Repairing concrete floor over cavity

[terryb]

I have a solid concrete floor that is laid over the cavity up to patio doors. Where carpet fixings have been nailed to the floor, the concrete has broken and fallen into the cavity. The broken area runs along most of the patio doors (8 feet). How can I re-concrete the floor.

The broken area is only about 2 inches wide - about the same as the cavity. I have considered using a cavity closer but these are very wide (over 5 inches) and I don’t want to have to remove a large amount of floor as this will make the problem worse.

There is very little exposed brickwork either side of the cavity on which to lay anything solid. Could I fill the cavity with insulating material and cement over that. What insulating material should I use. Would I need to use some kind of damp proofing. What should I use.