ABSTRACT
Using the ‘Clam’ machine[1], an air pressure of 0.1 bar was applied to the surface of the sample within a ring bonded to the surface. The rate of decay of this pressure was continuously recorded for a period in excess of 20 minutes. It is known[2] that this produces a relationship of the form-logarithm of pressure decay is proportional to the time. The proportionality constant is referred to here as air permeability index and may be directly related to the intrinsic permeability of the material. Two readings were taken on each cube, one on each of two opposite moulded faces. The results reported later are, therefore, the mean of four for each value.
