"Nick Read" wrote in message
...
My mum's house, built in about 1800, has a similar sort of arrangement,
the difference is its laid on top of the joists rather than down between
them. All the upstairs floors are solid. How it seems to have been done
was that bundles of hay were laid onto the joists, then layers of plaster
(apparently not concrete in this case) were poured on top gradually,

That will probably be lime/sand mix actually. Sort of between plaster and
concrete on the strength scale.