Concrete Strength Requirements - ACI 318-11 Table 4.2.1 1

[CBSE]

Has anyone else caught this note in the ACI code?

"Minimum f`c for exposure class F1 is 4,500 psi."

class F1 = Concrete exposed to freezing and thawing cycles and occasional exposure to moisture.

This is being specified for a duplex I'm doing the project management on for a contractor.

This seams pretty extreme to me.

 

[hokie66]

You should just get used to it. That is about 32 MPa, which for many of us is a minimum for structural concrete, regardless of whether freeze-thaw is involved or not.

 

[Archie264]

CBSE,

That requirement is not new with the 318-11 edition and it wasn't new with the 318-08 edition either. I'm not sure when it started but it wasn't new when I first leaned of it, and, like you I was surprised.

 

[CBSE]

So I read the commentary and the examples of Class F1 Exposure are

"exterior walls, beams, girders, and slabs not in direct contact with soil."

That being said, a typical foundation and stemwall could still use a 3,000 psi concrete mix correct? ACI is only classifying this for elements not in direct contact with soil?

 

[CBSE]

Also, wouldn't this requirement show up in the residential codes and require all residential structures to use the 4,500 psi mix? It seams that it is more geared toward concrete buildings.

 

[Ingenuity]

You should just get used to it. That is about 32 MPa, which for many of us is a minimum for structural concrete, regardless of whether freeze-thaw is involved or not.

Agree.

It was 1988 when AS3600 (Australian Concrete Structures Code) was first released and it required MIN concrete strength of 32 MPa for B1 exposures - and 99% of Australia is not freeze/thaw, but does have significant coastal chlorides.

If I recall correctly, the magnitude of the MIN concrete cover was simultaneously increased too.

 

[CBSE]

Yeah, but still, the code only refers to concrete that is not in direct contact with soil...unless I'm misunderstanding it.

99% of light framed would structures are probably not designed correctly if this is true.

 

[Ron]

You're thinking of strength as the only criterion for concrete acceptance. In order to achieve appropriate durability for long term performance, the concrete must be dense and have a relatively low water-cement ratio. This, in and of itself, causes the strength to be higher. In short, the higher strength is a by-product of the need for durability.

 

[CBSE]

Ron:

I do understand the durability requirements. What I'm getting at though, is that the IBC, many, many text books, the IRC, etc. state that 2,500 - 3,000 psi concrete is sufficient for light framed wood structure foundations.

The commentary in the ACI further suggests that the 4,500 psi minimum only applies to concrete that isn't in direct contact with the earth...so I would assume tilt up walls, concrete moment frames, parking garages, etc.

If 4,500 psi is the minimum, then I think many building departments are missing this requirement in the specifications.

 

[manstrom]

This has been there for a while, but not applied in my experience. It is conservative and costly.

I do apply the 4500 psi (over 3500 or 3000) requirement to areas like exterior garage slabs that will get ice and salt. Footings and stem walls, I still keep as 3000.

Consider what F1 really means as exposed to "freeze and thaw" exposure. I read it as an area that gets wet and freezes - not an underground footing, not a waterproofed wall, mostly just exterior slabs.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Archie264]

>>>...mostly just exterior slabs. <<<

...to include sidewalks, which, not altogether too many years ago used to simply be specified as 2,500 psi, hence many people's surprise.