Why does concrete take up moisture?

[N. Baufeuchte]

Hi, in this basement assembly, the external plywood catches moisture in the first years, half of the year >18M-%. The concrete soaks up moisture as well, both from inside and outside.

Where does the moisture come from, when there is a bituminous membrane outside of the concrete, and not much moisture on the inside. Is this due to infiltration alone?

I discussed the results with a tech rep from a concrete manufacturer and he said that moisture content should be stable at the initial value I set (46 kg/m³) - this seems to be a typical content, dependent on initial binder content. Still, concrete picks up moisture. Do I need to modify the concrete material data? How?

Thanks in advance for your help.

Kind regards,
Norbert