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hollow sounding radiant slab

  I poured a 14 x 16 foot slab for my new shop 5 days ago. It sounds like thumping a melon. Slab is 4 inches thick and reinforced with #3 rebar. It has 5/8 wirsbo tubing in it. My county has adopted the International energy conservation code so I insulated accordingly. The stem wall is insulated with 2 " of blueboard, The top   8" of the stemwall have 1 " blueboard embedded on the inner edge.There are 4 " radon tubes on the bottom, then 10 " crushed gravel tamped. Then 1 " of blueboard. Then another aprx 10 " of crushed gravel tamped, then a layer of 1/2 "insul tarp with the 4 inch slab above. The heating pipe is tied to the underside of the rebar grid. WHAT DID I DO. My concrete contacter is as baffled as me.


  • Funny sound

    Could it be the radon tubing somehow acting like a soundbox?

    The other possibility would be the rebar expanding enough to lift the slab off the gravel bed as the radiant tubing heats it up ( wild guess!)--NBC
  • Gordy
    Gordy Member Posts: 9,546
    edited November 2012
    Melon slab

    Does it sound hollow all over or in certain areas?

    Trying to understand the 10" of stone then 1/2" of foam then 10" of stone topped with insultarp. Why did you not just use xps between the stone ,and the slab?

    From what you described on the installation the ground should be stable if it was well compacted. With a 4" slab, 5/8 pex, radon tubing, and insultarp I'm willing to bet the insultarp, and radon tubing is giving you the melon sound.

  • james barnett
    james barnett Member Posts: 8
    sounds good to me

    Slabs with rigid insulation sound hollow. Also, where I work we don't have those radon tubes, and I don't layer rock, insulation, rock, insulation, slab. That might contribute to the sound... sometimes I pour a five inch slab when the slab is filled with radiant tubes, and other plumbing. My reasoning being is that a 5 inch slab is probably really a 2 to 4 inch slab in some places after all the tubing etc. goes in. You should probably have a couple of control joints in this slab, 12x 24 as I recall. But I'm fanatical about control joints and steel reinforcement as I usually polish the slab and use it as a finished floor material.

    If you didn't properly tamp the rock, that's the only situation where you could have a problem. But I bet you did tamp it well, the kind of person worried about a hollow sound I think would be the kind of person who tamps well...
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