Will this work? new RC slab

stone walls? just how old is this building?

Have a look for compressible material first and let us know if you find something suitable. I had a quick look in the past and the compressible materials I found readily-available were actually fairly stiff in comparison to (long-term) flexural stiffness of a slab. Even if you find something that's really compressible, the weight of the wet concrete will fully-compress it, then all live load and long-term deflection will load the ceiling.

We ended up having to specify an air void under the slab by removing the formwork but this was ground we didn't want to load, not a fragile (?) ceiling. I changed jobs between design and construction so don't know how well it worked out.

Yes, I was surprised by the how stiff the foam insulation actually is, so I would be quite surprised if it would actually work.

Another option might be a void form... or cardboard form under the slab. This has it's drawbacks as well and might not perform as intended.

You could also just tie the old and new together and size the new slab to limit the deflection under your new loads to something acceptable.

steveh49 said:

Even if you find something that's really compressible, the weight of the wet concrete will fully-compress it, then all live load and long-term deflection will load the ceiling.

Click to expand...

The 50 mm thick compressible insulation will not resolve the problem for the reason stated above, so it may as well be omitted.

What is the material of the existing ceiling? Is it capable of spanning from wall to wall under its own weight? The sketch provided would suggest otherwise. Is the existing ceiling capable of being jacked up sufficiently to offset the deflection of the new slab? What telepost spacing would be required below the ceiling to support a 200 mm slab, given the uncertainty of the ceiling strength?

Unless you can answer those questions with confidence, the proposed plan is not recommended. Another solution might be to place steel beams between walls. Forms would span between beams and be placed clear of the existing ceiling sufficiently to allow for deflection of the new slab.

Alternatively, you could use a precast hollow core slab clear of the existing ceiling instead of a poured in place slab.

BA

This is crying out for precast!

Precast is a good option only if the existing ceiling is self supporting over 5 m. If the ceiling is suspended from some existing roof framing, precast may not be an option.

BA