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March 15th, 2009

Manually counting concrete air voids?

Published on March 15th, 2009 | By: pwray@ceramics.org

Is this still the 1990s? I am no expert on concrete, but is it true that major highway departments still typically manually count air voids in concrete, as this story seems to indicate?

“They look at 1,300 points across the surface and count the number of entrained and entrapped voids,” she said. “It is a slow process and you cannot rush it, or there will be errors. It takes about three hours for each, depending on the operator.”

If the voids fall within the MTO standards, the mix is acceptable, she said. If the contractor disputes the findings, they can refer the sample to a “referee” who will re-examine it and report their findings.

Work has been done on automating the process, she said, but so far it takes more time to prepare the samples than to do it the old-fashioned way with a human operator and a microscope.

I quickly found research that asserts that at least some of the prep work for automated counting doesn’t take too long:

The sample preparation includes contrast enhancement steps ensuring white air voids in black concrete (aggregate and paste). For a well-lapped sample of good quality concrete the contrast enhancement procedure requires approximately 5-10 min to perform.

Does preparing a well-lapped sample really require more than 2.75 hours? Plus, there is the “small” problem of the reliability of manual counting and the use of referees, not to mention the risk of even a small amount of “operator bias” when relying on manual inspection:

Three samples were circulated to 7 different laboratories for automatic air void analysis. Prior to the automatic analysis the samples were analysed manually using linear traverse and point counting methods.

The results of the Round Robin study showed very good repeatability and reproducibility of the RapidAir system but large variations when using manually performed analysis.

I am not trying to promote RapidAir – there are probably several others that are just as good. But, is there really any good reason why we’ve had automated counting of blood cells for many years but can’t make an automated system work for air voids?


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