Is there a minimum depth beneath the surface that rebar must be placed to prevent any visible “bleed” to the surface?
IIRC, conventional wisdom suggests the optimum position (strength wise) is 1/3 of the way down though the slab.
But with a thin slab (2″, application is a cantilivered concrete countertop), and #3 rebar, there’s around 1/2″ of concrete above the “top” level of rebar. I’m laying out in a 9″ x 9″ grid so there’s 3/4″ below the “bottom” piece of rebar to the form substrate.
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Replies
Don't know what you mean by "bleed". Location of rebar in concrete is based on a structural analysis. In addition, you want a minimum concrete thickness to cover the rebar to protect the rebar from corrosion. Concrete against earth is 3 inches, for typical walls, beams or columns cover is 1.5 inches. It can be as small as 3/8 inch if you are using small bars or WWF for a shell structure.
I think that's usually called "ghosting". It's tough to get the rebar very far from a surface of a concrete countertop, as you know. I would just go for the middle, leaving 5/8 on each side of the rebar.
I don't think you'll get ghosting at that depth, but you want to avoid having the rebar smushed down closer to the surface.
zak
"so it goes"
Ghosting may well be the term. I'll follow your advice and position a 5/8 riser under the bottom bar (I'm casting in place, not upside down) and leave 5/8 to the top.
Thanks.
In addition to other concerns, if the concrete will be exposed to soil contact, weather, or especially salt, more thickness is needed to prevent rust and associated spalling.