Bridges help vehicles and pedestrians bypass obstacles. Bridges can span rivers, canyons or roads, and often cut travel time dramatically by providing a more direct route. Bridges are complex structures, and must be carefully engineered using strong, long-lasting materials that are also light enough for practical use.
Natural stone is one of the oldest bridge construction materials. It is strong and able to resist erosion caused by wind and water. Materials such as granite and limestone are naturally attractive and will last for centuries with little or no maintenance. At the same time, stone is heavy and has high installation costs. It is often used to build bridge piers and footers, which allow the upper portions of the bridge to be built from more affordable and lighter materials.
Concrete is popular for all types of bridge construction because of its affordability and strength. Concrete requires little maintenance, although it tends to hold up poorly against saltwater and erosion. Though it can be easily shaped and formed, concrete is often thought of as unattractive because of its dull, gray finish. When used on longer spans, concrete can be reinforced with steel bars or subject to a treatment known as "pre-stressing" to help increase its strength.
Steel is the one of the strongest bridge materials available, and can be used to span distances that are not possible with other products. It is 10 to 100 times stronger than concrete and weighs less. Steel bridges are susceptible to rust and corrosion, however, and tend to require a lot of maintenance. Many steel bridges are painted to improve their appearance. Aluminum is sometimes used in place of steel because of its anti-corrosive properties.
Wood is not as reliable as other bridge construction materials, and should only be used on relatively simple structures. It is one of the more affordable bridge-building materials, and is easy to work with using basic tools and equipment. Wood bridges are primarily chosen for their natural beauty, and are used for pedestrian access or light vehicle traffic. Because wood can swell and rot when exposed to moisture, wood bridges will last longer when protected from rain by a chemical treatment.
Composite products made of fiber-reinforced polymers (FRP) are one of the newest materials to be used for bridge construction. They weigh 70 percent to 80 percent less than steel yet are just as strong and durable. While even the strongest steel or reinforced concrete bridge will require substantial maintenance over the years, FRP requires virtually no upkeep. It will also not corrode in saltwater, making it the superior choice for construction in the water. This product has only been used to construct bridges since 1975, so it's long-term properties are still under evaluation.