Permeability in the context of concrete refers to how easily liquids or gases can pass through the material. This quality is crucial for assessing the water-tightness and durability of concrete structures and their resistance to chemical attacks. Concrete permeability can be determined through comparative laboratory tests. These tests typically involve sealing a concrete specimen from the sides, applying water pressure to the top surface with pressure, and measuring the amount of water passing through, often around ten days, until a steady state is achieved.

The permeability of concrete is not only influenced by the water/cement ratio and the degree of hydration but also by the porosity of the cement paste, which varies with pore size distribution. Capillary porosity plays a significant role, as segmented capillaries reduce permeability compared to interconnected ones. As the cement paste continues to hydrate, its permeability decreases due to the closure of voids initially occupied by water. For durable concrete, it's advantageous to have a lower water/cement ratio, which helps achieve lower permeability sooner.

It's also noted that the concrete mix should be dense and use well-graded aggregate for reduced permeability. Additionally, factors like curing methods and aggregate size can affect permeability, with steam-cured concrete usually having higher permeability than moist-cured, unless subjected to specific curing conditions like fog curing to reach desired low permeability levels.