCEDURE:
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) Choose suitable test location and remove plaster, if any,
at test location then make surface smooth and clean & mark gridlines as
20mm to 50mm apart in and area should be 300mm X 300mm
3) 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.
4) 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.)
5) Take the average of about 15 readings and calculate mean
of each set of reading after discarding extreme readings i.e. too low and too
high.
6) Apply correction factors and correlate strength &
rebound number correlation curve.
RANGE AND LIMITATIONS:
1. Smoothness of the test surface
Hammer has to be used against a smooth surface, preferably a
formed one. Open textured concrete cannot therefore be tested. If the surface
is rough, e.g. a trowelled surface, it should be rubbed smooth with a
carborundum stone.
2. Size, shape and rigidity of the specimen
If the concrete does not form part of a large mass any
movement caused by the impact of the hammer will result in a reduction in the
rebound number. In such cases the member has to be rigidly held or backed up by
a heavy mass.
3. Age of the specimen
For equal strengths, higher rebound numbers are obtained
with a 7 day old concrete than with a 28 day old. Therefore, when old concrete
is to be tested in a structure a direct correlation is necessary between the
rebound numbers and compressive strengths of cores taken from the structure.
Rebound testing should not be carried out on low strength concrete at early
ages or when the concrete strength is less than 7 MPa since the concrete
surface could be damaged by the hammer.
4. Surface and internal moisture conditions of concrete
The rebound numbers are lower for well-cured air dried
specimens than for the same specimens tested after being soaked in water and
tested in the saturated surface dried conditions. Therefore, whenever the
actual moisture condition of the field concrete or specimen is unknown, the
surface should be pre-saturated for several hours before testing. A correlation
curve for tests performed on saturated surface dried specimens should then be
used to estimate the compressive strength.
5. Type of cement
High alumina cement can have a compressive strength 100%
higher than the strength estimated using a correlation curve based on ordinary
Portland cement. Also, super sulphated cement concrete can have strength 50%
lower than ordinary Portland cement.
6. Carbonation of the concrete surface
In older concrete the carbonation depth can be several
millimeters thick and, in extreme cases, up to 20 mm thick. In such cases the
rebound numbers can be up to 50% higher than those obtained on an un-carbonated
concrete surface.
PRECAUTIONS:
This test is not generally recommended where accurate
strength estimate are needed. Because the rebound number is affected by the
near surface conditions, erratic results may occur if the plunger is located
directly over a course aggregate particle or a subsurface air void. To account
for these possibilities, ASTM C 805 requires that 10 rebound numbers be taken
for the test. If a reading differs from the average by more than seven units,
that reading should be discarded and a new average should be computed based on
the remaining readings. If more than two readings differ from the average by
seven units, the entire set of reading is discarded.
Comments