Just started re-fitting the bathroom in my 1910ish first floor flat.
After removing the carpet I find that what I expected to be a wooden
floor (as all the other rooms in the flat are) appears to be concrete.
Previously the bathroom would have been the kitchen when the flat was
originally built.

Closer inspection under the floor in the adjacent bedroom reveals that
just before the joists disappear under the solid wall into the
bathroom the gap between the joists has been filled with concrete.
This appears to continue on into the bathroom. I am guessing the
bathroom has then had a layer of concrete skimmed over the top to give
the solid floor appearance.

My question:

1) How on earth is this held up? If you poured concrete between the
joists onto a lath and plaster ceiling surely it would collapse almost
immediately. Would the concrete have been supported from below during
construction with the support being removed once the concrete had set
?
2) Was it a requirement that kitchens of the time had to have a solid
floor for fire protection hence this construction otherwise why bother
?

Just curious to know if this type of construction was common on older
properties ? Also, on a similar subject, how would the slab of
concrete in front of each fireplace have been held up in an upstairs
room ?