Modern civilisation is built on concrete and steel. Put the two together, though, and you can generate a problem.

Reinforcing concrete with steel rods called rebars is the basis of modern construction. But because water gets in through tiny cracks, the rebars rust. This causes them to expand, widening the cracks and weakening the concrete.

Hence such structures require constant attention and often have design lives of only 60-100 years. That is pitiful compared with, say, the concrete dome of the Pantheon in Rome—which was completed in 125ad and still stands.

Various ways of delaying or preventing concrete cancer, as this corrosion is known colloquially, have been tried. These include recipes for concrete that is less permeable to water, and rebars made from rust-resistant materials such as stainless-steel or composites. Such approaches work, but they can be expensive.