Repair to moulded concrete window surround?

[ZX81]

I'm looking to repair a moulded concrete window surround on an ex council house. The window surround - i.e. the window sill, mullions and lintel - appears to have been moulded in one piece. The bottom corners, where the sill meets the mullions, are damaged - chunks of concrete have come away exposing rusted rebar. I guess that water has got to the rebar, causing the rust and the subsequent expansion has caused the concrete to fail.

So I'm wondering what's the best way to repair this? I'm guessing that I need to cut a section out around the corroded concrete and rebar, but how do I patch the missing concrete to get a good bond between old and new, and how can I prevent water finding its way through the join and to the rebar and causing the same problem in the future?

I'm told that a repair to the neighbour's window involved the in situ moulding of an outer jacket of concrete to cover the whole surround - ie somehow pouring concrete into a mould fixed to the window surround. I can't quite picture how that would be done - I guess the mould would be built and filled in sections, sill first, then mullions etc. And there would still be the problem of ensuring a good bond to old the concrete.

Any ideas?

Thanks

 

[PrenticeBoyofDerry]

Have you any pictures you can upload?

Ideally you need a repair mix that is much like the original product, do you know if it's a wet cast or semi-dry product.

You can repair both by mixing silica sand and aggregate fines, to get the colour somewhere near you will have to tinker with cements(white and grey) and maybe some pigments, the mix should also contain a waterproofing agent.
If that fails to get the correct colour, you could consider filling in the repairs and paint the whole thing.