MMP20 SONO-DIS (option D)
Hauser
Page 27
Pitfalls in the laboratory and at the concrete plant
9.6.1
Pitfall 1: Problems mixing the concrete with dry aggregates
Depending on the rock, it can take some time for dry aggregates to become saturated after the
mixing process. This can range from 3-5 minutes for relatively absorbent aggregates, to up to one
hour for less absorbent aggregates. Given that SONO-WZ only "sees" one third of the core water,
we recommend that you wait a certain amount of time after mixing dry aggregates before checking
the water content with SONO-WZ.
Example: A dry, highly absorbent rock can absorb up to 30 liters of water per cubic meter in a
relatively short period of time. Due to the equilibrium moisture content, however, the rock that
is used and stored is not entirely dry. Rather, it has a water content typically of 7 liters/m
3
.
For a concrete formulation with 175 liters/m
3
effective water content, 197 liters (175 + 23) were
then used. Directly after the concrete has been mixed, SONO-WZ would measure approx. 185 liters
here. Shortly afterwards - approx. 3-5 minutes, depending on the rock - it would display a reading
of 175 liters.
For the SONO-DIS, two thirds of the maximum core water would have been entered for the G-Set
parameter. In this case, two thirds of the 30 liters maximum core water would have been entered
in SONO-DIS for the
G-Set
, i.e. G-Set = - 20 liters, if the effective water is to be measured.
When mixing with dry aggregates, it is important to wait a certain amount of time - depending
on the rock type - before taking a reading with SONO-WZ!
9.6.2
Pitfall 2: Problems with the subsequent addition of water to the concrete
The manufacturer has often been asked about problems and non-conformities during a laboratory
test, which was performed as follows:
•
The water content of approx. 8 liters of fresh concrete was measured in a bucket using
SONO-WZ. SONO-WZ gave a reading of 178 liters/m
3
, for example.
•
After this, 50 grams of water were added to the fresh concrete, which would correspond
to an increase in the water content from 178 liters/m
3
to 184.25 liters/m
3
, for example.
After mixing the concrete for approx. one minute in a small mixer, the concrete was then
tested with regard to the raw density and flow class. The concrete used to determine the
density and flow class was then poured back into the measuring bucket for the purpose of
determining the water content afterwards with SONO-WZ.
•
Then, the concrete water content was measured again with SONO-WZ. However, this time
the result was only 181 liters/m
3
and not 184.25 liters/m
3
as expected.
What happened here?
When the concrete is mixed in the small mixer, some of the water already escapes. This is because
when relatively small quantities of concrete are mixed in an open container, the water adheres to
the wall of the container over a large surface area and evaporates. If this concrete is then also used
afterwards to test the flow class and raw density, then no gravel and hardly any sand sticks to the
outer walls of the testing instruments but the water and fines "cling" to these surfaces through the
adhesion of water.
This effect can be easily checked. After the first SONO-WZ reading of 178 liters/m
3
, mix the
concrete again for approx. one minute and then check the water content again with
SONO-WZ. The reduction in the water content of 2-3 liters/m
3
is then an indicator for the
evaporative effect caused by mixing.
Conclusion: Subsequent mixing of the concrete causes considerable deviations in the water
content readings!
