Need to seal off slab heating ducts

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
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snipped-for-privacy@cox.net wrote:

I would not do anything about them for a year. See how well those attic vents work and how much energy they may loose. You may want to go back to the slab vents. At least most of the heat (and cooling) loose there is returned through the concrete.
--
Joseph Meehan

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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

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