Question of the Day - #120 Where does 5000 psi concrete go?

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Question of the Day - #120 Where does 5000 psi concrete go?

Postby RDavidson » Tue Dec 30, 2014 5:04 pm

Amended Table R402.2 requires all concrete footings to have a compressive strength of 5000 psi. The code gods have intimated that they didn’t really mean all footings. The stone tablets didn’t specify which footings were supposed to meet the requirement and which didn’t and which requirement applied to those that didn’t. But we still need to try figure out exactly which footings the code gods did intend for this rule to apply to. “Somebody from the city” suggested that it only applies to the exterior house footings with a basement. Ok, nice guess. Why doesn’t it apply to interior strip footings? Maybe it does. The contractor is unlikely to order two different mixes anyways and getting the trucks out in the right order could be a logistical headache. And who is going to know what concrete went into which trench anyway.

So let’s say we believe that the code gods only intended this rule apply to footings with basements because the footing is really, really, really close to the basement floor and the water would only need to penetrate the six inch thick footing and the 8 inch or thicker foundation wall to get to the interior and then squirt into the house. Therefore it wouldn’t apply to footings that were at locations other than really, really, really close to the basement slab.

But of course it can never be that easy, can it?

When you read the oxymoron “Statement of Need and Reasonableness”, there are a few sentences that attempt to explain the changes to Table R402.2 that read: “Footnote “g” permits a concrete mixture that is 2500 psi with an approved admixture (chemicals that can be added to concrete to change its moisture permeability), which provides a water- and vapor resistance equivalent to that of 5000 psi concrete. Footnote “g” includes language stating a minimum 3000 psi concrete strength is required when Tables R404.1.1(5), R404.1.1(6), or R404.1.1(7) are applicable regarding cantilevered concrete and masonry foundation walls. The 3,000 psi concrete will require an admixture that provides water and vapor resistance at least equivalent to 5,000 psi concrete.”

What this might be saying is that when using certain tables for designing cantilevered foundation walls that you have to use 3000 psi concrete for the footings and that an admixture is required giving it an equivalency to 5000 psi concrete. As pointed out in an earlier question, the connection between the 3000 psi concrete and the required admixture is still among the missing but I’m sure it will show up either in the chapter on roofing or as an “intended requirement”. Anyway, if you look at the footnotes for cantilevered foundations you find that the footing size must be larger than for non-cantilevered foundations. Footnote “f” in each of the three applicable tables requires the footing to be 20 inches by 8 inches. So, the water needs to penetrate an even thicker and wider footing in this situation before it squirts into the house. And then footnote “g” also requires the “top of footing shall be 16 inches below the bottom of the concrete floor slab minimum”.

Trying to put the SONAR and the code text and the “we didn’t intend that” all together seems to imply that footings must meet the 5000 psi requirement even when required to be a minimum of 16 inches below the bottom of the concrete floor slab. We are left in the lurch if the distance between the two components is 20 inches or 40 inches or 60 inches so the assumption can only be that the footings must meet the 5000 psi requirement regardless where they are located. And remember we are also dealing with a thicker and wider footing here.

Now we go back to the first paragraph. There we assumed, based on “we didn’t intend that”, that the 5000 psi rule only applied to dwelling footings that occurred right below the basement floor. Now we have code requirements that clearly contradict that idea. And now we seem to be back to the original theory, based on the specific text in the code, that all footings, regardless of location, must have concrete with a compressive strength of 5000 psi. How could someone come to the conclusion that “we didn’t intend that”?
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