Civil Engineering Reference
In-Depth Information
a scale and is visible through a small window in the side of the casing. The
rider can be held in position on the scale by depressing the locking button.
The equipment is very simple to use and may be operated either horizontally
or vertically either upwards or downwards.
The plunger is pressed strongly and steadily against the concrete at right
angles to its surface, until the spring-loaded mass is triggered from its locked
position. After the impact, the scale index is read while the hammer is still in
the test position. Alternatively, the locking button can be pressed to enable
the reading to be retained or results can automatically be recorded by an
attached paper recorder. The scale reading is known as the rebound number,
and is an arbitrary measure since it depends on the energy stored in the given
spring and on the mass used. This version of the equipment is most commonly
used, and is most suitable for concrete in the 20-60 N/mm
2
strength range.
Electronic digital reading versions of the equipment are available.
Procedure
The equipment is very sensitive to local variations in the concrete, especially
to aggregate particles near to the surface and it is therefore necessary to take
12 readings in the area of interest and to average the results obtained. The
surface to be measured should be smooth, clean and dry but if it is required
to take measurements on trowelled surfaces, the surface can be smoothed
using the carborundum stone provided with the instrument. This stone is
not
a calibration device and must not be used to receive an impact from the
hammer as the stone may shatter.
Theory, calibration and interpretation
The test is based on the principle that the rebound of an elastic mass depends
on the hardness of the surface upon which it impinges and in this case will
provide information about a surface layer of the concrete defined as no more
than 30 mm deep. The results give a measure of the relative hardness of
this zone, and this cannot be directly related to any other property of the
concrete. Energy is lost on impact due to localised crushing of the concrete
and internal friction within the body of the concrete, and it is the latter,
which is a function of the elastic properties of the concrete constituents,
that makes theoretical evaluation of test results extremely difficult. Many
factors influence results but must all be considered if rebound number is to
be empirically related to strength.
Factors influencing test results
Results are significantly influenced by all the following factors:
a Mix characteristics
Cement type
