Air entrained concrete, a modern marvel, is still the biggest headache in concrete production. A recent discussion we have been a part of involved the relationship between the pressure meter and the microscope. It may seem counterintuitive, but the microscope came first.
Many studies have been performed to show the relationship between the hardened air content determination by ASTM C457, a petrographic method, and the plastic concrete determined by the pressure method, ASTM C23.
The discovery that small air voids in hardened concrete will protect the concrete from damage due to freezing and thawing occurred in the 1930s by T. C. Powers. The analysis method used by Powers, microscopic examination and gravimetric testing, was too slow or too late to be useful in the control of air in concrete production and construction.
Air content of fresh concrete
The development of the volumetric air meter for determining the entrained air content of plastic concrete dates back to the later 1940s as detailed in ASTM STP 169B. Since that time numerous studies have been performed to confirm the correlation between the volumetric meter and the hardened air determinations.
One such study was performed by M. Muethel for the Michigan Department of Transportation in 1995. The figure you see to the right shows a close correlation between the two methods.
On the basis of the extensive literature on the topic, it is reasonable, to an engineering certainty, to conclude that the volumetric and hardened air contents are the same. Sometimes discrepancies between the plastic and hardened air contents present themselves.
There are a few things to keep in mind:
- The air void quantity and air void characteristics measured in hardened concrete are the final word.
- Conventional entrained air bubbles cannot manifest in concrete that has already been placed. The air was in the concrete mixture before it was placed even if the pressure meter reportedly measured something else.
- There is a class of entrained air bubbles that are too small to be measured with a pressure meter, but it is not likely they make up a majority of the entrained air void volume in a concrete.
Here’s the formal title of this 1995 report, which is available online: Muethel, R.W. “Investigation of the air content of plastic vs hardened concrete.” Michigan Department of Transportation Research Project 90 TI-1500, Research report No. R1332, 1995.