By itself, concrete is a very durable construction material. The magnificent Pantheon in Rome, the world’s largest unreinforced concrete dome, is in excellent condition after nearly 1,900 years. And yet many concrete structures from last century – bridges, highways and buildings – are crumbling. Many concrete structures built this century will be obsolete before its end.
Given the survival of ancient structures, this may seem curious. The critical difference is the modern use of steel reinforcement, known as rebar, concealed within the concrete. Steel is made mainly of iron, and one of iron’s unalterable properties is that it rusts. This ruins the durability of concrete structures in ways that are difficult to detect and costly to repair.
While repair may be justified to preserve the architectural legacy of iconic 20th-century buildings, such as those designed by reinforced concrete users like Frank Lloyd Wright, it is questionable whether this will be affordable or desirable for the vast majority of structures. The writer Robert Courland, in his book Concrete Planet, estimates that repair and rebuilding costs of concrete infrastructure, just in the United States, will be in the trillions of dollars – to be paid by future generations.
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Steel reinforcement was a dramatic innovation of the 19th century. The steel bars add strength, allowing the creation of long, cantilevered structures and thinner, less-supported slabs. It speeds up construction times, because less concrete is required to pour such slabs.
These qualities, pushed by assertive and sometimes duplicitous promotion by the concrete industry in the early 20th century, led to its massive popularity.
Reinforced concrete competes against more durable building technologies, like steel frame or traditional bricks and mortar. Around the world, it has replaced environmentally sensitive, low-carbon options like mud brick and rammed earth – historical practices that may also be more durable.