New garage concrete slab problem

Old 03-08-15, 04:42 PM
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Unhappy New garage concrete slab problem

I recently built a new 30' x 16' garage/work shop in my back yard. I built the whole thing except for the foundation, and slab. I couldn't be there when the mason poured the 5" thick concrete floor. Long story short, they poured the floor 4 inches to low, and now every time it rains, water runs into the garage from the driveway.

I have to fix this soon. I have plenty of block wall height to raise the floor. Question: Can I pour a 3" or 4" thick slab over the existing floor, to raise it, or will I have to spend a small fortune to have the slab removed?

Thanks Bill
Old 03-08-15, 07:52 PM
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As long as your new slab is going to be that thick you shouldn't have a problem going directly on top of the existing slab. I wouldn't even try to tie the two slips together but rather I would leave the new one floating on top of the existing one.
Old 03-09-15, 08:31 PM
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Deliberately making the new slab "float" on top of the old one is asking for trouble. Before too long, random cracking of the top section may start, and then get worse as temperature differentials do their thing--the underlying slab will not fluctuate in temperature as much as the top slab. If there are control joints in the lower slab, you should mimic their pattern in the top slab. Otherwise they will reflect through to the top over time. I would make an effort to bond the 2 slabs together, first cleaning and then scarifying the surface slightly. Using a neat Portland cement slurry (thickness of heavy cream) as a bonding agent between the new and old concrete will help glue them together.
Old 03-23-15, 04:12 AM
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imo, this is an old debate that doesn't have any common repeating standards to aid 1's decision/choice between bonded OR unbonded,,, as bdge post'd, you'll need to reference existing jnt pattern & transfer to new conc ( hopefully a sufficient pattern was placed ),,, all things being equal, i'd opt for unbonded using 15# roofing paper to keep slabs separate
Old 03-23-15, 05:54 AM
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Post a picture of the outside of the garage, there there may be an easier way.
The issues I see with poring over the old slab, it's not addressing the real issue and water will still be getting under the wall and at some point is going to rot out the bottom plate and bottom of the studs.
The walls would have to be formed up with pressure treated wood to keep the new concrete from making contact with the studs.
Things like adding gutters, regrading, flashing, adding a french drain would be a cheaper, easier ways of fixing it.

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