Residential Slab Code Minimum Reinforcing

[XR250]

I have a contractor building a basement with a monolithic slab. He wants to pour a continuous 11 1/4 inch thick slab as apparently it is easier for him to do that then to add more gravel. (i.e no transition from a thickened area to a 4 inch slab) I have not found in the IRC any reference to a minimum reinforcing in slabs. He has already installed #4 a 24 inches on center 3 inches from the bottom. The inspector has never seen anything like this (neither have I) and wants someone to sign off on it.
I told him unless he installs more rebar he's likely to get shrinkage cracking, however that would be the case if he had installed no rebar or mesh in a 4" slab (which seems to be allowed).
If there is no code minimum for reinforcing in a residential slab, should I sign off on this thing? I would put in the letter that shrinkage cracking is likely.

 

[phamENG]

Has he already added the required 4" base course of sand, gravel, or blast furnace slag (blast furnace - not EAF)? (R506.2.2)

From the looks of the residential code, rebar is not required at all. #4 at 24" isn't going to do much to minimize cracking or reduce crack width, but if it's going to be there I'd probably tell him to elevate it to be 2" below the top of the slab on taller chairs.

Oh, and for the last part - be careful. There's a story floating around here somewhere (I think it was Ron)...he put a similar clause in a letter about it being safe but a probable serviceability issue...the house was sold a few years later, the new owner found out about the letter, sued the city for accepting it and passing the inspection, and the city turned around and sued him. He ultimately "won"...but only after $10k in deductibles and lost time.

 

[XR250]

Thanks Pham for the info and heads up about the letter. He does have gravel below the slab. I don't get it but this is what he is doing and wants to pour tomorrow.

 

[Enable]

Unfortunately I'm no help on the code front. But I wouldn't see an issue with it. As you noted cracking will be a known issue, but more importantly so will curling especially if A) he used a high w/c mix (typical of residential basements) and/or B) a vapor retarder is placed under the slab.

I'd be very pushy about proper wet curing and a lower w/c mix with that kind of slab thickness in a residential application (more than pushy if a vapor retarder is present). Bonus is a low w/c mix with proper curing alleviates the shrinkage issue to a significant degree.

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[KootK]

I think it was Ron

That would be BAretired's terrifying anecdote.

I'd still be comfortable signing off on this with some kind of warning issued about serviceability. I don't feel that an engineer should even have to sign off on a light duty slab on grade but that's another discussion altogether I guess. You have to pick your battles as an SE and this wouldn't be mine.

 

[XR250]

Thanks everyone.

@Enable, Yes, there is a 10 mil vapor retarder in place. The slab is going to be carpeted so aesthetics are not important. Could he just use 4 mil dark poly as a "curing blanket"? (there is no framing above this yet and it is getting cold here)

 

[Enable]

Oh, yeah, winter...I'm pouring a slab today and it's +12 Celsius here so I forgot about that for a moment!

My protocol would really depend on the expected range for the next couple days (with the overnight lows being most important). Without knowing much more my general approach would be something like the following (temps in Celsius because you know SI units are great)

10+: Normal procedures = wetted burlap + 2mil poly
5-10: Normal procedures + insulated blankets
0-5: insulated blankets only but NO water/burlap (note: make sure accelerator is in the mix and day temps are higher than 5 degrees)
sub 0: tarped & heated with normal procedures OR electric heating blankets set to 7 degrees and no water/burlap

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[XR250]

Probably will be 10+.

How about wetted wheat straw and poly? That would be in his toolbox.

 

[Enable]

LOL farmers eh...

The key is to use a material that'll absorb the water and can be pressed in firm contact with the poly so it doesnt evaporate. If you dont have a flat surface the poly wont laminate itself and you'll get too much evaporation (BTW: I changed my post since I made a mistake, normal procedures are 2mil poly so it conforms to the burlap. 6mil is wayyy too thick for that).

You may have a problem with straight up straw since they'll overlap and hence create air pockets when the poly goes overtop. Though...cant say I've ever tried so might work just fine. But cheap filter fabric / landscaping fabric would do the job just the same and should be available from the local nursery or home depot (it'll leave a blue tinge but if the surface is being covered this is no problem).

CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[XR250]

Got it - thanks. Maybe his sub has curing blankets

 

[JoshPlumSE]

I don't know that I've ever done a non-structural slab on grade. Certainly not one that's 11 inches thick!

As a minimum for slabs, I want to use a 0.0018 reinforcement ratio. For thin slabs, I use just single layer. But, for thicker slabs like this I would probably split it into two layers top and bottom. But, I'd bias the reinforcement to smaller bars with tighter spacing. And, depending on the span, I would probably want to put in some control joints to force the cracking to occur in desirable / controlled locations.

All that being said, I don't like the idea of getting rid of the standard surface preparation that we do below the slab. Isn't a lot of that to allow for better drainage and prevent moisture infiltration from the ground? I would think that would be more important for basement slabs than other places.

 

[dik]

In these environs, it's not common to sawcut slabs, in particular at the correct time... essential for a thin slab and possible re-entrant corners in a basement.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Phil1934]

R405.2.1 Base

 

[phamENG]

Phil...he already indicated that the 4" layer is there.