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I’m not much of a concrete person (mostly finish carpentry) but need to pour a little garage slab. I want to go with a slab on grade, but need to have a 6″ stemwall to meet code. Can this be poured in one lift? How do I keep the bottoms of the stemwall forms together, yet removable after concrete is cured? (E.G. If I pour the footing, then the slab, then fill the forms, I do not want the stemwall forms to splay out from the pressure, nor do I want the concrete to “run out” from the stemwall and become part of the slab) This would be a 12″x12″ slab edge, 4″ thick slab, and a 6″thick x6″ high stem wall.
I am trying to avoid multiple short loads and pump fees.
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Here's one way........
Consider that code may also require rebar in the footer and stem as well as welded wire in the slab.
The stipple portion of the drawing is the concrete, shown as a monolithic pour. The heavy black lines are the forms and the red is depicting a homemade fixed clamp to hold the inner 6" form board. There are neat metal forms (not so bulky) that can be purchased or maybe even rented. Sorry I don't have a pic.
Use 3/4" plywood for the clamp and 1x for the inner form. Set your outer form first, putting in all the stakes and braces. Nail or screw the inner form to the plywood clamps so that when you drop the clamp over the outer form the clamps hits the side of each stake where it will be screwed in place. Shouldn't move after that.
The gap between the brace and the top of the stemwall pour as well as the space left at the bottom of the inner form board is to fit the trowel in for finishing.
You can get 24 clamps from a sheet of ply (12x16). You'll need enough of them so they aren't spaced too far apart where your inner form would bow out between them. I'd say between 12" to 16" max. oc. You can go for the overkill by getting some form ties and putting them across the forms as well.
Pour the perimeter first, up to but not the stem wall portion, then continue the pour into the slab, screeding as you go and making sure you keep the concrete level with the bottom of the inner form.
The time it takes to do the screeding of the slab should allow the perimeter to set up enough so that when you go around to fill the stem wall portion nothing will displace and be forced under the inner form. Rod the mix as you fill the last of the form so everything blends. Use as stiff a mix as you can also, for the job.
Have fun.
how do you screed - do clamps get in the way?
Just form it
You just form the exterior, with stakes and kickers like normal. It needs to be really solid. Then form the inside with stakes and 2X8 form boards which will leave the height 1-1/2 inch below floor grade, using spreaders and tie wire to hold the width between the two forms.
You place the concrete for the footer and stem wall first with no more than a 2-inch slump. By the time you work your way around the perimeter, you should be able to pull the inner form boards, and then pull the stakes out. Be carefull, the concrete is still green, and should still be in a plastic state, just stiff.
If you do it right you now have a 1-1/2inch edge of the interior slab adjacent to the wall. If you have enough guys and are moving fast enough you don't end up with a cold joint. The reinforcing steel will tie everything together.
The typical stem wall gets a footer, then "x" courses of block, capped with a lintel block and then pour the slab (steel as required) That is two pours
In your care are you saying you need 6" above the footer before you hit the bottom of the slab? (18" total assuming a 12" footer?)
Why not just dig a 16" trench, form the outside up to 22" and pour the whole thing in one shot?
You are trading extra concrete for the extra trip.