ASTM C231/C231M
Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method
Measuring how much entrained and entrapped air is in fresh concrete using a pressure meter, the standard check on freeze-thaw protection.
Who needs it
Field technicians and inspectors on any exterior flatwork, pavement, or structure exposed to freezing in service.
What the test involves
Fresh concrete is sealed in a calibrated pressure vessel, air pressure is applied, and the resulting volume change is read as a percentage of air. The method depends on the aggregate being dense enough that it does not itself compress, which is why it is the normal choice for ordinary crushed stone and gravel mixes and not for lightweight aggregate.
Common misconceptions
- Air content is not just a mix ingredient — it changes with pumping, dropping, and over-finishing, so the number at the truck may not be the number in the slab.
- More air is not automatically better. Air improves freeze-thaw durability but reduces compressive strength, so the target is a range rather than a maximum.
- The pressure method gives misleading readings on lightweight or highly porous aggregate, which is exactly what the volumetric method exists to solve.
Official source
We summarize scope and purpose only. The full text belongs to the issuing body — read it at the official source. ASTM International