ᐅ tough compound between masonry and concrete ceiling/slab

Hello everyone,

I have a question as well.
In our utility room, there is a small area about 5cm (2 inches) long in the corner between the exterior wall and the concrete ceiling where the white paint has suddenly turned dark and is slightly peeling. Behind it, there is a sticky black substance. If you scrape it a bit, it is malleable like bitumen/tar. Above this spot is the bathroom with the toilet cistern. My memory is a bit rusty after two years, but as far as I know, there were no bituminous membranes on the upper floor.
However, it’s possible that this was already present when the painting was done, and I may have painted over it without noticing. What surprises me is that it is now suddenly showing through as black and appears to have slightly spread.
Does anyone have an idea if bitumen/tar is commonly used for sealing in this situation, and if it is normal for it to become a bit soft again over time?

Possibly styrofoam dissolved by the paint solvent (?) – it has to be something that goes through both floors, right? In our case, the openings were clogged during the pour (to keep them clear).

Do you happen to have a picture of it?

I’ll try to take a picture tonight, but you probably won't be able to see much in it.

In terms of size, I would compare it to a 5cm (2 inch) long silicone joint that isn’t smoothed out. The material underneath is completely black and can be kneaded with fingers. It stains lightly but doesn’t stick to the skin. Initially it’s quite firm, but becomes softer when kneaded (due to temperature increase). It reminds me, as I said, of bitumen.

Could it be that there is a waterproofing membrane laid between the masonry and the precast concrete slab that has softened due to high temperatures and is now visible through the paint? The more I think about it, the more plausible this explanation seems to me.

That doesn’t make sense to seal the top stone before placing the precast concrete slab; how would water get in there? This is usually done only over the foundation slab. Your issue must have a different cause.

Bauexperte

Hmm, strange. I saw this on some images during an image search. That’s why it seemed the most likely to me.