How deep should mesh be in a concrete slab?

They won't tell you it wont crack because there are just too many variables. Poor preparation, shrinkage, a bad mix, etc. etc. The variables are even more of an issue when you get into a DIY situation. If I get called to place and finish a slab or floor for someone where I have not done the grading, compaction and underfill, all bets are off. The concrete is only as good as what you're putting it on and even if you rebar the daylights out of it you still can't be sure it wont just settle off level in one big chunk.

Ok, all that said, I've seen plenty of slabs that are decades old with no cracks in them. The one in my garage is nominal 4 inch psi mix on 6 inches of crushed stone, been there 31 years, one small hairline, no rebar, no welded wire. The only thing I know for sure about this one is that we puddled the crap out of it for days before we placed the stone and since the hairline exhibits no "lippage" from one side of the crack to the other, I'm gonna chalk it up to shrinkage. The stuff shrinks more than you would think. Take a look around the junction between your basement or crawl space walls and the floor... I't not uncommon to see a sixteenth of an inch space. Very interesting discussion and thank you to the guy who provided references to the technical publications about concrete reinforcement.

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medeek said:

1.) Best placement of the mesh in the slab, centered, 2 in. from top, or stomped on by construction workers until it is in the bottom.

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If it's a residential, 4" slab I put the mesh at mid-depth. If you're going to dodge the saw-cuts and get a modicum of cover, putting it in the upper third as some recommend is pretty tough. As I see it, the placement comes down to two issues as far as cracking goes. Firstly, the reinforcement helps with axial strains and cracking across the slab. For this purpose, position isn't all that important and mid-depth makes sense. Secondly, the reinforcement helps with flexural strain and cracking at the top of the slab where local hard spots may result in hogging moments. I say to heck with latter. In a 4" slab with saw-cuts, I doubt you'll ever get the reinforcing high enough in the section to be effective flexural reinforcement.

medeek said:

2.) Should the mesh size extend to the perimeter of the slab? Ground clearance from edge and from bottom? If its a 4" slab isn't the 3" rule for reinforcement violated?

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I'd extend it to the perimeter or at least the last saw-cut joint if there is one near the perimeter. Cover is routinely violated in thin slabs on grade, both on the ground side and below the saw-cut joints. That's what's done and, at least for common interior applications, reinforcement rusting doesn't seem to come to pass or cause any problems. Slab on grade is, technically, not structural concrete.

medeek said:

For more information, please visit concrete slab mesh.

3.) Most typical mesh size I've seen is 6x6-W2.9xW2.9 but I've also seen 6x6-W1.4xW1.4, any thoughts on what mesh size is best for a typical 4" and 6" slab (residential work).

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I've been using W1.4 for thing residential slabs. Frankly, I consider the reinforcing in these slabs to pretty much just be nominal rather than seriously purposeful. Where I'm doing non-calculated token detailing, I lean towards light.

medeek said:

4.) I've seen dobie blocks and wire chairs used to keep up slab bars, what should be used for mesh? Won't the workers step on the mesh and bend it out of shape or push it to the bottom?

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medeek said:

5.) #3 Bar, Mesh, or Fiber? What do you use and why?

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I like bars, fibers, or nothing from a performance/QC perspective as long as jointing is done properly. In my area it's almost always wire mesh because that's what contractors continue to prefer and expect.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

My opinion, presently:If it's a residential, 4" slab I put the mesh at mid-depth. If you're going to dodge the saw-cuts and get a modicum of cover, putting it in the upper third as some recommend is pretty tough. As I see it, the placement comes down to two issues as far as cracking goes. Firstly, the reinforcement helps with axial strains and cracking across the slab. For this purpose, position isn't all that important and mid-depth makes sense. Secondly, the reinforcement helps with flexural strain and cracking at the top of the slab where local hard spots may result in hogging moments. I say to heck with latter. In a 4" slab with saw-cuts, I doubt you'll ever get the reinforcing high enough in the section to be effective flexural reinforcement.I'd extend it to the perimeter or at least the last saw-cut joint if there is one near the perimeter. Cover is routinely violated in thin slabs on grade, both on the ground side and below the saw-cut joints. That's what's done and, at least for common interior applications, reinforcement rusting doesn't seem to come to pass or cause any problems. Slab on grade is, technically, not structural concrete.I've been using W1.4 for thing residential slabs. Frankly, I consider the reinforcing in these slabs to pretty much just be nominal rather than seriously purposeful. Where I'm doing non-calculated token detailing, I lean towards light. This . And yeah, it's a perpetual QC problem that routinely causes engineers to doubt the use of WWF for anything important.I like bars, fibers, or nothing from a performance/QC perspective as long as jointing is done properly. In my area it's almost always wire mesh because that's what contractors continue to prefer and expect.I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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