Concrete Chairs | Page 12 - TractorByNet

For more engineered stuff than a single family home; they have specific bends pre-made or field bent in the rebar, to place it where the load is specifically in the correct place. Bottom 1/3 across the span, but at other places, where the load changes, being in the top 1/3. if you cast a 1" diameter dowel of unreinforced concrete, you could literally pull it in a straight line, with some grunt needed, and pull it apart; if you add even a single 16 garage wire into that same dowel, it would take a truck to pull it apart. That 16 ga steal wire had an approx yeiled strength of lbs in tension. I may have went in a whole rant for nothing; if we are talking a driveway slab, or something similar, 6" of psi concrete, poured on top of a well compacted subgrade, where the expansive materials and organics have been removed; does not need rebar; and the wire is there to both control curing cracks, and to hold the cracked pieces in place. A rebar grid will maybe help to distribute the point loads, but we are still looking at more than 12 sq inches of load evenly spread, and you should be fine. A building slab, bridge, inlet top, ect, are different animals, and that's why they nearly always call for rebar.

I have a dumb question: if the rebar is supposed to be in the lower 1/3 of the slab why are the chairs or dobies 2.5-3" high when most slabs for non industrial use are in the 4-5" range?

They sell chairs in different heights. For a 3 1/2 inch slab, I buy 1 1/2 inch chairs.

When I did my garage addition in , I used these little plastic chairs. Pretty easy addition, cheap, and I know the rebar isn't laying on the bottom or 1/4" from the top. They are currently .29 each. Not sure what they were 4 years ago.

EDIT: They were .17 each in .

Nice thing about Menards... they keep your receipts on-line for quite a while.

Plastic Rebar Chair

The 2-1/4in plastic rebar chair is a durable plastic rebar support system that is ideal for concrete applications. The chair is designed so that the rebar snaps securely into place allowing the rebar to sit above the ground while concrete is poured. The chairs are easy to work with and...
(click to enlarge).

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Regarding how deep... these chairs are 2.25" tall. I used 2X6 for depth, so real number is 5.5" thick slab. So they are 0.4" shy of being in the bottom 1/3 of the slab.