Concrete is made of cement, aggregates like sand and gravel, and water that provide density and strength when cured. Steel reinforcement is often added to concrete structures to increase their tensile strength and allow them to withstand greater stresses and make taller buildings possible. The two main concrete construction methods are sitecast, where concrete is poured on-site, and precast, where concrete elements are produced off-site - precast has advantages of speed and quality control but limits design flexibility.