This article is about the construction material. For the philosophical term, see Concrete (philosophy). For the comic book, see Concrete (comics).
In construction, concrete is a composite building material made from the combination of aggregate and cement binder. The most common form of concrete is portland cement concrete, which consists of mineral aggregate (generally gravel and sand), portland cement and water. After mixing, the cement hydrates and eventually hardens into a stone-like material. When used in the generic sense, this is the material referred to by the term concrete.
The Assyrians and Babylonians used clay as cement in their concretes. The Egyptians used lime and gypsum cement. In the Roman Empire cements made from pozzolanic ash were used to make a concrete very similar to modern portland cement concrete. In 1756, British engineer John Smeaton pioneered the use of portland cement in concrete, and used pebbles and powdered brick as aggregate. In the modern day, the use of recycled/reused materials as concrete ingredients are gaining popularity due to increasingly stringent environmental legislation. The most conspicuous of these are pulverised fuel ash (pfa), recycled from the ash by-products of coal power plants. This has a significant impact in reducing the amount of quarrying and the ever attenuating landfill space.
Concrete has great compressive strength, but little tensile strength. To overcome this limitation, concrete is most often constructed with the addition of steel reinforcement bars (rebars), steel mesh, cables, or steel fibres to produce reinforced concrete. Concrete can also be prestressed, allowing for beams or slabs with a longer span than is practical with reinforced concrete.
Concrete is also made with asphalt or epoxy as a binder.