Concrete Slab prep - chair spacing for mesh

[Usafexplorer]

We have an engineered cold formed building going up shortly, the drawings specify #4 rebar in the footings (we are using #5) and wire mesh in the field on a monolithic slab. Contractor initially tried telling me they would "Pull up" the mesh but I insisted on chairs at the suggestion of the county inspection department. My question is proper chair spacing as it's not specific on the drawing. The drawing references the following standard: ASTM A185 (Fy MIN. OF 70 ksi) but so far I can determine what the industry standards are for spacing. Any ideas out there on the interwebs??? Thanks.

 

[dcg9381]

Mesh is likely "not engineered" so you won't find standards on it.

Use enough spacing to keep the mesh "reasonably straight" and off the surface below (IE, watch the sag).

They do that "pull up" **** around here... Or even worse, they offset it with rocks.

 

[billconner]

I'm following. I've read 24". I'd seen adds for plastic chairs that are suppose to collapse under a load and spring back. Are these the standard or new? I've used bars - the little triangular truss things - last slab. Used dobies long ago.

 

[Usafexplorer]

I've seen too many videos of failed mesh "Pull ups" as once the concrete starts flowing nobody seems to concerned with pulling up the mesh and then it becomes too late. Just seems crazy there isn't a published standard, I've seen a couple manufacturers that have 18 - 24" suggested spacing amounts on their sites but I was hoping for an actual standard to give my concrete guy.

 

[Usafexplorer]

I'm following. I've read 24". I'd seen adds for plastic chairs that are suppose to collapse under a load and spring back. Are these the standard or new? I've used bars - the little triangular truss things - last slab. Used dobies long ago.

I'm not sure, can't remember the last time I saw dobies used but I still see them in the big box store.

 

[ConCretin]

Support spacing depends on the gauge of the wire mesh you are using. If it's 2.9 0r better, 3' is a good start. Honestly with anything less you are probably better off hoping for the best by pulling it as you go cause it's so flimsy. Figure 2' if you want to try and hold it up.

I prefer using rebar because it's more rigid and easier to hold in position. #4 bars at 18" or so can be supported at 4' OC and will stay where you put it.

 

[Usafexplorer]

Support spacing depends on the gauge of the wire mesh you are using. If it's 2.9 0r better, 3' is a good start. Honestly with anything less you are probably better off hoping for the best by pulling it as you go cause it's so flimsy. Figure 2' if you want to try and hold it up.

I prefer using rebar because it's more rigid and easier to hold in position. #4 bars at 18" or so can be supported at 4' OC and will stay where you put it.

Thanks for the suggestion, I appreciate the guidance from someone in the business.

 

[Dagny]

skip the mesh spend the money on rebar.

 

[billconner]

I lean toward no remedy nor rebar, fiber mesh, heavy vapor barrier, and don't restrain edges do it can shrink without cracking.

 

[Rusted Nut]

If you’re using 6x6 10 ga wire mesh, I’d chair it @ 24”. When rebar is chaired, you can walk on it, not with mesh. Chair the first 4’, then lay the mesh flat on grade and chair up as you go, otherwise it gets mangled. This is not pulling it up as you go! This is chairing it 2’ ahead of concrete, there is a difference! Me, I’d go with #4 bar; preferably 2’ OC, but 3’ OC works pretty well.