Impact Products CN652 User Manual download pdf (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

• 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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Impact Products CN652 User Manual Page 23