If the concrete is entrained with a significant amount of very small bubbles, the pressure meter will give you a false reading since the strong, small bubbles will not register a change in pressure.

The laboratory testing performed on hardened concrete uses a microscope to count and measure the size of each air bubble. 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.

A specific surface of 1200 in2/in3 or greater is a very small air void system. Typically, the air content conducted in the laboratory is slightly higher than the field test, as the air void size distribution will always provide some bubbles the pressure meter can't measure but the microscope can.

To correct for some of these concerns, producers can run the volumetric air test, although it does take significant effort and time. An easier method for tracking your mixtures would be to measure unit weight. Air content has the largest effect on unit weight and it is difficult to fool a scale.

Now back to the comment about the “gray area.” Total air content is not an indicator of concrete's durability. As for the air void system, it requires a spacing factor of 0.008 inches or a specific surface greater than 600 in2/in3 to be considered durable.

These specifications should be applied at targets for concrete production, since there is much evidence that indicates a spacing factor of 0.01 inches and specific surface of 450 in2/in3 are sufficient. The spacing factor is the maximum distance from any point in the paste to the nearest air bubble. Basically, it is the distance water will move through the paste to expand into an air bubble when freezing, without damaging the paste.

So if total air content is not the property most important to concrete durability then why do we measure it with the pressure meter? It is the only way we have of measuring air in the field. There are other methods under development, although they have not shown much promise. The pressure meter is still one of the best tools we have.

Also, air content doesn't act alone. The water-to-cement ratio, curing, and environmental exposures greatly affect the performance of the concrete. I have seen results from concrete pavements that measure 0.014 inches on the spacing factor and 400 in2/in3 on the specific surface. After 14 years, the concrete is in great shape. In the lab we have seen passing freeze/thaw results for non-air entrained concrete which contained a water-to-cement ratio of 0.32.

So don't be too quick to remove concrete based on only on field test results. There are analytical techniques which 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 valid or not.

Contributed by Braun Intertec. Visitwww.braunintertec.com.