Clear Cover to Flexural Reinforcement in Structural Slab on Grade 1

[Matetu]

I am peer reviewing the design of a structural slab on grade. It has two conditions: one is an office, the other is a covered parking structure....both are enclosed and the parking structure is ambiently heated to 55+ deg year round.

The design engineer has utilized 1" of bottom cover and 3/4" of top cover thru out. I assert that since the slab is cast on grade (with clean engineered fill material as a form) that the bottom cover could be as much as 3-inches since it is "cast against and permanently exposed to earth". In reviewing the Guide to the ACI, I am willing to concede that the 3-inch cover was increased by an inch to account for an uncontrolled bottom of excavation and that a sub-base prep is likely to be much more consistent. Therefore, I would assert that 1 1/2-inch would be the absolute minimum for the bottom rebar with or without a vapor barrier as it is still "exposed to earth".

In the office, I would agree with the 3/4-inch top cover, but in the parking, as this project is in the NorthEast and subject to auto runoff and deicing salts, this constitutes exposure to "weather" and therefore would require 1.5-inches as well.

Can you comment on this?

Matthew Etu, P.E.

 

[cvg]

1.5 inches is the minimum I require for any exterior concrete work. for exposure to road salts I would increase it. And is that measured from the centerline or to the outside edge of the rebar...

 

[Matetu]

All cover dimensions are referenced to the outermost surface of the steel per ACI 318.

 

[ATSE]

Matetu,
I agree that the parking garage slab is exposed to weather. Even worse, cars track in salts. 1.5" would be an absolute minimum. Go to concrete repair companies websites to see photos of what salt does to reinforcing steel with small cover.
2" absolute minimum for bottom cover for structural slab (I would use 3").
This design engineer is a crack-pot.

 

[dik]

I generally use 2" with 1-1/2" deep sawcuts and fill the sawcut joint with a flowable polyurethane.OK with a 6"-8" slab. The secret is timing the sawcutting...

 

[hokie66]

dik,
The OP said that it is a "structural slab on grade". I think he must mean that it is suspended, thus no sawcutting.

 

[Matetu]

it is a structural slab on grade, but not "suspended" in the truest sense. It has no form other than a sculpted subbase stone. Should the high organic levels in the earth actually consolodate over time, a gap could form between the underside of slab and the earth, but this would be fractions of an inch, maybe. The slab is supported by a grid of helical piers acting as "columns" for a two way flat slab with drop panels. The rebar I am referrring to the positive flexural reinforcing on the bottom of the slab.

 

[hokie66]

I thought so. In my terminology, that is a suspended slab. You are just casting it on the ground for convenience. Agree with those who would not use less than 2" cover.

 

[dik]

hokie66... I'd still use 2" cover and sawcut it... Dik

 

[hokie66]

dik,

I don't understand the philosophy of sawcuts in a flat slab. That just reduces your effective depth, unless you are sawcutting along the support lines. Along the support lines is where you have the most top steel, so the sawcuts would not likely make the cracks occur there. The 2" is to the bottom bars, while the top bars would only have maybe 1". Any more would significantly reduce the slab strength.

 

[dik]

The flexural compressive strength is reduced by the sawcut with marginal transfer through the joint filler also marginally augmented by the reinforcing steel. The flexural tensile strength is only marginally affected. The shear transfer is not normally a problem and the load is transferred by aggregate interlock. Any tendency for it to move is resisted by increased soil bearing. The 'forced' crack pattern if done properly allows most cracks to be reduced or intercepted with less problems for corrosion. As a last comment, unless there is a 'real' bunch of reinforcing steel, the flexural steel likely has little merit in the first place.

Why not break it up to establish a cracking pattern in the first place?

 

[Matetu]

It has been our experience that sawcutting a formed structural slab is wholly unnecessary as the steel ratio is significantly higher than any minimum shrinkage reinforcing requirement, the square footage of slab cast in one day is usually less due to the higher overall volume of concrete, and, in general, there is less "drag" on the forms than there would be on a subgrade to create opportunities for shrinkage cracks to form. Moreover, CRSI recommends using a minimum of #4@12 top wherever flexural reinforcing is at a minimum for constructability as much as shrinkage. This provides more than enough shrinkage compensation for slabs up to 8-inches in thickness (ignoring the bottom bars). The only crack we have ever had in a structural slab was off a rentrant corner and it happened, in large part, because the contractor jumped the gun and released the shores too early.

That said, as a follow up to our current condition is that, despite our best efforts, the design engineer is sticking firm and has only suggested increasing the 3/4-inch cover to 1-inch on the top of the parking slab.

For all those reading this, ACI and Bruce Suprenant (Mr. Concrete) agree that 1 1/2-inches of cover on the bottom of a slab "exposed to earth" is the ABSOLUTE minimum WITH A VAPOR BARRIER! 2-inches is recommended without. There is no specific criterion for cover on rebar in an enclosed garage, but ACI suggests 2-inches for "exposure to de-icing salts". ACI qualifies "exposed to weather" in the commentary as "direct exposure to moisture changes and not just temperature changes...unless subject to alternate wetting and drying, including that due to condensation conditions or direct leakage from exposed top surfaces, run off, or similar effects."

Bruce acknowledged that both sides of the arguement could be valid, but the conservative approach would be to consider the salt run-off from the autos as "direct leakage" and go with 1.5-inches.

ACI acknowledges that this clause suggests that a vapor barrier is equivalent to 1/2-inch concrete cover and references this as an example of "alternate protection" as indicated in ACI 318 R7.7. Other forms of alternate protection include surface sealers and coatings, but the owner should be made aware of the risk vs reward aspect of voluntarily disregarding the code requirements and that corrosion or other forms of damage may occur as a result. It is also up to the building code official to accept (good luck there).

 

[hokie66]

That's the thing about building a slab on the ground, suspended or ground supported. There are more ideas about how to do it than Carter has little pills, and everybody thinks his way is best.