Arch bridges were first built by the Romans more than 2,000 years ago. They have several advantages over other types of bridges. Elegant and strong, many bridges built by the Romans are still standing today. They don’t require a central support, so they’re a sensible choice for bridging long, open spans. In addition, they are built with simple materials like wood, stone and concrete.
Building a stone arch bridge is difficult because the bridge remains completely unstable until the two spans are joined together. Wood is traditionally used to build a frame in the shape of the arch. The stones are put into position around the frame, which supports the spans until they’re joined in the middle. At that point, the frame is removed. Wooden timbers can also be used to make vertical posts, or piles, that help support the bridge under water.
Weather-resisting stones like granite, gneiss, porphyry, diabas and crystallized limestone are optimal choices for bridge construction. Granite is especially good at resisting erosion by sandy water. Stones can be worked in different ways depending on bridge requirements and the qualities of the stone being used. Different effects can be obtained by mixing different colors or using bright or dark joint-filling. The texture of different rocks also affects the appearance of the bridge.
Concrete is an artificial building material that’s made from a filler such as gravel, broken bricks or chunks of stone and a binding agent that joins them. The binding agent can be made from mud, but usually lime or gypsum is used. Cement is a modern-day binding agent that’s made from earth substances. Because concrete sets under water, it can be used build the foundation the bridge rests on. It can also be used to build the arches.
Because stone is expensive, masons don’t build stone arches entirely from stone. Gravel, sand and rubble are used to fill empty spaces in the building. Various substances such as tar or puddled clay, which is a mixture of clay and sand, may be added for waterproofing, and to ensure that the fill isn’t washed away by water.
Mortar is used to bind the stones in the bridge together so they form a single, solid mass. Mortar can be made from sand, lime and water, from earth and clay, or from portland cement. Because it’s semi-liquid when wet, it can be used to fill in spaces between the stones, which helps to distribute weight. It also keeps seeds and roots from growing between the stones, thus preventing the damage they would cause, and it keeps water out.
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