ᐅ Dark spots on a lightweight concrete slab

Damp concrete darkens evenly.

The pictures clearly show mold growth.

Bauschu schrieb:
Wet concrete darkens evenly.

Yes, if you wet it... When concrete dries, it is definitely not uniform anymore...

The pictures clearly show drying concrete... and that is exactly how it looks...

I clearly do not see any spores!

Jokes aside: clearly ... at least for me, let’s just remove that word.

Still, I do not see any spores in the photos. Maybe a close-up image could help with a remote diagnosis, but only maybe.

Regards, Yvonne

Hello, I would like to bring up this topic because I also have stains on the ceiling that I cannot identify.
A precast concrete ceiling was delivered and installed about 1 year and 2 months ago. The garage roof was sealed approximately 1 year and 1 month ago. Just before the sealing was done, there was heavy rain and the ceiling was fairly wet. Since the garage door could only be installed 6 months ago, the ceiling was still able to dry out quite well. The screed was poured about 9 months ago, and the garage remained open for around 2 months after that.
I have been waiting for the stains to disappear for a while now before painting the ceiling, but they do not change. Compared to other stains I have seen in the forum, these do not seem to be caused by leaks, as they always look the same regardless of heavy rain or not. Also, they are not really stains but more like streaks on the ceiling, and oddly, not on all precast concrete panels.
Does anyone have any idea what this could be?

Best regards, Philipp

To shed some light on this 🙂 Precast concrete slabs are cast in a mold at the factory, and these are residues from the form release oil. So nothing serious and completely normal 😉

Hisearnest schrieb:

I have been waiting for a few days now for the stains to disappear before painting your ceiling, but they are not changing.

Just paint over them; you won’t see them later. It was the same for me.