Slab of Grade Reinforcement Coverage

[ars001]

If I am using #3 bars at 18" o.c. in my slab of grade am I required to provide 3" of cover as ACI 7.7 specifies? Or are concrete slab on grades a different ball game than what 7.7 covers? I wouldn't usually worry about it but I'm using it as a diaphragm so technically I suppose its a structural element. Thanks

 

[SteveGregory]

3" cover is required for concrete cast against soil. I would assume that your slab on grade is cast on top of a level course of compacted crushed stone and not directly on the soil.

 

[ars001]

SteveGregory
ACI reads "concrete cast against and permanently exposed to the earth". Isn't compacted granular fill considered earth?

 

[frv]

Read the commentary.

"The condition "concrete surfaces exposed to earth or weather" refers to direct exposure to moisture changes and not just to temperature changes. Slabs or thin shell soffits are not usually considered directly exposed unless subject to alternate wetting and drying..."

Pretty clear to me.

 

[ars001]

Sounds good. I guess I need to open my eyes. Thanks

 

[DonPhillips]

If the SOG is for habitable space, a vapor retarder is needed and I usually interpret this as the concrete not being cast directly against the earth.

Don Phillips

http://worthingtonengineering.com

 

[Teguci]

ACI 318 does not govern for Slab on Grades (1.1.6)

ACI does have a publication for SOGs but I do not have access to it. I do know that it was standard practice to spec a 5" slab with #4 at 12 mid depth for a very large consulting firm. This provides for only a 2" cover on bottom.

 

[ThePenguin]

I believe the document Teguci is referring to is ACI 360 "Design of Slabs on Ground." The only guidance given for placement of reinforcing within the slab is as follows:

For Crack-Width Control - At or above mid-depth, typically 1.5" to 2" cover below the top surface of the slab.

For Moment Capacity - At the centroid of the tensile area of the uncracked concrete section.

If the slab is taking vertical or lateral loads from the structure then it is covered by ACI-318, and as you had mentioned you are considering it to be a structural element. I would imagine you would take the 3" value for cast against earth, as a slab covered by ACI-318 would be acting almost like a footing in some regards. However I am not entirely sure what the typical value would be as the slabs on grade I have encountered, as Taguci mentioned, are not covered by ACI-318. My guess is that since the steel reinforcement is not as critical for a SOG (in many cases its sole purpose is crack control) that the cover requirements are not as stringent.

frv-

I hate to nit pick, but the commentary to 7.7 reads "Slab or thin shell soffits..." (slab not slabs) which I take to apply only to soffits, as would appear consistent with the remainder of the sentence.

Teguci-

2" does seem to be a common minimum cover for slabs on grade. As you mentioned slabs on grade that do not take loads from the structure are exempt from ACI-318. I have not read anything that specifies a minimum cover for these slabs.

 

[msquared48]

For a 4" thick slab-on-grade, it is normal to put the reinforcing at the center of the slab, WWF or rebar.

Mike McCann
MMC Engineering

 

[ThePenguin]

In addition to my above post, a "fact sheet" on the WRI website says the following:

"In the case of the lower layer, when the concrete slab is placed on a well-constructed base course (normally graded,
compacted and porous), many who design floors consider 1.5” of clear cover below the steel to be adequate. Additional cover should not be necessary unless the governing building code requires a bottom cover of up to 3 inches."

I have provided a link to this document:

http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF 702-R-08.pdf