Gas permeability tests measure the flow of a gas into the concrete under the action of a pressure gradient.  It overcomes some of the disadvantages of measuring water absorption that include:
– In concretes that are not fully hydrated, water alters the permeability to be measured
– A measurement cannot be repeated in the same location
– The viscosity of water is approximately 100 times greater than that of air, with volume flows being far smaller
– With water, only overpressure can be measured

Tests include the Schonlin air test, Surface Airflow Test (SAT), Vacuum test,  Torrent test, Hong Parrott, Figg air test, and Autoclam.