8 .3
Creating New Conversion Curve
Experience has shown that deviations from the normal
conversion curves occur under the following circum-
stances:
- Artificial stone products with an unusual concrete com-
position and small dimensions. It is recommended to
perform a separate series of tests for each product in
order to determine the relationship between the rebound
value R and the resistance to pressure.
- Aggregates made from low strength, lightweight or clea-
vable stone (e.g. pumice, brick rubble, gneiss) result in
a strength value lower than shown on the conversion
curve.
- Gravel with a preponderance of smooth, polished surfa-
ces and spherical shape results in values for compres-
sive strength which are lower than those ascertained by
the rebound measurements.
- A strong, dry mixed concrete (i.e. with low sand content)
which has not been worked adequately may contain
gravel agglomerations which are not visible from the
surface. These affect the strength of the concrete
without however influencing the rebound values R.
- The concrete test hammer gives inadequate rebound
values R on recently applied, green concrete or con-
crete which has hardened under water. The concrete
must be dried before the test.
- Very high values for compressive strength (> 70 N/mm²)
can be achieved by adding pulverized fuel ash or silica.
However, these strengths cannot reliably be ascertained
© 2017 Proceq SA
using the concrete test hammer.
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.
Appendix
.
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