Why Do Bridges Corrode?

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When stories of corrosion control issues make the headlines, it usually refers to bridges, in part because they are major structures that are both heavily used and need to be extremely robust to stay safe.

At present, there are several high profile bridge repairs taking place which were required as a result of metal corrosion. One of the most significant of these is the closure of Hammersmith Bridge in London, in part caused by decades of untreated metal corrosion.

Here is why bridges are prone to corrosion and what can be done to mitigate or slow its progression.

Metal Supports

Most major bridges and other structures use huge amounts of steel in their construction. This is not just seen in suspension bridges where huge metal cables support the load of the many pedestrians and drivers on the deck below.

Most concrete that is used in construction has steel rebar embedded in it during the pouring processes as they resist forces that pull a structure in directions that regular concrete would be weak.

However, if rebar is exposed to water or the open air, the resulting flakes of rust expand and crack the concrete from within.

A Bridge’s Purpose

Many bridges are built across bodies of water, which create particular challenges to any metal element of a bridge.

The reason for this is that seawater contains a high level of salt compared to normal water, and this can have a devastating effect on exposed metal if it splashes on or soaks into metal rebar and suspension cabling.

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