Floor crumbling at edges

I'm looking at replacing some carpet with some laminate flooring. I've pulled up the carpet/underlay and found that the surface underneath is crumbling in some corners.

I'm not sure what this stuff is, it's black and kind of rubbery feeling. I've attached a picture (the cracking is hard to see. It starts in the corner and runs along the wall). Do I need to worry? Should I try and make good and how would I do that?

Any insights would be appreciated.

If you raked out & cleared away any loose crumbly debris, and then posted a photo it would help.
How far does the crack in the concrete extend - are you sure that it a crack and not the edge of the concrete slab?
If its a monolithic slab (ie. one single large slab) does the slab go under the wall?

The black stuff looks like bitumen - it could have been used to "glue" down some kind of flooring eg. wood block or parquet. It would not have been used for carpet or carpet underlay.
Or it could be a kind of Damp Proof Membrane (DPM).

Plaster should never be in contact with a solid floor - it should stop short by a min of 25mm.

Do you have any signs of damp in any lower walls? Is there now a musty smell?

Thanks for the response.

I'll try to get a better picture later. To clarify, it is the black stuff that has the crack - not the concrete. I believe it could well be bitumen. It is very difficult to photograph but it appears to have a faint impression of squares in it so it would appear it was used to fix some sort of tiled flooring.

Also searching for bitumen on this forum has found some threads with similar issues:
//www.diynot.com/diy/threads/how-best-to-repair-bitumen-asphalt-floor.237327/
//www.diynot.com/diy/threads/bitumen-asphalt-floor-over-concrete.214259/

There does not appear to be a consensus on how to repair it...

More pictures from a different corner.

One shows a chunk of floor removed and a bit zoomed in, you can see the concrete below is intact. The other shows the piece replaced and zoomed out - you can see a double socket for scale.

There doesn't seem to be an issue with damp at the moment...but my research suggests the bitumen flooring does act as a dpm.

Your cracks are not serious, probably due to expansion since heating was installed but if you want to repair them:
Your asphalt is a DPM, and can be repaired by using liquid or "pitch" bitumen.
No need to fill the gaps with bitumen, just seal the concrete and when the bitumen goes off


a bit, fill and level off with a packing of semi-dry sand & cement.

The square marks are probably from squares of parquet definitely not tile.