Impact Products Digital Concrete Test Hammer CN652 - Manual (Page 23)

The relationship between the rebound value R and the compressive strength of
the concrete fck is determined using test specimens of a certain type of concrete.
Proceed as follows:
• Take test specimens following the regulations applying in the particular country.
• In a pressure testing machine, apply force on the test specimens with a force of
40 kN, vertical to the direction in which the concrete was poured.
• Then measure the rebound hardness by applying as many test impacts as
possible to the sides of the test specimen.
The only way to achieve a meaningful result is to measure the rebound values R
and the compressive strength of several test specimens.
Concrete is a very inhomogeneous material. Test specimens made from
the same batch of concrete and stored together can reveal discrepancies of ± 15
% when tested in the pressure testing machine.
• Take the mean value R
m .
• Determine the compressive strength of the concrete by applying force to the
test specimens using the pressure testing machine, until the test specimens
break.
Then ascertain the mean value fckm.
The pair of values R
m / fckm applies to a certain range of the measured rebound
value R.
It is necessary to test samples of differing qualities and / or ages in order to
prepare a new conversion curve for the entire range of rebound values from R =
20 to R = 55.
• Using the pairs of values R
m / fckm, calculate the mathematical shape of the
conversion curve, i.e. determine the parameters a, b and c of the x 2 function (e.g.
EXCEL in the RGP function).
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