Structural Slab Frost Protection-Questions

build4u · Nov 18, 2022

I have structural slab that was poured about a week ago in mild weather, so it experienced a few days of warm (above freezing) temperatures. It is a 20x40 foot slab that has 2" insulation underneath as well as the perimeter grade beam also has 2" around it. The slab is on 3/4 clear stone compacted that is minimal 6" on the grade beam and probable 2 feet plus on the center slab area. The grade beam is 2ftx2ft with lots of steel and steel 6" mesh tying it to the slab portion. The slab portion is minimum 6" and all in one pour. Framing is due in the next week or so and we're just up and around the freezing mark day2+C and night -2C as an example. I under the impression that I'm to get the horizontal 2" insulation installed as soon as possible and then backfill. Some are saying you should backfill with 3/4 clear, then cloth followed by soil? Others have said get some heat into the house once framed and wait until spring to finish the outside grade? Just looking to see what makes sense for now. Also, the contractor has used 2" styrofoam board both under the slab and on the outside walls. I had wanted silverboard on the outside and intend on using silverboard on the the horizontal board that is to be buried.
Any advise would be appreciated.
Thanks!

dik · Nov 30, 2022

Can you post a section that shows the slab, grade beam and insulation?

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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

KevinValenta · Feb 27, 2023

Getting heat in the house once framed won't help the slab if you have temperatures cold enough for long enough in the meantime.

SlideRuleEra · Feb 27, 2023

build4u said:
...structural slab that was poured about a week ago in mild weather, so it experienced a few days of warm (above freezing) temperatures.

The slab is probably ok, and nothing special needs to be done...

"All concrete must be protected from freezing until it has reached a minimum strength of 500 pounds per square inch (psi), which typically happens within the first 24 hours... Once concrete has attained a compressive strength of around 500 psi, it is generally considered to have sufficient strength to resist significant expansion and damage if frozen."

Quote from "What Happens When Concrete Freezes?" by the Portland Cement Association.

dik · Feb 28, 2023

Concrete can be frozen, in it's plastic form, with little effect (not a good idea, though). It's when the freezing occurs during the gel formation that serious problems occur. Not part of your construction, but the presence of insulation below the slab, does not help the freezing process. It allows the concrete to 'get colder'. As SRE notes, there is likely no problem. The insuation should be some form of extruded polystyrene or something of that ilk.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

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