Special Cases - Proceq N Mode D'emploi

2 .5 .4

Special Cases

Experience has shown that deviations from the normal con-
version curves occur under the following circumstances:
- Artificial stone products with an unusual concrete com-
position and small dimensions. It is recommended that
a separate series of tests should be performed for each
product in order to determine the relationship between
the rebound value R and the compressive strength.
- 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 extremely smooth, polished surfaces and
a spherical shape results in values for compressive
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 placed adequately processed may
contain lumps of gravel which are not visible from the
surface. These affect the strength of the concrete with-
out however influencing the rebound values R.
- The concrete test hammer gives inadequate rebound
values R on concrete from which the form has just been
removed, which is wet or which has hardened under
water. The concrete must be dried before the test.
- Very high values for compressive strength (> 70 N/mm
can be achieved by adding pulverized fuel ash or sili-
ca fume. However, these strengths cannot reliably be
ascertained using the rebound value R measured by the
concrete test hammer.
14 Measurement
2 .5 .5 Conversion Curves for Special Cases
The recommended course in special cases is to prepare
a separate conversion curve.
• Clamp the sample in a pressure testing machine and
apply a preload of about 40 kN vertically to the direc-
tion in which the concrete had been poured in.
• Measure the rebound hardness by applying as many
test impacts as possible to the sides.
The only way to achieve a meaningful result is to mea-
sure the rebound values R and the compressive
strength of several samples.
Concrete is a very inhomogeneous material.
Samples made from the same batch of con-
crete and stored together can reveal discre-
pancies of ±15% when tested in the pressure
testing machine.
• Discard the lowest and highest values and calculate
the average R
m
• Determine the compressive strength of the sample
using the pressure testing machine and ascertain the
average value f
The pair of values R
of the measured rebound value R.
It is necessary to test samples of differing qualities and / or
2 )
ages in order to prepare a new conversion curve for the
entire range of rebound values from R = 20 to R = 55.
• Determine the curve with the pairs of values R
(e.g. EXCEL).
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© 2016 Proceq SA
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