ASTM C231 - Concrete Air Content Test, Pressure Method

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Aggregate Testing Limitations

This test method cannot be used for testing lightweight concrete because the aggregate in lightweight is too porous. See ASTM C173 Air Content by Volumetric Method.

Two Types of Air

  • Entrapped (naturally occurring in the mix)
  • Entrained (intentionally added to the mix)

Remember, entrained air is necessary for concrete that will be directly exposed to freeze-thaw cycles throughout its lifetime. The air voids allow for collected moisture to expand and contract through freezing without causing the concrete to spall.

And as air increases, the ultimate strength of the concrete decreases, just as the increase of water decreases strength.

Two Types of Pressure Method Air Meters

  1. Type A: Not commonly used. Uses pressure to displace water into concrete.
  2. Type B: Most commonly used. Uses equalization to determine air content in the concrete.

Once the valve is depressed and held, equalization occurs when the known air in the air chamber releases into the unknown air content in the concrete sample in the bowl below. The apparatus equalizes and provides you with an air reading (how much air it took to equalize the system).

Additional Important Information

  • Maximum 2" aggregate permitted for use with the Type B bowl size
  • Pressure gauge should read to the nearest 0.1 percent
  • Mallet mass same as described under ASTM C138
  • Tamping rod dimensions same as described under ASTM C143
  • Strike-off bar made of metal at least 1/8-inch thick, 3/4-inch wide, and 12-inches long or strike-off plate that is at least 2-inches wider than diameter of bowl
  • Metal, at least 1/4-inch thick or glass or acrylic, at least 1/2-inch thick