Most people think of concrete as a solid material. In reality, however, it's a porous material. The pore volume, size, and arrangement affect how fast concrete absorbs water and how long it holds the water in the pores. Permeability is the rate at which water or other liquids flow through concrete. The more permeable concrete is, the less durable it is. Highly permeable concrete is more likely to be damaged by freezing and thawing because water enters it easily, expands as ice is formed, and causes destructive pressures. Sulfates, acids, and other aggressive chemicals penetrate faster. Reducing permeability is an effective way to improve durability. Permeability is determined largely by water-cement ratio, curing, and chemical and mineral admixtures. For the most part, to control concrete permeability, you must control the mix proportions and curing. THE INFLUENCE OF THE WATER-CEMENT RATIO Not only does the water-cement (w/c) ratio affect compressive strength, it also affects permeability. Initially, the space between cement grains forms a continuous water-filled network of capillary pores. As the cement grains hydrate, they block off the pores. This makes concrete less permeable. Small pores are blocked more readily than large pores. More cement grains (lower w/c ratio) lead to greater blockage. Greater blockage results in less permeability and more durable concrete. THE INFLUENCE OF CURING To obtain capillary pore blockage, the cement grains must hydrate. This requires curing, which is the control of concrete moisture and temperature after placing or finishing.