Determining Time Coefficient - Proceq DIGI-SCHMIDT 2000 ND Operating Instructions Manual

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 direc-
tion 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 measu-
re 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 pres-
sure testing machine, until the test specimens break.
Then ascertain the mean value fckm.
The pair of values R / fckm applies to a certain range
m
of the measured rebound value R.
© 2001 PROCEQ SA
.
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
mathematical shape of the conversion curve, i.e. deter-
mine the parameters a, b and c of the x
EXCEL in the RGP function).
8.4

Determining Time Coefficient

The age of the concrete and its carbonate penetration
depth can significantly increase the measured rebound
values R.
It is possible to obtain accurate values for the effective
strength by removing the hard, carbonate-impregnated sur-
face layer using a manual grinding machine over a surface
area of about Ø 120 mm and performing the measurement
on the non-carbonate-impregnated concrete.
The time coefficient, i.e. the measurement of the increased
rebound values R, can be ascertained by taking additional
measurements on the carbonate-impregnated surface.
R
m carb.
Time coeff. Z =
f
R
m n.c.
R : Average rebound value R, measured on
m carb.
carbonate-impregnated concrete surface
R : Average rebound value R, measured on
m n.c.
non-carbonate-impregnated concrete
surface
Appendix
/ fckm, calculate the
m
2
function (e.g.
R
m carb.
⇒
R =
m n.c.
Z
f
21