Mesh Placement in Slab on Grade 4

[medeek]

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.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

My opinion, presently:

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.

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.

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?

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.

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).

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.

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?

This. And yeah, it's a perpetual QC problem that routinely causes engineers to doubt the use of WWF for anything important.

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

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.

 

[dik]

If you use mesh, use it in flat sheets, chaired using support bars/conc bricks, and make sure it stays in place. I don't spec mesh.

Dik

 

[FusionDan]

First Post. I make my living off mesh, imo 10 gauge(aka W1.4) does not stand up to human traffic, I refuse to sell it...I see that it's heavily used to the east of the Rockies, which tells me 6x6-10/10 has become a creature of habit more than anything. The only place I would bother w/ 10 gauge is in thin Metal Deck slabs, but you have to specify Cont. slab bolsters (PRICEY). Buyer beware, if your Metal Deck floor isn't stiff enough, it will vibrate (tickle your toes) and drive the hot heads/impatient types insane. I wish Mesh makers would just eliminate 10 gauge completely. The fact the MDI (Metal Deck Institute) suggests it bums me out.

 

[thaidavid40]

If you are looking to get flexural reinforcement ratios for a 4" S.O.G., then good luck - especially with wire mesh. All we ever use mesh for in the South is for typical T&S steel. If you truly need flexural capacity, then use bars. If you do though, move up to a 6"-thick slab: its more practical, more workable, and more forgiving of slight reinforcement depth displacement. Space the bars at 16" o.c., so workmen can walk without stepping on the bars. And never, ever, let the workmen "hook and lift" ANY reinforcing - ever. As KootK suggested, if you want to keep the 4" slab thickness, opt for fiber reinforcing instead - it more than pays for itself in saved labor, and headaches about big shrinkage cracks forming because the wire was lying on the ground at the bottom of the slab, and working to exacerbate the cracking. The old joke around here is to use a "standard #6 slab": 6"-thick, #6 wire mesh, with 6" x 6" spacing. ;-)
Dave

Thaidavid

 

[Ron]

Agree with dik....rolled wire mesh is worthless. Use sheets if you use it at all. In my nsh opinion, T&S steel is unnecessary in properly jointed slabs on grade. Fiber can be used to enhance the properties of the concrete, but don't let it influence your jointing decisions. Sawcut joints early and keep the spacing to 12 feet or less, each direction with an aspect ratio of no more than 1.2. Use larger aggregate in the concrete and keep the W/C to less than 0.55. And.....control the thickness of the concrete by seeing that the subgrade does not vary in flatness in short distances.

 

[BigH]

Could you use fibre reinforcement instead? It is easy to spec location but actual "on the job" always screws it up.

 

[MotorCity]

Best place for wire mesh is on the truck it came on, keep it out of my slabs. .

I use #3 bars instead.

 

[medeek]

How does fiber mesh really work. Once the concrete is cracked the fibers are rendered useless. Then you could potentially have differential settlement of your slab (case in point my garage has a nasty edge jutting up that I need to grind down but have never gotten to).

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[SlideRuleEra]

How does fiber mesh really work?

Here is what a fiber manufacturer says "Understanding Fiber Reinforced Concrete"

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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