Hi all.
Here’s a couple of questions for all you concrete experts out there. In order to finish my basement, I have to move my garden tractor and yard tools out of the garage area. I’m considering two options: building a shed or building a storage area under my back deck (there’s about seven feet of headroom). In the second case, I want to pour a 10 x 12 foot concrete slab for the floor. So here are the questions, how thick should the slab be and does it need to be reinforced (i.e. rebar, mesh, etc.)? Thanks for your help!
Replies
Hi Stonemason....
4" nominal is the least thickness I'd pour a slab. It doesn't *have* to be reinforced but I'd add 6x6 welded wire mesh none the less. The important thing is to make sure the substrate is firm native ground. If you need to fill or level any low spots I'd use pea gravel since it's self compacting. I'd also pour a thickened edge maybe 6" deep x 8-10" wide around the perimeter where the shed walls sit. Ideally I'd want to keep all the wood at least 6" above any earth around the perimeter to avoid any wood damaging organisms from invading but you might get away with using pressure treated sills directly on the slab. If it were me, though, I'd put one course of CMU around to set the sills on and use pressure treated material as well. If you decide to place the sills directly on the concrete slab, have them at the ready when you pour the slab, anchor bolts drilled and placed in the sills then embed and level them in the wet mix. Once cured (a day or two later, snug up on the AB nuts.
Dennis in Bellevue WA
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I agree with Dennis but, instead of using 6 x 6 - 6/6 wire mesh, I would use #3 (3/8") resteel placed about 18" on center both ways with the tops of the bars 1" below the surface of the slab. It's easier to keep the reinforcing in the proper location since the resteel is much stiffer and you can step between the bars while placing the concrete.
Clint's actually correct with respect to the rebar thing. You can get little chunks of concrete called "dobbies" that you place under the bar to hold it to the proper elevation in the slab. I would have mentioned using bar instead of mesh except for a shed, and for the casual concrete worker, mesh is easier to work with. If you get your ww mesh in sheets (so it's relatively flat) and chair it up frequently the same as for the rebar, you can get it to stay put while you place the concrete.
When all is said & done, I prefer rebar over mesh in most cases myself.
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Dennis in Bellevue WA
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What are the pros and cons of dobbies vs wire chairs? With the talk on another thread about snap-ties being a conduit for corrosion, it would seem that the chairs would be equally vulnerable - except that I have never heard of this being a problem.
I assume that with dobbies you would want them to be damp before slopping the mud around them.
Casey -
Wire chairs are used more often on elevated decks, that is, decks poured over an elevated plywood form. They're practically invisible from the underside of the slab when its exposed. Plus the 'proper' kind have plastic covering on the ends of the wire to prevent or minimize corrosion.
There a little hard to use with a slab on grade since they might get punched into the ground or definitely get mashed in if the slab is being poured over gravel, pea gravel or polyiso insulation.
There's no need to dampen them for a slab on grade since they only draw minimal moisture out of the concrete and are (or should be) 2" or more below the surface anyway. I've got conc. dobbies all over under my shop slab that was poured a couple months ago and there's absolutely *no* evidence of where they are.
I've never had anyone in our area (Seattle - the rain capital of the contiguous 48) be too concerned about snap ties. You don't want to try snapping them off too early while the concrete is still green or they'll 'spin' on you and could then cause more of a problem. Myself I wouldn't worry about them since they're self draining and don't collect weather to any degree.
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Dennis in Bellevue WA
[email protected]