Concrete - Air Entrainment Not Meeting Specs

[P1ENG]

I'll admit that concrete knowledge is the weakest amongst building materials, so I only get involved in very simple concrete things. Well, I am being asked for a letter of approval for the concrete used in sonotube piers used for bearing and lateral resistance of a single-story modular building. The piers have weld plates for anchoring the building and are only 4' deep on the East coast (Mass). So freeze/thaw is expected. The drawings specify 4,000 psi and 6% +/- 1.5% air content. I received documents of (2) samples which tested (3) cylinders each. One sample says that % air (T152 or T196) is 3.0% and has 28 day strengths of 6250, 6050, and 6150 psi. So the air is lower than specified but the strength is higher than required. The other sample meets strength and air specs. Is there some way to decrease the required air because of the extra strength in the concrete? If not, what is my next step? I've already stated that I can't write the letter because the concrete that was produced was observed to not meet the specifications and that I would only write a letter if all involved were made aware of the missed specification and if the concrete producer provide written acceptance of liability for issues caused by freeze/thaw cycles. That is unless there is another avenue...

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant

 

[Once20036]

What's your role on the job?
Based on your AWS/CWI certifications, I`m assuming that you`re a third party inspector? In that case, it wouldn't be appropriate for your to review or approve anything. That should be left for the engineer that spec'd the concrete.

 

[P1ENG]

@Once20036,
I am an SE/PE that sealed the modular foundation plan. Thank you for bringing my signature to attention. I haven't looked at it in years and I let my CWI expire since I never used it. I'm going to go update that now.

Juston Fluckey, SE, PE
Engineering Consultant

 

[flight7]

What is the function of the modular building? Is it a temporary structure? A truly temporary structure might not need the long-term durability which air entrainment should help provide, but temporary structures do have a way of becoming permanent.

 

[P1ENG]

@flight7,

I never assume temporary structure unless very apparent. Most of my work is in the modular world and, like you said, many will claim a life at site at 2 years or less, but that doesn't mean they don't intend to leave it there much longer. Since there are poured concrete and weld plates involved, I'm very sure they don't want to claim temporary either. It would be more likely with a temporary type of anchor system such as HUD straps and ABS bearing pads... but not on this job.

Juston Fluckey, SE, PE
Engineering Consultant

 

[dik]

What is the weather/climate like? Do you get a lot of freeze thaw conditions? and is the concrete exposed to the elements so it can be wetted and dried? Can the Contractor provide you with a letter from another engineer attesting to is suitability?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[P1ENG]

@dik,

Massachusettes, so yes, freeze/thaw would be common. The building is on piers and the bottom of the building will likely be skirted like a mobile home. So, the foundation is under unconditioned space with no real protection from water.

Juston Fluckey, SE, PE
Engineering Consultant

 

[PT99]

As building codes requires you to comply with the requirements of ACI 318, I think you will find that ACI 318 says "concrete exposed to freezing and thawing shall be air entrained", with specific percentages listed. So it appears to me, this is not subject to engineer's judgement, this is simply not building code compliant.

 

[P1ENG]

@PT99,
That's where I am at too, but my lack of in-depth ACI knowledge might have kept me from another avenue that could justify this. It is 3/4" aggregate and could be might be able to be classified as F1, which ACI 318-14 Table 19.3.3.1 says the target air content is 5%. F1 is "Concrete exposed to freezing-and-thawing cycles with limited exposure to water". The site should be well graded to keep water from running under the building, but there is no actual water proofing provided by the skirting.

Juston Fluckey, SE, PE
Engineering Consultant

 

[WesternJeb]

I concur with PT99 and don't know of any exception. Reject it as it isn't structurally acceptable, and put the liability on the contractor if they don't want to redo it. Extra strength is nothing when exposed to water freezing as those pressure far exceed any design pressure.

 

[JedClampett]

Let me the disagreeable one on this. Measuring air is flaky. I've asked at ACI seminars what to do if the air is not in compliance and got a confused look from the instructors.
I might note it on the submittal and ask for an explanation. I would likely chalk it up to the nature of the air content testing.

 

[dik]

With problems, that is generally my last approach... Can cores be taken to determine the in situ air? If tests are done, there has to be a clear determination of the results of the test, before they are taken. Can you accept 5%? 4%? Can the concrete be coated to prevent the ingress of moisture? Will skirting provide enough protection?

I generally don't like the 'cut it out' approach.

I used to joke that in Winnipeg that we got one freeze-that cycle a year, and the concrete warmed up July 1... Toronto, likely is subject to greater freeze thaw cycles, due to the milder climate.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[P1ENG]

Well, sorry everyone. It seems my wasted time has wasted your time. Apparently the test reports that I was sent were historical tests for the mixture, not the actual in-situ concrete. The actual concrete hasn't even been poured yet, so this "letter of approval" request doesn't even make sense now. I was sent shop drawings of the rebar layout which I stamped with approval, so maybe these shop drawings are the "letter of approval" they are wanting, but I don't know. I'm at the tail end of a game of telephone I suppose.

Juston Fluckey, SE, PE
Engineering Consultant

 

[dik]

Not wasted time... still a learning process...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bridgebuster]

As air entrainment goes up compressive strength goes down slightly. Anyway, I agree with dik. Can you take cores and perform freeze-thaw testing? Typically 3% air is bare minimum for F-T durability.


I know this is a moot point but are you saying that the plant inspector observed a batching problem and let it go, or did the site inspector check the air and let it go?

 

[JAE]

I've seen in ACI documents (sorry can't exactly cite them here) that air content tolerance is usually between 1% and 1.5% of that specified.

I've not seen higher tolerances than that.
Also, the use of averaging cylinder strength tests (avg of 2 cylinders = 1 strength test) doesn't apply to air.

Higher-strength concretes do have greater freeze-thaw resistance so at 6,000 psi I'd think the tolerance shown as 1.5% is probably correct.

 

[dik]

The problem I have with testing (cores) is that you have to establish, before hand, what the criteria is. Too often when this is done, you are sitting at the end with data that no one is sure of what it means.

JAE... air sampling generally occurs with the plastic mix before test cylinders are made. If important, it should be flagged at that point.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

dik,
Not sure why you seem to suggest I was saying air sampling is only done at the time of placement.
I do know that and have taken 1,000's of air tests in my past.

Having said that, you can take small cores of hardened concrete, send them to a lab and have a petrographic analysis run on them to determine the air content of that sample.
For a small modular building with a few 4 ft. piers, this sampling/testing would be unreasonable in my opinion.

 

[dik]

Sorry JAE... didn't mean a slight...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

no problem.