The scourge of modern concrete is, perhaps surprisingly, rust.
Most concrete structures are laced with steel rebar to add strength, but if the metal corrodes, it can cause the concrete to fail prematurely. Bridges, which are exposed to water and salt, are some of the most vulnerable. About one third of bridges in the United States need to be repaired or replaced, which could cost nearly $400 billion over the next decade.
There are plenty of ways engineers cope with rust, from coating the rebar with epoxy to pouring extra concrete to buy some time before seeping water reaches the rebar. Eventually these measures fail, too.
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