Q.: How do you control and measure the amount of air in a slab?

A.: The amount of air-entraining agent used in the first batch of concrete on a job is based on the experience of trial mixes. The air actually obtained can be determined with an air test meter in about 5 minutes time following the procedure given in ASTM C 231. If the air content is outside the range specified, adjustments are made in the amount of air-entraining agent added to the next batch. This process can be repeated throughout the job.

It is generally assumed that the amount of air in the fresh concrete is the same as the amount of air in the hardened concrete slab. This is not always true because sometimes air is knocked out of the concrete by improper job practices such as overwetting or finishing the concrete too soon. If necessary, a measurement of the air in the hardened concrete can be made by cutting out a sample and sending it to a laboratory for a linear traverse test, as specified in ASTM C 457, but this is an expensive test.