Frequently Asked Questions
Q: What are advantages and disadvantages of the different air tests?
A: The three common
methods to test for air content in a concrete mixture are (see
left to right): the pressure method (ASTM C231), the volumetric
method (ASTM C173), and the Gravimetric method (ASTM C138). Occasionaly
used as a quick check for the presence of low, medium or high air
content: pocket-size air indicator (AASHTO T 199).
The pressure method uses one of two devices: the
A meter or the B meter. Both of these meters rely on the relationship
between pressure and volume to determine the air content of a concrete
mixture. The pressure methods are not suitable for testing the air
content of lightweight aggregate concrete and other porous aggregate
concretes, as they would measure the air void system of the aggregates
and not just the air content of the paste in the mixture. The A
meter is sensitive to altitude and must be calibrated to accommodate
the altitude at which it will be used, while the B meter uses the
change in pressure of a known volume of air and is not affected
by altitude variations.
The volumetric method requires the removal of
air from a known volume of concrete by agitating the concrete in
an excess of water. This test is typically used for lightweight
and porous aggregate concrete mixtures. Care must be taken to assure
that the sample has been agitated sufficiently to remove all of
the air from the sample.
The gravimetric method is a comparison of the
actual unit weight of the concrete versus the theoretical weight
of all of the concrete constituents. The actual specific gravity
of all the materials in the mixture must be known to avoid errors
with this method. It can be used as a convenient way to assure against
variation in air contents as significant changes to the unit weight
would identify a change in the air content of the mixture.
For more information see Design
and Control of Concrete Mixtures (EB001).