Steel, concrete, cement and timber are used to build bridges. Other materials include asphalt, aluminum, stone and aggregates, which are a composite of gravel, sand and other materials. Sometimes plastics, titanium and aluminum are used, and, although uncommon, bridges made of tree roots and vines exist in parts of Asia.
Different bridge-building materials have varied strengths and durability, which determines which materials are used in construction. Because steel is the strongest material, it is most often used in bridges with a longer span. Steel is prone to corrosion caused by salt spray from waterways, as well as humidity and other atmospheric conditions. Corrosion is typically prevented by painting the bridge with a corrosion-resistant material.
Stone was primarily used in ancient times by Romans and other groups. The material proved durable as some ancient bridges are still in use, such as the Stone Bridge in Regensburg, Germany, which was built in the 1100s.When used in modern bridges, stone is typically used in surface abutments, for piers and arches. Crystallized limestone, granite, porphyry and similar weather-resistant stones are best for bridge construction. When designed and built well, stone bridges are long-lasting. They are also nearly maintenance-free, but can be expensive to build.