I wouldn't fill them up completely. I would only fill the last 4" or so.
Make a form, out of wood, to hold up the concrete while it cures. Put in a
couple of anchors into the sides of the opening so that the new has
something to grab onto. It would be best if you could chisel out some type
of keyway to support the new slab instead of anchors.

ASCII Art follows


Floor Floor
___________ __________
| |
| | cut into concrete here
|------|
| ^ |
^
build form here




wrote in message
ups.com...
I recently bought a 40 year old house that has heating ducts in the
slab. It occasionally has water leak into the ducts after a lot of
rain. It dries up within a couple of days and there is no evidence that
there has ever been any mold problems.

I just finished rerouting the ducts into the attic (MAJOR job), and
removed the slab ducts from the supply feed. Now I need to close off
the slab ducts. I have seen other forum posts suggesting filling them
with concrete. Filling the entire slab duct would be a lot of concrete
and a real pain.

I am looking for other (simpler/cheaper) alternatives. One would be to
seal off a couple of feet each side of each register with conrete. An
even more attractive solution I think would be to use expandable foam
(like "Great Stuff") to fill around each register.

Does the expandable foam solution have any problems?? Are there other
alternatives??

Thanks,
JJ