forming shallow holes in surface of wet concrete
I want to set a few decorative 4" slate tiles in a small concrete slab. The tiles will not be ready for weeks, and I'd like to pour the pad this weekend, so my plan was to cut some 4.25" squares from 1/4" plywood, and press these into the wet mix flush with the surface. After it cures, I'll glue some little handles on the back with 15 minute epoxy and pull them out, leaving cavities to glue/grout the tiles into later.
Is this a mistake? Will the wood tend to float out of the wet mix, or other basic problems? Any better ideas for how to do this?
I planned to cut the edges with a 10° bevel to let them release. Is that enough? How tightly will the concrete adhere to the wood, particularly the porous edge grain? I thought I might cork on a bit of paraffin. Would some other release agent be better? Will residue interfere with the later adhesion of the tiles?
Finally, how long should the concrete cure before pulling the plugs? This is in Seattle, so average temp will be 50°, roughly.
Sounds like you're putting the cart before the horse, but there's no reason it won't work.
Don't waste your time or money with epoxy. Use a couple screws.
Use some sort of form release on the plywood. No need to bevel the edges. You can also wrap your pieces of plywood in packing tape instead of using form release oil....A very common tactic when making plumbing voids concrete countertops. That way you have a pure concrete surface to bed your tiles into without worrying about form oil or paraffin breaking the bond.
The wood won't necessarily float in the concrete like you asked. If you use too much water and make it too soupy, I guess this is a possibility. Use the least amount of water possible to make the concrete workable. Less water equates to stronger concrete as well. The wood will fit right in and you can finish around it.
I'd pull the plugs before the concrete really sets. You'll place it in the form and get it close with a trowel, and an hour or so later you'll come back and finish trowel or broom the surface to achieve the desired finish. After an hour or two, you should be able to pull the plywood plugs out, but the edges will still be green enough to smooth the rough edges if necessary. Just remember, don't use too much water.
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