Rebar supports in a slab

[3Ts]

I have a slab pour coming up and I'm wanting rebar chairs to hold the rebar off the ground during the pour. The slab will have 26,000# on it so the contractor is going to pour 5.5" thick with 8" at the edges. Question is:

1) How far apart are the chairs supposed to be? I've seen 2' on highway projects but that is usually some pretty heavy duty rebar. I've seen 2 chairs used in 400 sq ft, but I don't think that spacing provided any benefit.
2) I've seen pieces of brick used as a chair, but seems to me all that does is make the concrete thin above the brick and more likely to crack. The plastic chairs seem to come in 2 varieties - one will push the rebar back up when stepped on, and one that won't and may actually break. I'm assuming the push up type is better.
3) The slab is quite large and the contractor is talking about driving the trucks into the slab area and setting the rebar as the truck pulls out. Is this really practical?

 

[Streetcar]

What are dimensions of slab
There are metal chairs

 

[rademamj1]

Definitly never use bricks to elevate your rebar. Typically for standard 1/2" rebar, a placement interval of about 16" is just fine for the rebar grid. Normally 6' to 8' apart for the chairs is good. Any longer and the rebar starts to sag between chairs. Make sure you have either the 2.5" chairs or 3" chairs. Also recommend after placing your rebar grid, and depending on your budget, placing some welded wire sheets directly ontop of your rebar.

Double check to make sure your base below the concrete is well compacted. Also, some like a vapor barrier at base of concrete, others ignore a vapor barrier.

 

[ultrarunner]

Any downside to adding welded wire other than expense?

I've seen one or the other but never both...

We built a new imaging center 5 years ago and all the rebar was fiberglass... could not have any ferrous material in proximity to MRI machine... it was interesting just how involved and different the build was.

 

[Oldpath05]

2013 mine was about 4' apart holding 1/2"rebar for a 26'x36' 6" thick slab, had to use shute to get concrete back that far.

 

[rademamj1]

Any downside to adding welded wire other than expense?

I've seen one or the other but never both...

We built a new imaging center 5 years ago and all the rebar was fiberglass... could not have any ferrous material in proximity to MRI machine... it was interesting just how involved and different the build was.

The only downside really is budget. The real strength comes from the rebar grid, but no doubt adding the welded wire mesh certainly adds to the overall strength of the slab. Typically you want 2" of concrete cover over any rebar, and with 2.5" chairs holding the rebar, and the OP planning for 5.5" slab thickness, he has plenty of room for adding the welded wire mesh. As long as he has the budget.

Here are 2" chairs from Amazon, you need the 2.5" chairs.

https://www.amazon.com/dp/B071FR963N/ref=cm_sw_r_cp_apa_fabc_Z59D990NKB4GDAPQCH94

 

[ultrarunner]

Learned something new... always used little cement blocks with embedded tie wire.

 

[robstaples]

Consider a pump truck. My 6” 12x 14 patio with footings and wire and chairs was a breeze using a pump truck and saved landscaping in between truck and the pour. Enabled minimum spread labor.

 

[user.69169]

Consider hiring a genuine Concrete Placer who knows how to do de job as well as how to use a vibrator on de pour and make sure rebar is where it belongs in pour.

 

[3Ts]

The yellow is the slab to be poured. It slopes down from top to bottom and left to right. The arrows indicate where the trucks have access. The one to the right will require a short drive (an extra 1/2 mile) on a county then forest road to get to the site. I want this to be done right so I don't have to do it again. His quote is right at our budget, but if I need to spend a little more, I'll get it from my wife's projects since she wants this concrete more than I do. I'll have a chat with the concrete contractor again. He's poured concrete for me before and seems to be one of the better guys in this area.