Concrete is a mixture of cement, water, sand, gravel, and sometimes admixtures. The cement reacts with water to form a paste that coats the aggregates and hardens into a rock-like mass. It is the most widely used building material on earth, and for good reason: it is strong, durable, moldable, and relatively cheap.

Concrete is strong in compression but weak in tension. That weakness is solved by adding steel reinforcement — rebar, mesh, or fibers — to carry tensile forces. Reinforced concrete can span, cantilever, and resist earthquakes. Prestressed concrete takes it further by compressing the concrete before loading, allowing longer spans and thinner sections.

Mixing and curing matter. Too much water weakens the concrete. Too little makes it unworkable. Curing — keeping it moist and warm while it hardens — determines its final strength. A good concrete mix design balances workability, strength, durability, and cost.

Concrete also has an environmental cost. Cement production is a major source of carbon dioxide. Researchers are exploring lower-carbon binders, recycled aggregates, and carbon capture to reduce the material's footprint.