ACI 318-08 exposure class and compressive strength for foundations
ACI 318-08 exposure class and compressive strength for foundations
(OP)
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!
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!






RE: ACI 318-08 exposure class and compressive strength for foundations
RE: ACI 318-08 exposure class and compressive strength for foundations
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.
RE: ACI 318-08 exposure class and compressive strength for foundations
RE: ACI 318-08 exposure class and compressive strength for foundations
RE: ACI 318-08 exposure class and compressive strength for foundations
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.
RE: ACI 318-08 exposure class and compressive strength for foundations
RE: ACI 318-08 exposure class and compressive strength for foundations
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
RE: ACI 318-08 exposure class and compressive strength for foundations
Credit where it is due. That was not my post, but rather a younger (I think) fellow Hokie.
RE: ACI 318-08 exposure class and compressive strength for foundations
RE: ACI 318-08 exposure class and compressive strength for foundations
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
RE: ACI 318-08 exposure class and compressive strength for foundations
RE: ACI 318-08 exposure class and compressive strength for foundations
Also combating ASR problems, which bring back the flay ash issue.