Repairing external window sill

[Glastoun]

Part of the external window sill (presumably wood) has rotted off, exposing the metal bar within it. I'm not sure what function the bar plays, but is this repairable or would the sill have to be removed and replace with a new one?

If repairing, how would I go about it? My first thought is to build it up with some kind of filler but I'm guessing that wouldn't be a long term fix.

 

[Burnerman]

It's a concrete sill, and what you see is steel reinforcing bar. It has rusted, probably due to a crack letting the water in, expanded and blown the concrete away.
I've only done this once, so best to hang on until the experts appear, but I constructed a timber shuttering around the entire sill, and cast a new one.
John

 

[crank39]

That's exactly what I've done in the past, quite a simple repair. Don't forget to pin a length of round wire or coax cable to the shuttering so it forms your capillary groove

 

[Glastoun]

Okay, so I would be fixing some timber around the existing sill to make a mould of what it should end up like, and then pouring concrete in to fill the gaps?

What timber, and how would I fix it up in place?

I've mixed plaster before but never concrete, what bags would I be buying? (probably from B&Q, and the quantities are going to be small)

Would I have to do anything with the existing bar? I've read that the metal bars are more for keeping the concrete in one piece during transit, rather than acting as a support once the sill is in place.

I'm happy to take this on, but at the moment I have no idea how to start.

 

[Burnerman]

Create your shuttering box with 22mm ply, so it doesn't distort, fix timber to the outer sides so you can fix it to the wall with rawlplugs. Prop it up too, if you can.
Rebar gives great strength to concrete especially when its under load.....chip away the existing rust (even cut the bar away if its completely exposed) add some rust preventative treatment if you can.
The mix - 3:3:1 or 4:4:1 sand / fine gravel / cement with a little pva just to help it along, maybe....the ready mix concrete repair stuff from the sheds will probably do you fine.....don't forget Cranks drip lip!
Stops the water from running underneath.
John