12.4 : Measurement of Air Content in Concrete

Air content measurement in concrete is critical for ensuring structural integrity and durability of concrete structures, especially in environments prone to severe weather conditions. Accurate air content analysis optimizes concrete's resistance to freeze-thaw cycles and enhances its workability and strength. Several methods are standardized under ASTM guidelines to measure the air content in fresh concrete, each suitable for different concrete types and conditions.

The pressure method, specified in ASTM C231, is the most common and involves pressurizing a concrete sample in a sealed chamber to measure the air volume based on the pressure change. This method is reliable for non-porous and normal-density concrete. The volumetric method, detailed in ASTM C173, is preferred for lightweight concrete or mixes with highly absorptive aggregates. It measures air content by observing the volume change in a concrete-alcohol mixture.

The gravimetric method calculates air content by determining the specific gravity of the concrete and is suitable for precise scientific analyses, though it is less common in field applications. Lastly, the Chace Indicator method offers a quick, approximate measure using a syringe-like device to assess air volume changes in concrete, which is useful for immediate field tests.

Choosing the right method depends on the concrete's properties, project requirements, and available tools, with strict adherence to ASTM procedures ensuring accuracy and consistency in measurements.