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
Why does concrete take up moisture?
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N. Baufeuchte
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Why does concrete take up moisture?
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Christian Bludau
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Re: Why does concrete take up moisture?
Hi Norbert,
The moisture content of a construction assembly will generally not remain at its initial value indefinitely. Instead, the materials gradually adjust towards a long-term equilibrium condition that is influenced by both the interior and exterior climate.
In your model, the concrete was initialized at approximately 47.5% RH. If this initial condition is lower than the moisture level associated with the long-term boundary conditions, the concrete will absorb moisture until a new equilibrium is reached.
It is also important to note that freshly cast concrete typically has a relative humidity close to 100%. However, not all of this moisture is available for drying, as part of the water becomes chemically bound during cement hydration. For simulations of new concrete structures, we therefore often start with an initial relative humidity of around 95%.
For existing concrete elements that have already had time to dry, an initial condition in the range of roughly 70-80% RH is often more realistic, depending on the age of the structure and its exposure conditions.
The moisture increase that you observe does not necessarily indicate liquid water intrusion. In this case, the most likely source is moisture diffusion from the indoor air into the concrete. Even with a bituminous membrane on the exterior side, water vapour exchange can still occur from the interior side, and the concrete will gradually adjust to the prevailing hygrothermal conditions. The permanent temperature gradient between indoors and outdoors also affects this moisture balance.
Therefore, I would not recommend modifying the concrete material properties solely based on the observed increase in moisture content. Instead, I would first verify whether the selected initial moisture condition is representative of the actual construction. If the moisture content stabilizes after some years rather than continuing to increase, this generally indicates that the simulation is approaching its equilibrium state rather than showing ongoing moisture accumulation.
Christian
The moisture content of a construction assembly will generally not remain at its initial value indefinitely. Instead, the materials gradually adjust towards a long-term equilibrium condition that is influenced by both the interior and exterior climate.
In your model, the concrete was initialized at approximately 47.5% RH. If this initial condition is lower than the moisture level associated with the long-term boundary conditions, the concrete will absorb moisture until a new equilibrium is reached.
It is also important to note that freshly cast concrete typically has a relative humidity close to 100%. However, not all of this moisture is available for drying, as part of the water becomes chemically bound during cement hydration. For simulations of new concrete structures, we therefore often start with an initial relative humidity of around 95%.
For existing concrete elements that have already had time to dry, an initial condition in the range of roughly 70-80% RH is often more realistic, depending on the age of the structure and its exposure conditions.
The moisture increase that you observe does not necessarily indicate liquid water intrusion. In this case, the most likely source is moisture diffusion from the indoor air into the concrete. Even with a bituminous membrane on the exterior side, water vapour exchange can still occur from the interior side, and the concrete will gradually adjust to the prevailing hygrothermal conditions. The permanent temperature gradient between indoors and outdoors also affects this moisture balance.
Therefore, I would not recommend modifying the concrete material properties solely based on the observed increase in moisture content. Instead, I would first verify whether the selected initial moisture condition is representative of the actual construction. If the moisture content stabilizes after some years rather than continuing to increase, this generally indicates that the simulation is approaching its equilibrium state rather than showing ongoing moisture accumulation.
Christian