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Concrete
In construction, concrete is a building material made from sand or other small stones, water, and cement. After mixing, the cement hydrates and eventually hardens into a stone-like material. Concrete was invented in the Roman Empire, but the knowledge of its making was lost, to be reinvented only in modern times. Concrete has great compressive strength, but little tensile strength. To overcome this limitation, concrete is most often constructed with the addition of steel bars (rebars) or cables, to produce reinforced concrete, a composite material with more balanced strength properties.
The Assyrians and Babylonians used clay as cement. The Egyptians used lime and gypsum cement. In 1756, British engineer, John Smeaton made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powdered brick into the cement.
Something is concrete if it is not abstract: it is both particular and an individual, and (hence?) occupies some space and time. To say that a physical object is concrete is to say, approximately, that it is a particular individual that is located at a particular place and time.
See particular; individual; abstract.
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