The rates at which liquids and gases can move in the concrete are determined by its permeability. Permeability of Concrete is often referred to as root cause for lack of durability. Permeability affects the way, in which concrete resists external attack, and the extent to which a concrete structure can be free of leaks. Theoretically, the introduction of aggregate of low permeability into cement paste. it is expected to reduce the permeability of the system, because the aggregate particles intercept the channels of flows and makes it take a circuitous route. Compared to neat cement paste, concrete with the same W/C ratio and degree of maturity, should give lower coefficient of permeability. But in practice, it is seen from test data it is not the case. The introduction of aggregate, particularly larger size of aggregates increases the permeability considerably.

The use of pozzolanic materials in optimum proportion reduces the permeability of concrete. This is evidently due to the conversion of calcium hydroxide, otherwise soluble and leachable, into cementitious product.

Though air-entrainment makes the concrete porous, when used upto 6%, makes the concrete more impervious, contrary to general belief.

High-pressure steam cured concretes in conjuction with crushed silica decreases the permeability. This is due to the formation of coarser C-S-H gel, lower drying shrinkage and accelerated conversion of Ca (OH)2 into cementitious products.