I've got a garage access door (32" width) whose wood/aluminum sill
chronically allows leakage beneath it, into the garage when it rains.
I've tried all kinds of caulking, rubber seals, improved drainage, etc
and nothing works - I think it was just a poor installation that needs
to be ripped out.

I'm thinking of replacing it with qwikrete instead of installing another
wood/aluminum sill.

Good idea, bad idea? It doesn't seem like it would be too difficult to
make a form and then level it out to be flush with the existing door
bottom, but I haven't tried this before and I don't want to make an
annoying situation into an actual bad one.

Thanks