Mesh Placement in Slab on Grade
Mesh Placement in Slab on Grade
(OP)
I know we've beaten this topic to death but I thought I might revisit it one more time. The questions that are still nagging are:
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.
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?
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).
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?
5.) #3 Bar, Mesh, or Fiber? What do you use and why?
For reference the image below shows a typical or proposed SOG with mesh reinforcement.
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.
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?
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).
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?
5.) #3 Bar, Mesh, or Fiber? What do you use and why?
For reference the image below shows a typical or proposed SOG with mesh reinforcement.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com





RE: Mesh Placement in Slab on Grade
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.
RE: Mesh Placement in Slab on Grade
Dik
RE: Mesh Placement in Slab on Grade
RE: Mesh Placement in Slab on Grade
Dave
Thaidavid
RE: Mesh Placement in Slab on Grade
RE: Mesh Placement in Slab on Grade
RE: Mesh Placement in Slab on Grade
I use #3 bars instead.
RE: Mesh Placement in Slab on Grade
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Mesh Placement in Slab on Grade
I'm a fan of fiber reinforcement and started using it, instead of wire mesh, when plastic fibers were first being introduced in the 1980's. However, neither fiber nor wire mesh are going to "make up" for shortcomings in design/construction.
The first thing is to have suitably prepared subgrade. (Poor subgrade is probably the real cause of the problem in your garage.)
Second, design the slab per Ron's recommendations. (Then you don't really need either fiber or wire reinforcement.)
Third, continuously wet cure the concrete for many days - preferably 7. (Minimize concrete shrinkage to reduce uncontrolled cracking. If unexpected cracks appear, they should be smaller because of reduced shrinkage... then, aggregate interlock works.)
As a final step, take the money "saved" by not buying/installing either fiber or wire mesh and "spend" it on extra concrete. Make a 4" slab, 5" thick... make a 6" slab, 7" thick. The differential cost to do this very low. Subgrade preparation, forming, concrete placement, finishing, and curing are all virtually unchanged, regardless of slab thickness.
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