Of the three methods for measuring air content--pressure, volumetric, and gravimetric--which one is the most reliable?

We think the pressure method is most reliable for normal-weight concretes, provided that you keep the air meter in calibration and use the aggregate correction factor. In the gravimetric method, air content is determined based on differences in actual and theoretical (air-free) unit weights of the concrete. Determining the actual unit weight incorrectly--for instance, by failing to properly strike off the concrete after the mold is filled--can cause a relatively large error in air content. Theoretical unit weight is affected by mix proportions used and the specific gravities of the mix ingredients. Small variations in specific gravity or large variations in mix proportions can again cause relatively large errors in air content. The volumetric test requires diluting the concrete with water, then repeatedly rolling and agitating the volumetric air meter to free the entrained and entrapped air. Although this test is required for determining air content of lightweight concrete, the air content results aren't very reliable indicators of air content in the hardened concrete, especially in the higher air content range. One reason for this is difficulty in dislodging all concrete in the air meter base so it mixes with the added water. Failure to dislodge the concrete causes an air-content reading that's lower than the true value. Concretes containing more than 600 pounds of cement per cubic yard may require up to 60 minutes of repeated rolling and letting the air meter stand before a stable reading is obtained. Another problem is difficulty in dispelling foam in the neck of the apparatus when the concrete has a high air content. This can cause erroneous air-content readings. The pressure method isn't entirely free of problems. If the concrete isn't completely consolidated in the bowl, apparent air content will be higher than the true value. For example, when low-slump concretes are tested, failure to tap the sides of the bowl after each rodding can leave rod holes that the meter reads as air voids. Also, any leaks at gaskets or valves in the pressure meters will cause erroneously high air-content readings.

References ASTM C 138, "Standard Test Method for Unit Weight, Yield, and Air Content (Gravimetric) of Concrete," ASTM, West Conshohocken, Pa., 1992. ASTM C 173, "Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method," ASTM, 1994. ASTM C 231, "Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method."ASTM, 1997. Lawrence R. Roberts, "Air Content, Temperature, Unit Weight, and Yield," Significance of Tests and Properties of Concrete and Concrete-Making Materials,ASTM STP 169C, ASTM, 1994.