building in NW Ga, a 24 x 36 slab will be a passive solar design with Sips

I was going to use 4x8 by 2" foam around perimeter floor the footing will be IFC

As far as the slab itself I'm getting different opinions on rebar, mesh wire or just adding the fiber mesh in the concrete mesh

What's the code or best option, can't have an expose solar mass floor show cracks.

Concrete will crack. Control joints are a must. We use 5000 psi fiber reinforced concrete with 6-6 x 10-10 wire over our snow melting PEX tubing. The wire is overkill and the fiber will make polishing impossible. My new slab in the shop has zip strips, which are nearly invisible at the moment...

The slab will crack. You can make the cracks occur via control joints in places where it will not be obvious, like in doorways or under interior partitions. Cheers, Eric

Do you have expansive clay soil there? If you do then post-tensioning is recommended and rebar wouldn't be required. But whatever method you go with, the reality is that concrete will crack. Even when you have control joints you're likely to get some cracks where you wish you didn't.

The biggest factor is an exposed floor slab after install/reinforcement is CURING. So many contractors just walk away after they float the slab and don't do proper curing. The reason WHY is simple. Curing requires a lot of tedious labor that they don't want or like doing. Most contractors will just hose down the slab with some water the next day and call it a day. That is NOT proper curing.

I believe wet curing with a concrete blanket is easiest and inexpensive. Once it is rolled onto the concrete one doesn't have to do anything until it is ready to be pulled back 7 days later. Proper curing results in a stronger slab with less cracks.

For sure the basics need to be done:

• MINIMUM 4" of compacted AB
• Rigid EPS 2" + (based on climate)
• Vapor barrier
• Rebar (based on engineering)
• Good psi mix
  
• Control joints
• Wet curing

I have pretty much the same size slab (24x32) with this stackup:
- compact engineered rock fill
- 6" gravel
- 4" rigid foam under the entire slab and 2" vertical foam separating the heated slab from the foundation wall.
- vapour barrier
- wire mesh
- PEX tubes for infloor heat

The slab is 3" thick. However, I do not have any control joints in the slab. We will have porcelain tile installed on the slab and we opted not to put a decoupling membrane (Ditra mat) under the tile due to cost (about $3500 extra).
We figured that if any tile grout cracks, we can just replace that tile/grout as needed. Plus if my slab cracks, I would want to kniw about it and address it rather than hiding it under the Ditra mat.
Now, this talk of every slab cracking has me worried.
Should we request control joints to be cut into the slab or is our slab relatively safe based on how it's installed?

eljay, simple cracks are not much concern as they are usually just at the surface. This is mostly a cosmetic issue, but lay people do tend to freak out when they see concrete cracks. If there is no difference in height of the slab from one side of the crack to the other, then it's typically nothing to worry about. If there was a difference, then this could indicate that a void had opened up in the supporting soil underneath.

I'm not clear whether your slab is already finished or still in the planning stage. But if it hasn't been poured yet, and it makes you feel better, then why not add joints. They can be tooled in while finishing, or they can even be sawed in within the first 24 hours. Now if the slab is already there and has been cured for more than a couple of weeks, whatever you do now is not going to have any impact.

Posted By Jelly on 03 Mar 2015 08:38 AM
eljay, simple cracks are not much concern as they are usually just at the surface. This is mostly a cosmetic issue, but lay people do tend to freak out when they see concrete cracks. If there is no difference in height of the slab from one side of the crack to the other, then it's typically nothing to worry about. If there was a difference, then this could indicate that a void had opened up in the supporting soil underneath.

I'm not clear whether your slab is already finished or still in the planning stage. But if it hasn't been poured yet, and it makes you feel better, then why not add joints. They can be tooled in while finishing, or they can even be sawed in within the first 24 hours. Now if the slab is already there and has been cured for more than a couple of weeks, whatever you do now is not going to have any impact.

Thank you. The slab has been poured over 2 weeks ago, so I guess that's done.
Yes, I do hope a few small surface cracks will not cause any major damage to the tiles. I think that, although small, there should still be some elasticity in the tile mortar not cause the finished floor to lift etc.

The tile can be isolated from the slab by a tile membrane.