#verylong time due to #rain fall
It's a very old structure. As per my observation this is roof slab. Water percolated through this concrete for a long time and that water corroded the rebars. Everyone knows when corrosion on rebar happens the rebar expands. When rebar expand it creates force on concrete. As concrete is a rigid materials it spall out. And this scenario happens.
If rebar corrosion is major than add rebar and clean out the corroded rebar surface. Apply protective coating on affected rebar surface. After that cut 3" dis core @3' spacing. Then apply epoxy bonding agent and do shuttering work asap. Then pou microconcrete through those cores .