Air Content by the Pressure Method
Pressure meter procedure
The pressure method measures air by compressing the concrete in a sealed bowl and reading the gauge. Fill the bowl in three layers, rod each layer 25 times, tap the sides, strike off, and clean the flange. Clamp the lid, close the valves, and pump to the specified pressure. Tap the gauge gently while pumping and again before reading, then read the air content. The meter must be calibrated and in good condition, with clean seals.
Aggregate correction
The aggregate correction factor accounts for air trapped in the aggregate or for the compressibility of the aggregate. It is determined for the aggregate combination and is subtracted from the meter reading. Do not ignore it, and do not use a correction factor from a different aggregate source. The pressure method is not suitable for highly porous or lightweight aggregates that compress or absorb air.
Traps and worked example
Common traps are a leaking lid, concrete on the flange, not tapping the gauge, and reading before pressure stabilizes. Another trap is using the pressure meter as if it needed no correction.
Rule of thumb: seal clean, tap twice, read once, subtract the correction. Worked example: the gauge reads 6.5%; the aggregate correction factor is 0.8%; air content = 6.5 − 0.8 = 5.7%.