Concrete flat roof missing pieces!

Hi all

I have a concrete flat roof partly joined to the house that makes up a utility area and outside workshop/dumping ground.

Part of the underside of the concrete roof is deteriorating and falling down gradually due to a leak.
The leak and roof have been fixed and all has dried out, however I could do with advice on how to make good the bad areas.

I've attached a photo (hopefully) Do I just 'skim' a new mix of concrete on the area or is there other issues I should be aware of before doing this?
Will the reinforcing bars be compromised in the state they are?

Cheers guys