Concrete is simply a mixture of Portland cement, aggregate like stones and sand, and water. The water reacts with the cement and forms a bond which binds the cement to the aggregate to form a very hard structure. Concrete was used thousands of years ago but has improved dramatically in the last 100 years. It is used in roads, sewers, buildings, bridges, tunnels, and even boats. Protecting it and enhancing it has become an industry all its own.