Concrete Chairs

[jjeff]

Those concrete rebar chairs have a striking resemblance to a flintstone tv lol

 

[paulsharvey]

My last large pour, (40 yards, 15+ years ago), I used chairs made from concrete with a built in wire tie. Very handy. I searched and cannot find them now. It seems like plastic or wire are the only choices. I used #4 rebar, 24" on center in a grid format. I have no cracks, I did not use fiber in the small aggregate 4000 psi concrete.

Search Dobie brick. Looks like Hime Depot lists them, but may not stock them; place like Whitecap should stock them.

 

[zzvyb6]

I don't view rebar, chairs, or other support elements as giving better bending strain relief. And it's strain not stress that's the problem. The rerod is just there to keep the broken pies in relative position (IMHO). In order to minimize slab stress, the rebar needs to be pre-stretched as is done in bridge beam construction. Otherwise, the non-concrete pieces just provide discontinuities in the slab, which to me means pre-assigned fracture locations. Examine a bending moment diagram of any type of slab, beam, or truss. That's where the 1/3 rule comes from.

I went with a thicker slab, higher bag mix in my driveway with suitable crack 'assignment' slits, and use post-cure water proofing solutions to keep the water out. I prefer to use a rotary broom for snow removal, because it completely gets the water, snow, and ice off the surface. Plows do an ok job but leave a lot of residue for freeze thaw action. My broom even cleans out the slits.

 

[jjeff]

What I don't like is the lack of a loop on those wire ends for consumer box store dobie blocks, you can't use a twister. Imo it would get old in a hurry on a big pour bending over tying all those blocks with a lineman's or similar pliers.

 

[lilranch2001]

My broom even cleans out the slits.

This is a line I can honestly say I’ve never seen on TBN before

 

[paulsharvey]

Zzy, that's not correct about rebar, but is for WWF. Rebar, in the lower 1/3rd of the concrete is what takes the tension load. I know, your thinking what tension? A shear load, (close proximity of compression in opposite directions) creates tension. IE. down force on your exterior wall, next to static load, creates shear and tension. 3000 PSI concrete only has about 300 PSI of tension capacity. Steel is the opposite, it doesn't resist bending/reforming in compression Nearly as well as in tension, But the two together acr as one material; creating a material with the strength needed. Your rebar has 60,000 PSI of tension strength, and the concrete (with Vastly more Sq inches) resists compression and bending; creating a single structural element that meets the design loads.

 

[paulsharvey]

For the expense question, look at a monolithic slab, by far the most common foundation. Your thick edge is going to be 16-20" thick, with 2-#5 rebar, but the rest of the floor is only 4" thick, maybe with WWF but maybe not. Imagine the cost of going to something like a 36" thick edge, vs 2-#5 parameter bars.

 

[ericm979]

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?

 

[paulsharvey]

Don't know if I really explained well how it's a single structural element. The concrete can't fail in tension without breaking (causing to fail, that includes stretching) the rebar. The rebar can't bend without the concrete first failing in compression. Because the rebar has deformations, it can't move inside the concrete.

 

[paulsharvey]

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?

That's because you don't typically use rebar in a 4" slab for structural use; the rebar is used in 10"+ parts, ie, the thickened edges.