Cracked and powdery constructional hearth, can I replace concrete?

[ArdentDIY]

I want to raise the level of my hearth to ultimately install a wood burning stove. However, the current hearth (below floor level) is cracked and powdery, and has a big piece missing right underneath where the old fireplace was. It's clearly been damaged by all the thermal stresses over its lifetime. I should add that the hearth is not on the ground floor.

I would like to remove the old damaged concrete, and recast it. However, I'm very afraid that it would fall through to the floor below if it were damaged further. I can't find any resources detailing how constructional hearths are designed. Is there wooden joists/boards underneath the concrete? What supports it? Is it supported over it's entire area, or just around the edges?

If I can't get a definite answer, then I'll probably just cast concrete over the top of it to be safe.

 

[blup]

Don't know whether this is acceptable now, from memory the concrete is cast in situ over a timber board that sits on battens within the trimmed joists. It will be very heavy. A HETAS registered installer could probably advise if it's permitted, and if so what dimensions/thicknesses of concrete are required.

Blup