Sealing between two layers of concrete underground?

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    We have four 7″ conduits running 10 ft between buildings (for heating/hot water pipes, etc.) below the frost line. They are assembled in 4ft sections with couplers that now take on water when it rains hard (50 yrs old). We will be encasing them in cement to seal them up again – 4″ all the way around. We were advised to do the cement in two pours to avoid making them sag in the middle from the weight. The idea is to pour the first half just up to the center line and let dry so the pipes are now supported, then pour the rest on top the next day. See attached.

    The concern is future water entry where the two pours meet. Is it necessary to seal between layers? If so, how?

    Also, what material would you suggest for the ‘expansion joint’ where the new slab meets the basement walls?

    The surrounding dirt is all clay.


    Owatonna, MN - Minnesota

    If the conduits and existing concrete are in good shape and on a solid base, doing it in a single pour should not be a problem. Concrete weighs 150 pounds per cubic foot so the concrete on top would only be 50pounds per square foot, which is not a great load, floors in your house are designed for more than that.

    If you do pour in two pours, you would need to use a waterstop between the pours.

    As far as an expansion between the walls, I would use a good grade of foam expansion with a tear off strip so can seal the joint with caulk well and then waterproof it also.

    You may be able to get by making your pours and spraying or troweling a good waterproofing on the exterior of the concrete to keep water out. It would also bridge any future cracks that may develop. I would probably do that anyway.


    Thanks, Kurt. Let me see if I can clarify.

    We’re not worried about the conduit getting crushed, only pushed down in the middle into the wet first pour.

    There is no existing concrete (besides the basement walls) and no base. Conduit is just surrounded by dirt/clay right now. We will have to dig out 4″ below the conduit so they will essentially be suspended in ‘mid air’ until the pour. We’ll probably suspend with wire at the middle (see attached) until the concrete is worked underneath, then pull the wire out and work out any gaps.

    We’ll carefully pour the concrete near the wall ends and work it under and toward the middle.

    Do you think we can just complete the top pour right away while that first (half) pour is wet? Will the wet cement support the conduit from sinking with the weight of the top pour (50 lbs/ft) on it? The conduit walls appear to be 3/8″ thick concrete so it’s assumed they have a lot of weight on their own.

    If not, what ‘waterstop’ do you recommend?

    Fort Lauderdale, Florida

    are the couplings failing because of corrosion or movement?

    Owatonna, MN - Minnesota

    Hanging the conduit like that should keep it from sinking in the concrete, I don’t know that I would worry about it to much. I f you hang them like that I would still do one pour and clip the wire once the concrete has set up.

    I would also think about the conduit floating up in the concrete. That may be as much a problem as it settling

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