How is the air content of freshly mixed concrete determined?

Q. How is the air content of freshly mixed concrete determined?

A. Three ASTM methods are available:

1. The gravimetric method (ASTM C138) in which the air content is obtained by subtracting the sum of the absolute volumes of the ingredients in a batch from the measured volume of the mixed concrete as calculated from its density.
2. The volumetric method (ASTM C173) in which the air is removed from the concrete by agitation in a closed water-filled vessel and the air is measured by a drop in the level of water.
3. The pressure method (ASTM C231) in which the amount of air is found from the change in volume of the concrete under the application of a known pressure.

These last two procedures have some advantage over the gravimetric method. When concrete with low-density aggregates is tested, the pressure method cannot be used because of air in the pores in the aggregate, so the volumetric method is used.

References: SP-1(02); ASTM C138; ASTM C173; ASTM C231

Topics in Concrete: Cold Weather; Concrete Fundamentals; Testing of Concrete