When the field test results arrived soon after concrete placement on a recent project, we were told the mix had low air content. By then, the truck was empty and the crew had completed the work. The owner wanted us to remove and replace the concrete. We sent a sample of the material to a petrographer and an air-void analysis showed that the air content met the project spec. Why didn’t the field test match?
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—specifically if the concrete will be durable under freeze/thaw conditions.
There are two types of air meters. The pressure meter is the most popular because it provides quick results—apparently not quick enough results in your case. The second, the volumetric meter, is a slower test to perform.
The pressure meter has an upper chamber filled with air and a lower chamber that the technician fills with concrete. To measure the air content of the concrete, the upper chamber is pressurized and then the volume of air is released into the lower chamber. The pressure gauge on the top of the meter indicates the 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. So the pressure change is correlated to the air content of the concrete.
Entrained air is formed in concrete using a surfactant. 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. If the concrete is entrained with a significant amount of very small bubbles, the pressure meter will give you a false reading because the strong, small bubbles will not register a change in pressure.
Petrographic testing on hardened concrete uses a microscope to count and measure the size of the air bubbles. With proper magnification, even very small bubbles are counted. The test also provides a measure of the average bubble size called the specific surface. The specific surface is the surface area of the bubble divided by the volume. The larger the specific surface, the smaller the average size of the bubbles.
To correct for some of the concerns with the pressure meter, testing technicians can run the volumetric air test, although it takes significant effort and time. An easier method for tracking air content is to measure unit weight. Air content has the largest effect on unit weight and it is difficult to fool a scale. So if total air content is not the property most important to concrete durability then why do we measure it with the pressure meter? Simply because it is still the best way we have of measuring air in the field.
So don’t be too quick to remove concrete based only on field test results. There are analytical techniques that are more accurate and provide a better picture of concrete durability. Especially on large quantities of concrete, it may be worthwhile to investigate further to see if the field tests are accurate.
Contributed by Braun Intertec. Visit www.braunintertec.com.