Concrete Dobies 1

[eatapeach]

Not sure where I have read this but I'm sure I did somewhere. Are concrete supports required to meet the strength of concrete being placed? Dobies are generally 4000psi and I am inspecting concrete to be placed at 6,000psi. I may remember this from a USACE specification or is this in an ACI code?

 

[phamENG]

I'm not sure if it's specifically written in ACI, but it's something I've always specified. If the block is in a tension region, it won't matter - that's what the reinforcement is for. But if it's in a compression zone, then it needs to be at least as strong as the concrete around it or it could crush prematurely.

 

[r13]

Good point above. But why not use metal or plastic, or combination, rebar chairs?

 

[eatapeach]

I did find it in ACI 301-2010 however it was removed in the 2016 edition and ACI now references CRSI and it does not state the requirement. As for why the contractor doesn't use chairs, the dobies come with a wire already attached therefore securing the reinforcement is much faster.
Thanks,

 

[phamENG]

I haven't seen those in practice, but have heard of them. I usually encounter them when WWR is being used. Easier to lay the blocks down and rest the mesh on it than try to support the sheets of mesh on individual chairs. Didn't think to check 301.