Testing Air Content in Concrete

A Type B pressure meter is used to determine the air content of normal-weight concrete. The air content is read at the dial, which is calibrated for each apparatus. The aggregate correction factor (explained in ASTM C 231) must be subtracted from your reading to obtain the net air content. (Photo courtesy of PCA.

Air-entrained concrete is typically specified in areas of the country where frost-related damage can occur. The measurement of air content in fresh concrete of normal density is typically performed using the pressure method (ASTM C 231). Another useful test is ASTM C 173. However, the pressure method is frequently preferred because it is relatively fast.

You should begin the test within 15 minutes after obtaining the composite sample. Start by filling the 0.25 ft³ base of the air-content test device in three equal layers, and rod each layer 25 times. After rodding, strike the outside of the base with a mallet 12 to 15 times to close any air voids. After completing the three equal layers, strike off the bowl flush at the top to completely fill the 0.25 ft³ volume. At this point, it can be weighed as part of the calculation to determine the fresh concrete unit weight.

Next, latch the top of the air-content test device over the base and fill the air gap between the top of the struck-off concrete and the underside of the top of air meter with water. The meter top is then pressurized with the built-in hand pump until zeroed out (or as calibrated). After a stabilization period, release the pressure in the top and read the air-void content on the dial on the top of the meter. Subtract the aggregate correction factor from the dial reading and report the final value.

Testing tip: A typical air content for concrete with a ¾-inch maximum-size aggregate is about 6%, and specified ranges in air content are typically minus 1 ½% and plus 1 ½% of the target value.