Old building floor slab - only gravel under 3cm cement

Hello,

we removed a partition wall (1.2m high) in the basement next to the washing machine. Unfortunately, it was not made of simple stone but, once again, had been concreted by the previous owner. Unfortunately, something unsightly has now been revealed underneath – see photo.
The concrete under the tiles is only 3 cm thick and there is gravel directly underneath.
Question: what is the best way to seal the whole thing?

Thanks in advance for tips and advice.
Best regards, Markus

The house is from 1952.

Hello questioner. What you describe as a problem is not easy to solve. Therefore, the most important question first: Why should it be sealed there? It is true that moisture can rise from the back here. However, if a moisture-resistant covering (like the adjacent tiles) is to be used on the strip, I would forego sealing. A tight connection to the black vapor barrier, or whatever that may be, is not possible in my opinion. This transition is and remains a weak point in the structure. If, yes if a moisture-sensitive covering is to be applied over it. It will be a special construction where no one can say for sure that it will be a 100% moisture barrier! I would excavate the strip about 40 cm deep, fill it with glass gravel (it is a bit more expensive, but no capillary transport occurs). Pay attention to good compaction. On top, at the height of the underside of the tiles, a water-resistant, load-bearing gypsum board or other (water-resistant) construction panel as a laying base. This strip will always have a "life of its own" under pressure stress. So plan an elastic (not rigid) transition on both sides of the excavated length so that the later covering does not break/crack. Once again: all this is and cannot be a "standard-compliant" structure! It remains an emergency solution where you have to see whether it works like this or not. -------------------------- Good luck: KlaRa

Hi, first of all, thanks for the reply.
I think I should add a few more parameters.

[*]The room in question is the laundry basement
[*]There is a dehumidifier in the basement
[*]Without the dehumidifier, the humidity rises above 70% in some spots during summer
[*]The basement has the usual single-glazed basement windows
[*]In the end, a tile should simply be placed on the spot
[*]Otherwise, the basement is only used for storage and not as living space
[*]The black coating on the screed is probably just paint from the 1950s

Hope this helps for a better assessment.
Best regards, Markus

Don't make it so complicated.

Remove everything loose, maybe remove about 2 cm of gravel. Then fill in about 1 cm of sand and then lay a somewhat more stable construction foil. Then glue this to the bitumen layer with assembly adhesive and then pour in concrete.

Done.