Affects of Air Entrainment

[SperlingPE]

Problem: Air entrainment spec. 5%-7%. Actual air entrainment 1.5%

Exterior footings for a one story wood framed building. Project located in midwest (freeze-thaw cycle is expected).

70 yards of concrete was placed. The field testing was done somewhere around the 50th yard. Late testing was done because of bad scheduling by the contractor. The field testing was done twice to verify.

The concrete has some air. Is this enough to do the job? Is there any research or other information that you can point me to to help evaluate this situation?

 

[mbullism]

Without looking it up, I'm pretty sure 1.5% AE can be expected without an admixture, just due to the mixing process...

you say the concrete is for footings... aren't they buried away from the weather, and below the frostline?

 

[SperlingPE]

Trench footings are being used.
Bottoms of footings are below frost.

 

[SlideRuleEra]

Perhaps some ideas to investigate from this link

http://www.concreteconstructiononline.com/industry-news.asp?sectionID=718&articleID=249268

http://www.SlideRuleEra.net

 

[henri2]

Does the local residential building code require air-entrainment for the footings or is this a requirement in the job specifications? Check IRC Sec 403 and table 402.2

Due to the delay in performing the test, the lower than expected air-content might not necessarily reflect the actual amount of air-entrainment in the in-situ freshly placed concrete. Refer to Section 4 of ASTM C 172 Sampling Freshly Mixed Concrete.

To get an estimate of the entrained air consider
ASTM C457 "Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete"

The measured air content of freshly mixed air-entraied concrete will include contributions from both entrapped and entrained air. Freshly mixed non-air-entrained concrete will contain between 1 to 2% of entrapped air; air entrainment will increase the total measured air. So there is a possibility that in fact no air entraining admixture was even used.

 

[SperlingPE]

IRC does not apply. Building is a commercial building and was designed under the IBC.

The test was performed on the concrete in the second to last truck due to circumstances related above.

My thoughts are
1. no air entrainment ad mixture was used
2. entrained air was lost due to any number of things.

My question is about the concrete and whether or not it will hold up. Also, is there any research or documentation available that addresses this problem.

henri2 - Thanks for the ASTM C457 info.

 

[Ron]

SperlingPE....what you have is minimal air and likely not entrained air, but entrapped air. There's a significant difference.

Entrained air is specified so that very small (microscopic to sub-macroscopic)"bubbles" or air voids are created and dispersed throughout the paste matrix. When the moisture in concrete freezes, these numerous small voids absorb the strain from the expanding water as it freezes, thus preventing spalling of the concrete.

I recently finished an investigation of air content issues on a project in Nevada. I did a petrographic examination of the concrete, including the ASTM C457 procedure. One of the specimens had a 2.1 percent air content. That core represented concrete that had extreme surface deterioration and spalling in an exterior, exposed slab. The concrete adjacent to this had an air content of 6.6 percent with no spalling or deterioration. All of the concrete was 4 years old.

Entrapped air, which is likely what you got with no addition of air entraining admixture, creates air voids that are quite a bit larger than entrained air. They are readily visible in a prepared specimen. They will actually hold water and when the water freezes, the concrete around the void will break away from the pressure (spalling).

I would suggest that you get several cores from the affected concrete and other areas as well. Have a petrographic examination done, including the ASTM C457 procedure. It isn't cheap, but it is fairly definitive. We can tell a lot about concrete by looking at a prepared specimen under a microscope. It will cost about $800 to $1000 per core to get this done.

It is also possible that in a 70 cy placement, the tested specimen was an anomaly.


Good luck.

 

[henri2]

Is it common practice to use air-entrained concrete for these types of footings in your area..when constructed at this time of year? Check IBC Table 1904.2.1 footnote b

 

[cvg]

It appears that your structure is probably considered to be moderate exposure. For moderate exposure, assuming you used 3/4" aggregate, PCA recommends your specified air entrainment should be 5% with allowance of -1 to +2 for an allowable range of 4% - 7%. I would suggest that your concrete is unacceptable, unless you can somehow prove that the earlier batches had air entrainment.

Note that there are a number of production, construction and placement variables that will cause the air entrainment to vary, even if air entraining admixture was used.

http://www.cement.org/tech/cct_admixtures_AEA.asp

http://www.cement.org/tech/pdfs/PL981.pdf