ACI 318-08 exposure class and compressive strength for foundations 1

[cliff234]

Table 4.2.1 in ACI 318-08 lists four "F" category exposure classes. It appears that if the top of an exterior foundation is above the frost line, then the concrete in that foundation must be at least 4500 psi (per table 4.3.1) and must be air entrained (per table 4.4.1). We have always air entrained our foundations, however we have previously always used 3000 psi concrete for below grade foundations. It appears that ACI 318-08 is now mandating a minimun compressive strength of 4500 psi for all foundations when the tops of the foundations are above the frost line.

I would like to get the opinions of other engineers. Is everyone now specifying 4500 psi concrete for their foundations? This requirement seems to be solving a problem that I never knew existed.

Thanks!

 

[JedClampett]

I'm in the sun belt, so this isn't an issue here. But if the code requires it, I think the verdict is in. It may seem excessive, but why open yourself up to plan reviewers and lawyers by fighting it?

 

[concretemasonry]

The strength of the concrete is usually not major cost factor and strengths below 4000 or 4500 may be justified in rare cases.

Recently, the common strengths even for "minimal" concrete have been increased by suppliers and contractors. The days of 3000 psi are gone.

In some areas, a supplier will refuse to sell or deliver concrete below 4000 psi and is air entrained (driveways, patios and exposed foundations). The call is made by the driver of the truck based on his opinion of the use and exposure. That is an example of the minimal effect the cost has on concrete strength needed. The cost of cement is minimal considering all other costs.

Unfortunately, it is easy to show minimal material cost savings to an owner by using lower strength concrete and not really enforcing proper placement practices and curing, so the engineering community suffers.

I have an associate that has over 300 R/M delivery trucks and many plant locations and his opinion is that it is cheaper to sell proper concrete for the application/exposure rather than get into a legal issue on the suitability without having any control over the placement and a curing. The incremental volume and liability is not worth the possible legal/representative costs, so turning down the questionable sales is logical from a business sense.

The increased usage of higher strength concrete by suppliers and contractors is the "tail wagging the dog" and eventually later gets reflected in standards.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[steellion]

Is this to limit the water-cement ratio for freezing? It makes sense to set a max W/C ratio than a min f'c. I can't think of any other correlation between f'c and frost damage.

 

[JAE]

cliff234 - I think that it says what you fear it says. I too usually use 3000 psi concrete for footings and while most traditional footings are "mostly" below frost, recent use of trenched footings, where the top of the footing is only 8 inches or so below grade, would seem to fall into the 4,500 psi requirement.

 

[Hokie93]

This very issue was addressed in the May 2011 edition of Concrete International magazine, published by ACI. In the "Concrete Q&A" article, the response indicates "concrete below ground should be selected only on the basis of strength and workability requirements". The response goes on to say "Although footings certainly can freeze, they are below ground and are not exposed to frequent cycles of freezing and thawing because of the high thermal mass of the soil".

It's still a good idea to keep the water-cement ratio low, in my opinion, and provide air entrainment if you are particularly concerned about freeze-thaw action, but the requirements of ACI 318-08 Chapter 4 don't appear to apply to typical below grade members.

 

[JAE]

hokie66, good info. Thanks for posting. I kept imagining my trenched footings having freeze-thaw damage on the upper exterior corner - an area I really don't care much about as it is underground, not part of the load distribution path, and simply is extraneous concrete that allows me to avoid the cost of formwork.

 

[msquared48]

Thanks for the link Hokie.

In my opinion, 4500 psi is ridiculous for amall project foundations where there are no piling or grade beams. I have used 2500 to 3000 psi for years with a frost of 12" to 18" for years with no problems (sound like a contractor, right?).

As for higher values for concrete extending above the grade line, this could only apply to the stem wall or columns, not the strip or spread footings, pile caps or grade beams.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

JAE,
Credit where it is due. That was not my post, but rather a younger (I think) fellow Hokie.

 

[JAE]

OK - wow. How many "hokies" are there around here? Thanks Hokie93!

 

[slickdeals]

One more "hokie" here. I would be HOKIE05.

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[hokie66]

There are a few of us. fattdad comes to mind.

 

[emmgjld]

I imagine this also has to do with curing issues (slow strength gain during the 1st 2-3 months) due to higher amounts of fly ash.
Also combating ASR problems, which bring back the flay ash issue.