Slab Repair after trench for plumbing


Old 02-24-15, 07:23 PM
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Slab Repair after trench for plumbing

I've just cut several trenches (2-3 ft wide x 25 ft long) in my existing concrete slab to run plumbing. The slab was about 4in thick with wire mesh and a plastic barrier between the concrete and the wet clay like soil (home located in New Orleans). I also had to break up footings in a few locations as well. I used a concrete saw to score the trenches and then a jack hammer to break the concrete. Most of the trench edges have 1-2 inches of smooth saw cut with the rest of the edge jagged. The plumbing is done, and now I'm ready to repair the slab.

I was planning to use a company that mixes short loads of concrete on site and then I would wheel barrow and poor the concrete. A few questions.

Do I need to put wire mesh?
Do I need to dowel into the existing slab or would adhesive painted on the edges be adequate?
Do I add a plastic barrier over the PVC pipe/below the concrete.

The plumber dug the trench out after I removed the concrete, placed the pipe, then back filled with the hard clay like soil (very clumpy). It appears like he mashed the clay like soil manually, but nothing else was done to compact it. It is pretty hard and doesn't move much when I stand on it. Is that ok or is it critical that this is compacted better? If so, any recommendation how without messing up all the plumbing slopes already installed.

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Old 02-24-15, 10:31 PM
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Based on the pix, I think the best thing you could do would be to manually remove at least a few inches of that lumpy clay, replacing it with crushed gravel that you should then compact with either a manual or mechanical tamper. Doing so will result in your new concrete being less likely to settle and break free from the original concrete.

Wire mesh isn't necessary and won't do much good for such small areas. Neither is doweling into the existing concrete slab. The best bonding agent available would be to use a neat Portland cement slurry, brushed onto the cleaned (but damp) concrete slab edges--can't go wrong at $9 a bag. I don't think I'd bother wrapping the pipes with plastic either, as I'm not sure what it would accomplish.

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