• Sydney Harbor bridge.sydney harbour bridge image by Xavier MARCHANT from Fotolia.com
  
  Initially, bridges were made with wood and were often as simple as a log across a stream. The Inca then started using rope bridges to get from one mountain to the other. In medieval times, stone arch bridges gained popularity. By the 4th century B.C., the use of iron chains to suspend bridges had begun in India. Modern bridges however, need special construction materials to support their record-setting spans and heights, and to offer the strength necessary to ensure the safety of commuters.
  
  

Artificial Stone and Brick

• Historically, stone bridges have proven among the strongest and most durable of all bridges, but cutting and processing the stone required also makes them the most expensive to build. Therefore, stone is being replaced by artificial replica stone. Again, due to cost constraints, its use is limited to the surface for finishing touches and aesthetics, with brick often replacing stone where the surrounding structures also predominantly use brick.
  
  

Concrete

• Despite looking bland and dull on the surface, concrete has become the de-facto construction material for nearly all structures in the world and bridges are no exception. The ability to pour concrete into any shape or size makes it ideal for constructing bridges as it needs no cutting or molding. To add more strength to it, the concrete is often pre-compressed and reinforced with steel. On the surface, concrete is prone to corrosion by salt-water and pollutants in the air like carbon dioxide and sulphur dioxide. This is remedied by using other materials to cover the surface.
  
  

Steel

• Apart from being used to reinforce concrete, steel is also heavily used as a primary construction material for bridges. From cables for suspension bridges to the suspension towers themselves, it is the primary material used in many bridges. The compressive and tensile strengths of steel are 10 and 100 times that of average concrete respectively, enabling longer spans of bridges to be supported by fewer columns. Also, being a metal, steel has a remarkable ductility, or ability to bend, stretch or otherwise deform without breaking, unlike concrete. However, corrosion is a major concern, requiring an alloy coating for protection from the elements.
  
  

Aluminum

• While not as strong as steel, aluminum or one of its strengthened alloys are used as a substitute for bridges not requiring the strength of steel. It is also corrosion-resistant and more aesthetically appealing. Aluminum sheets are at times used as the surface material for bridges constructed with other materials, as the "Influence of Bridge Building Materials" article mentions.