Why was cement poured in two layers
I am cutting through the basement floor of my 1940 colonial to relocate some of the plumbing and also add a sump pump as part of a bathroom renovation.
When I chipped through the floor I found it had two cement layers. The lowest layer is about 4" of very hard cement and in very good shape. The upper layer, which varies from about 1/2" to 1" is softer cement that seems to have a fairly high sand content. In several areas it is not strongly adhered to the concrete underneath. The height of the closet flange appears to line up with the top layer, so it looks like the both layers may have been poured before the fixtures were installed.
Under the cast iron pipes I also ran into a charcoal black layer of dirt. At first I thought it was discoloration from contact with the pipe, but it seems more like it might have been something to bed in the pipes before pouring concrete. However, I can't figure out what the material is.
Any suggestions about why they used two layers of concrete, and/or would that have been common? Could it have simply been a way to level the floor?
Also, any suggestion about the what the black material would have been under the pipes?