Concrete Chairs

[groundcover]

Pretty sure it was one of Eddie's posts a while ago that made me over rule my builder and concrete guy and put my rebar up on small blocks of broken brick. They played the whole " we will pick it as we go " thing, yea, no ! Slab on the garage addition has been perfect, nary a crack !

 

[jjeff]

Used chairs once before had to put them in place during pours between laser screed and the screed board it was pulling they were made of plastic specifically for there intended use, I believe they were used with #8 bar. Those are heavy. Normally used this hook in picture to pull rerod or mesh on "come along" when puddling, that hook is not for hanging it up but prob is used to lol. Usually tied all rerod to hold it in place during pours overlapped mesh and tied it also. Used fiber a few times almost 20 years ago steel trowel finished but was a struggle and those darn fibers would would usually stick out. Last flatwork I did was my own, poured a 5" thick (in case i ever got a bulldozer lol) 24x36' slab for my pole barn up north. Used wire mesh pulled it as I placed the concrete also spent the few extra dollars per yard more and paid for fiber. Very limited fibers exposed with a smooth steel trowel finish probably over jitterbugged it idk? Going on ten years no cracks anywhere in that slab, and I didn't cut in control joints.

 

[jmc]

Anybody ever bring up stress risers in slabs? Seems like those chairs will leave some significant ones in the tension side of the (brittle) slab. Maybe positioning rebar at the right height makes up for them...

 

[dodge man]

I’ve watched a lot of concrete poured and tested quite a bit of it. In the engineering world there is often a trend of not using rebar for flat work. Parking lots and highways are often that way and if done properly will last. Often on highways the pavement will not have continuous reinforcement but have joints that are reinforced. The concrete is also very thick.

For residential I agree, mesh almost always ends up at the bottom and rebar needs to be on chairs.

 

[wagne223]

One of the things I look for in pictures and videos of people pouring concrete, is if they use concrete chairs to hold the rebar off of the ground. If they are not using them, that's a huge red flag to me. The same thing if they are using wire instead of rebar, that's a red flag.

While watching some YouTube videos last night, it hit me that 100% of every commercial concrete job that I saw, they use Concrete Chairs.

The only people that do not use them are residential jobs, or home owners doing their own pours. To me, biggest lie told in concrete work is that they will pull it up while spreading it, and it will remain in the middle of the slab after being walked on.

What's your opinion on post tension slabs?
I did not even know it was a thing until a few years ago.
My initial thought was it seems like a good technique with the only downside being the equipment required for the tensioning.

 

[MossRoad]

One of the things I look for in pictures and videos of people pouring concrete, is if they use concrete chairs to hold the rebar off of the ground. If they are not using them, that's a huge red flag to me. The same thing if they are using wire instead of rebar, that's a red flag.

While watching some YouTube videos last night, it hit me that 100% of every commercial concrete job that I saw, they use Concrete Chairs.

The only people that do not use them are residential jobs, or home owners doing their own pours. To me, biggest lie told in concrete work is that they will pull it up while spreading it, and it will remain in the middle of the slab after being walked on.

I know this is a sore point for you, as you've mentioned it several times over the years.

So when we did our garage addition a few years ago...

I USED CHAIRS FOR THE REBAR!


Thanks!

 

[scootr]

What exactly do you mean?

It should have read 10g wire 6x6" oc (on center)

 

[newbury]

What are the pros thoughts on using "cattle panels" for walks etc?
/edit - suspended on chairs or small pieces of concrete for blocks

 

[ponytug]

Anybody ever bring up stress risers in slabs? Seems like those chairs will leave some significant ones in the tension side of the (brittle) slab. Maybe positioning rebar at the right height makes up for them...

Having rebar 1/3 up in the slab maximizes its ability to provide tension strength (bending strength) to the concrete to keep it from fracturing. Since it is cast into the concrete, stress riser effects are minimal, if and only if, there is enough concrete below it. That's why "lifting rebar" is so detrimental to the strength of the slab. From a practical perspective, nobody is going to get the rebar at the right height consistently by lifting it, resulting in a weaker slab. My motto is use enough chairs that walking around on the rebar doesn't bend the rebar, or shift it.

I’ve watched a lot of concrete poured and tested quite a bit of it. In the engineering world there is often a trend of not using rebar for flat work. Parking lots and highways are often that way and if done properly will last. Often on highways the pavement will not have continuous reinforcement but have joints that are reinforced. The concrete is also very thick.

For residential I agree, mesh almost always ends up at the bottom and rebar needs to be on chairs.

I agree. I would point out that the compaction and subsurface grading for roads and highways often are several feet thick, and commonly below the frost level to enable straight pours. However, as far as I am aware adding rebar to the concrete on that prepared subsurface will yield a stronger concrete road. I think for many places it is a cost / benefit call; they could spend 10x more on the road and it would last 20x longer, or they could do it cheaply and come back in two/five/ten years and redo it, depending on actual usage, and degradation.

I've watched many folks on the county and state highway commissions wrestle with that one over the years...

All the best,

Peter

 

[ultrarunner]

With our MRI center all the rebar is fiberglass rods and plastic chairs... Nothing ferris/magnetic allowed anywhere near MRI

We had an electrician and repeatedly caution him and he said got it...

The ladder he carried was flung out of his grip sticking to the MRI...

One other contractor also didn't comprehend... I heard shouting and his steel tool boots pulled his feet right it from under...

I believe the fiber regards are stronger with better longevity than ferris bars...