Q: We placed concrete that had low air content because the field test results were not provided until the truck was empty and the crew had completed the work. The order came to remove and replace the concrete. We retrieved a piece of the material and an air-void analysis showed that the air content met the project spec. What happened?
A: This proves that concrete is not black and white, but many shades of gray. Testing performed on the fresh concrete during placement does not always tell us what we need to know about concrete properties.
There are two types of air meters. The pressure meter is the most popular because it provides quick results. Here, it may not have provided quick enough results. The second, the volumetric meter, is a much slower test method to perform.
The basis for this question is the difference in air content between the laboratory test and the field test. But there are additional issues.
The method the pressure meter uses to measure the air content is the underlying issue in your question. Each pressure meter may be slightly different, but the concept is the same. There is an upper chamber, which is pressurized. That volume is released into the lower chamber, which is filled with concrete.
The gauge on the meter measures the pressure in the upper chamber and its change in pressure as it's released into the concrete. The measurement is from the bubbles in the concrete compressing under the increase in pressure. The only ingredient in the concrete that is compressible is the air. Cementitious material, water, and aggregates do not respond to the small rise in pressure.
Entrained air voids are formed in concrete by using a surfactant and the smaller the bubble, the stronger the bubble. As the bubbles get smaller, concrete becomes more resistant to the increase in pressure from the pressure meter.