Clear Cover to Flexural Reinforcement in Structural Slab on Grade
Clear Cover to Flexural Reinforcement in Structural Slab on Grade
(OP)
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.
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.





RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
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.
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
The OP said that it is a "structural slab on grade". I think he must mean that it is suspended, thus no sawcutting.
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
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.
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
Why not break it up to establish a cracking pattern in the first place?
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade
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).
RE: Clear Cover to Flexural Reinforcement in Structural Slab on Grade