shallow footing concrete strength

[jgajary]

We are having a disagreement in my office about the required strength of shallow footings at the perimeter of new buildings. It appears from chapter 4 of ACI that the concrete strength should be a minimum of 4,500psi due to freeze/thaw and moisture conditions (type F2). Is this correct? Or is there an exception taken for footings, as they are not exposed and (strip footings expecially) often have excess capacity, so can tolerate some deterioration? Contractors seem to balk at 4,500psi concrete for a 2-3 story building.

 

[concretemasonry]

I do not have a copy of the refernce handy, but there seems to be an error in interpretation.

In the real world, the compressive strength is no guarantee of freeze/thaw resistance! You can have 7000 psi concrete fail under the same conditions that 3500 psi would perform well. All too often, the compressive strength, because it is a classic test and is easy to measure, is used as an easy solution to a perceived problem/situation.

A prudent (and obvious) measure would be to place the footings below the required frost depth. Concrete in the soil does not go through the freeze/thaw cycles that exposed concrete does, consequently the conditons are radically different.

 

[JAE]

jgajary,

I have 318-05, chapter 4 here and it suggests in the commentary to Section 4.2.1 that footings don't really qualify for the 4500 psi because they are not in the continual presence of moisture like bridge decks and parking garages are.

I would say your 4500 psi requirement doesn't apply to footings. I never use that strength for footings. Usually use 3000 to 4000.