41
Handling the Lancehead-Shaped Probe
SONO WZ
Translation of the Original Instructions for Use
Measurement problem 2: subsequent
adding of water into the concrete
Laboratory testing often results in prob-
lems if carried out as follows:
1. The water content of approx. 8 l of
fresh concrete was measured in a buck-
et with the SONO WZ. For example,
178 l/m³ was measured here with the
SONO WZ.
2. Following this, 50 g of water was added
to the fresh concrete, which correspond-
ed to an increase in the water content
from 178 l/m³ to, e.g. 184.25 l/m³. After
mixing for about one minute in a small
mixer, the concrete was then checked
for raw density and slump. The tested
concrete was then poured back into the
bucket to determine the water content
with the SONO WZ.
3. The concrete-water content was once
again measured with the SONO WZ;
however, the result was not the expected
184.25 l/m³ but only 181 l/m³.
What happened here?
When mixing relatively small quantities in
an open container, the water adheres to
the container wall over a large area and
evaporates. If the concrete is then addi-
tionally used for the slump and raw density
test, no gravel and virtually no sand will
remain on the outer walls of the test
equipment, but the water and the fine
particles remain “stuck” on these surfaces
due to water adhesion.
It is easy to check this effect. After the
first SONO WZ measurement result with
178 l/m³, the concrete is mixed again for
approx. one minute and the water content
is then re-checked with the SONO WZ.
The decrease in the water content
by 2 – 3 l/m³ is then an indication of
the evaporation effect through mixing.
This means subsequent mixing of concrete
causes significant deviations in the water
content.
