Concrete Chairs

   / Concrete Chairs #111  
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.
 
   / Concrete Chairs #112  
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 3060 lbs in tension.
 
   / Concrete Chairs #113  
I may have went in a whole rant for nothing; if we are talking a driveway slab, or something similar, 6" of 3000 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.
 
   / Concrete Chairs #114  
Lol meanwhile I started following this thread expecting to see elaborate and ingenious ideas for building forms for concrete furniture. Anyway I'll continue using my methods for concrete reinforcement and placement cuz they work.
 
   / Concrete Chairs
  • Thread Starter
#115  
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.

 
   / Concrete Chairs #116  
When I did my garage addition in 2020, 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 2020.

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


(click to enlarge).

View attachment 850529
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. 🙃
 
   / Concrete Chairs #117  
Is fiberglass rebar a good thing or something to avoid? I've never used it, but Menards sells it for less than steel.
 
   / Concrete Chairs #118  
Is fiberglass rebar a good thing or something to avoid? I've never used it, but Menards sells it for less than steel.
I think that it has the big big advantage that it doesn't corrode. Just make sure you get the right size/strength.

I also think that it has the disadvantage that cutting it requires saw blades that are compatible, and good dust control.

All the best,

Peter
 
   / Concrete Chairs #120  
Is fiberglass rebar a good thing or something to avoid? I've never used it, but Menards sells it for less than steel.
It’s all we used for the MRI center…

It was a simple grid pattern… no bending required.
 

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