Sheet Metal Box Under Slab

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  #1  
Old 09-22-17, 03:45 AM
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Sheet Metal Box Under Slab

There will be a 10" x 8" sheet metal box a few inches under the slab where the fresh air intake enters the back if the masonry chimney (chimney will be enclosed by new garage). Is it okay to bury a thin sheet metal box in concrete? I don't want there to be issues with the slab cracking in the future because that area doesn't have proper support. It will be a structural slab with grade beams, so maybe it's a non- issue.
 
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Old 09-22-17, 03:55 AM
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Any other ideas for how to join a 3" PVC pipe to a 2"x8" opening in the brick? Maybe chisel a round opening, so the pipe can fit inside an inch or two, then install flashing around the junction so when the concrete is poured it doesn't seep inside the intake channel?
 
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Old 09-22-17, 05:25 AM
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Yes, there is a good chance the concrete will crack over the sheet metal box. The chance is reduced the thicker the slab. You could lay some rebar or wire mesh in the area to help reinforce it and hold the pieces in place if/when it does crack.

I would look at HVAC duct fittings. There are many sizes of round to rectangular. If it's just to keep concrete out it doesn't have to be pretty and gaps can even be taped over with duct tape.
 
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Old 09-22-17, 05:36 AM
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I think I'd rather have them cut the brick out and place the pipe in (no sheet metal box), and flash around it. Then reinforce around the area with welded wire mesh. Does this sound like a good approach? BTW, this will be a structural slab with w.w. mesh and #4 rebar spaced 12" apart and supported by grade beams.
 
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Old 09-24-17, 10:11 AM
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I found a way to secure a 4" round PVC pipe fitting to the opening and flash around it with sheet metal. I took some more accurate measurements, and as-is, the top of the PVC pipe will be about 1" into the bottom of the concrete slab, with the lower 3" buried in gravel. Would it be better to bury the pipe deeper in the gravel or will it not make a difference? 3" of concrete over top seems like it would be sufficient. No vehicles will be parked in this area of the garage and there will be rebar and wire mesh over top of the pipe.
 
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