MMP20 SONO-DIS (option D)
Page 28
Hauser
9.6.3
Pitfall 3: Sampling in the concrete plant
Before the concrete was transferred to a truck mixer, a concrete sample was taken directly from
the twin-shaft mixer and transferred to a bucket. The concrete sample with a normal-distribution
grading curve and a water target value of 170 liters/m
3
was measured with the SONO-WZ and
170 liters/m
3
was displayed as the reading. After that, a 5kg concrete sample was kiln-dried.
A kiln-dried value of 149 liters was determined, i.e. there was a difference of -21 liters between
this value and the SONO-WZ reading of 170 liters/m
3
.
What happened here?
As the concrete was mixed in the twin-shaft mixer without being continuously mixed again in the
truck mixer, the kiln-dried sample during the first sampling contained a lot of large gravel pieces.
These large gravel pieces resulted in a considerable error during sampling: there were simply too
many large pieces of gravel in the sample which "drove down" the kiln-dried value to 149 liters/m
3
(gravel pieces have no water content). The cement paste, which was very high as a result, caused
the SONO-WZ reading to deviate from the (actually incorrect) kiln-dried value. The question is
then whether this was a disadvantage, as the water content of the cement lime was too high in
this case (and would not have produced good concrete).
