Screed still has 5% residual moisture after 5 months

Hello,

a floating cement screed from Knauf was installed in the basement. After 5 months, we still have a residual moisture of 5% - far too high.

According to our Bauherren-Schutzbund expert, there are 3 possibilities:

1. the measuring device is defective (you can hardly rule anything out)
2. there was insufficient heating and ventilation
3. the watertight floor slab is not watertight.

The floor slab is made of WU concrete as is the entire basement, additionally there is a black tub with KMB coating around it. During the groundwater lowering, a pump failed briefly, but the underpressure was not completely disconnected yet (according to the lowering company). Therefore, I do not assume it is the floor slab. Especially since our site manager also has problems with drying on other construction sites.

What do you think about this?

Regards,
StarAce

Addition (unfortunately cannot edit?): It is a calcium sulfate screed. It was also installed in the bathroom and shower, where it is dry (thanks to underfloor heating). So it is probably really due to the screed mixture.

Hello,

....
According to our Bauherren-Schutzbund expert, there are 3 possibilities:
1. the measuring device is defective (nothing can really be ruled out)
2. there was too little heating and ventilation
3. the waterproof floor slab is not waterproof.

He is right here! An explanation can be found by determining the moisture progression (component, room air) over time. Also, the basement does not seem to be heated, so no underfloor heating! So use building dryers. If the moisture increases again after their use, there are usually two possibilities:
- condensation from supplied warm outside air
- point 3
It is a misconception to assume that WU waterproof is! Read the relevant DIN standards about this.
Certain problems are usually almost preprogrammed. For example, when groundwater lowering is necessary for basement construction.
This can be taken into account accordingly in the planning of the building structure beforehand.
Also, 100% dry components do not exist!

Regards

Hello,

€uro has already said a lot of the right things about this ... although I am not quite as sure about your expert as €uro is.

The floor slab is made of waterproof concrete as is the entire basement, additionally there is a black tank with KMB coating all around the outside.

That sounds more "high-flown" than it actually is. WU means nothing other than "waterproof" and the so-called "black" tank is nothing other than a thick bitumen coating.

During the groundwater lowering a pump failed temporarily, but the negative pressure was not yet fully reduced (according to the lowering company). Therefore, I do not assume it is the floor slab. Especially since our construction manager also has problems with drying on other construction sites.

You built (almost) in the groundwater and had to take no further measures except a normal WU concrete basement on a likewise normal WU floor slab? I can hardly imagine that! Are you sure that your soil expert did not make any requirements regarding additional sealing of the WU concrete?

As an example here is an excerpt regarding rising seepage water:

Basement sealing

1. According to point 7 Building sealing of the soil expert report, the load case "rising seepage water" according to DIN 18195 T6 A9 must be applied to the construction project. The recommendation of the soil expert is .....

Concrete basements are always a matter of their own; unheated ones especially. Before you seek your salvation in the supposedly "wrong" screed, take a look at the soil expert’s recommendation and in the meantime set up building dryers. Only when it is 100% certain that all requirements from the soil expert report have been met can you proceed with further troubleshooting.

Rhenish regards

This is what our site manager writes about it:

- Floor slab according to WU guideline with a thickness of 35 cm (without a blinding layer, but with a Delta dimple sheet installed on the underside)
- Vertical connection of the floor slab to the concrete double wall elements (d=25 cm) using coated Master joint sealing sheet
- Filling of the double wall elements with WU concrete of grain size 0/8 mm
- Fillet with HKS
- Sealing of the basement outer walls using bituminous thick coating according to DIN 18195-6

As already mentioned, the combination of the floor slab design with the sealing of the basement walls using bituminous thick coating is not regulated in the DIN, but corresponds to the recognized rules of technology.According to the soil report, sealing according to section 8 of DIN 18 195-6:2001-12 is to be provided and must be permanently secured against backflow. Alternatively, the construction of a waterproof basement made of WU concrete is possible (exposure class 1 according to section 5.2 (2) and usage class A according to section 5.3 (2), no masonry basement outer wall.

Hello,

before any false impression arises here - I do not presume to know everything; how could I? Only when it comes to water, especially in the basement, do all alarm bells usually go off for me. Especially since a few months ago I encountered a comparable situation at one of the construction projects we supervise. In the case mentioned, the basement builder simply did not consider it necessary to comply with the specifications from the soil report regarding water pressure.

As already mentioned, the combination of the slab design with the sealing of the basement walls using bituminous thick coating is not regulated in the DIN standards, but complies with the recognized rules of technology.

That is correct:

"There are no DIN standards for the construction of white tanks, but there are for the quality of the concrete and for the calculation by the structural engineer. Furthermore, there is sufficient specialist literature with the DBV leaflets (German Concrete Association), the DafStb guideline 'Waterproof concrete structures – 11/2003' and other recognized printed media to reliably plan and/or assess the correctness of the white tank as a construction."**

but also:

"Since the publication of the DafStb guideline 'Waterproof concrete structures – 11/2003', white tanks are thus considered regulated components. Professionally planned and executed white tanks are recognized rules of technology."**

** Wording borrowed: Mark A. Carden from the HwK OWL in Bielefeld, publicly appointed and sworn expert

It should therefore be possible, based on the specialist literature, to track down the cause of the moisture problem.

Rhenish greetings