From when can a screed no longer cup?

Hello everyone,

our screed (fiber-reinforced cement screed) has now been in place for 4 weeks, since yesterday the heating is finally running and gradually warming up.

From when is there actually no longer a risk that the screed will shake while drying? After 4 weeks it is considered cured.
Can dehumidifiers now be set up to support drying?

Background:
We have two basement rooms with no windows (they will later be ventilated via controlled residential ventilation, which is of course not running yet), so drying by ventilation is naturally difficult there (currently constantly 90%) and I would like to help a bit.

Our site manager keeps putting the brakes on me and would prefer to leave the screed for at least 12 weeks...

Thanks and best regards,

Andreas

Hello "andimann". Bowing occurs increasingly with cement screeds during the drying stage. The question of up to what point a cement screed can bow cannot be satisfactorily answered as such. Why? Because it depends on the surrounding drying conditions. So, to give an example, if the screed were kept for over 1 year at 90% relative humidity, it could not dry and therefore could not bow. Bowing occurs in fact with every cement screed more or less noticeably as soon as it dries. One should not overestimate this issue. Screeds (if it is a cement-bound heated screed) should be slowly heated after 21 days. This is referred to as "functional heating." Shock ventilation through windows tilted open for hours is absolutely necessary (where else should the water go?). If the cement screed cannot be heated due to its design, shock ventilation must also be provided after the 3rd day to allow for drying. In the case you described, there are no window openings. Then forced drying must be carried out. In multiple rooms with only one condensate dryer but with many fans (for good air movement in and between the rooms). --------------------------------------- Regards: KlaRa

Hi KlaTa,

thanks for your reply. We have almost complete underfloor heating in the basement, so the screed in one of the two windowless basement rooms is now being heated. The second windowless basement room is the utility room, which has no heating.

In both rooms there are now drying units and the humidity hovers around 65-75 % at about 22°C. The drying units do not run continuously (the tanks fill up after about 5-6 hours), so I think the drying is not happening too quickly.

Best regards,

Andreas

Hello "andimann". The forced drying currently being carried out is the only correct way given the described structural situation! That the relative indoor humidity has settled at 65-70% shows two things: Firstly, that the drying measure is working (otherwise the value would be even higher) and secondly, that this measure is also necessary (otherwise the value would settle lower than currently measured). All in all, the right path has been taken with this measure! Regards: KlaRa

Hi KlaRa,

Current status:

Currently, we have a large construction dehumidifier (Trotec 170 Eco) in the 45 sqm basement room and a household device (Comfee something) in the utility room in use.

Two more Trotec TTK 105 units are on the way.

The large construction dehumidifier seems to be very effective; last night the humidity in the basement dropped below 50%, albeit at 27°C. The heating is now running at a supply temperature of 38°C. The dehumidifier is currently extracting slightly more than one liter per hour from the air, the 6-liter tank is full every 5 hours.

Incidentally, it is noticeable that the small household dehumidifier only manages about 1/5 of the water amount of the large one, even though it consumes half the electricity. The efficiency seems rather mediocre...

I am now curious whether we will get the screed dry in time. We want to start with the parquet on October 31. We still have a good 4 weeks until then...

Best regards,

Andreas

Note on the further inclusion of a drying device:
Caution with drying too quickly!!
Drying (especially of a screed) takes its time.
If you dry too quickly by using several condensation dryers, the capillaries in the upper screed edge zones break after the first successes. They dry out.
As a result, further drying can only take place by diffusion (even though there is still enough "food" in the screed structure for capillary moisture release).
The drying devices can "try as hard as they want": the further drying successes are moderate.
This means:
The builder accustomed to success looks at the drying device’s collection container, is pleased with the low yield, and (wrongly) assumes that everything is now technically dry. Until the point where the installer (in this case the parquet layer) brings him back to reality by the unexpectedly high result of his CM residual moisture measurement.
This means: "More is better" applies in many cases of daily life.
Technically, regarding drying and with regard to a cement screed, however, this can be completely wrong!
--------------------
KlaRa