A Roman recipe for underwater concrete still matches modern practice
Writing around 25 BC, the Roman architect Vitruvius advised mixing one part lime with two parts volcanic ash from Pozzuoli, near Naples, for concrete meant to sit underwater. That proportion is essentially what builders still use for marine concrete today. For buildings on land he preferred one part lime to three of volcanic sand.
Concrete is a composite: two or more materials with very different properties combined so that each stays distinct while the whole behaves in a new way. Usually one ingredient acts as a binding matrix and another as filler. In concrete, loose stones sit in a cement matrix. It shrugs off crushing loads but cracks under tension, which is why steel bars, strong when stretched, are embedded to make reinforced concrete. By 2009 the world was pouring about 7.5 billion cubic metres a year, making it the most common artificial composite of all.
People have been combining materials this way for millennia. Wattle and daub may be over 6,000 years old, straw-and-mud bricks appear in Egyptian tomb paintings, and Mesopotamians were gluing wood at different angles into plywood by around 3400 BC. Egyptians layered linen or papyrus with plaster into cartonnage for death masks. Nature got there first: wood is cellulose fibres held in lignin, and bone is a mineral reinforced with collagen.
Fibreglass shows the logic neatly. Glass fibres are stiff and strong but brittle; the polymer around them is weak and bendy but tough. Together they make something stiff, strong and forgiving. The first artificial fibre-reinforced plastic combined glass fibre with Bakelite in 1935. Sandwich panels use another trick, bonding two thin, stiff skins to a thick, light core so the result resists bending without much weight.
Lightness is the main selling point for pricier fibre composites. The wings and fuselages of the Boeing 787 and Airbus A350 are largely made of them, as are bicycle frames, rowing shells and most hockey sticks. Carbon-carbon brakes stop aircraft and racing cars, carbon composites shield spacecraft on re-entry, and metal-loaded plastics can match the density of lead, letting them replace it where it is banned.
Source: Composite material
