2. Passive and active cables, guy wires and tie rods

There's one area of civil engineering where the king is no longer concrete, but steel: cables, guy wires and tie rods. The reason is simple. Of all the widely-used materials, steel is the archetypal one associated with tensile strength, and at a price compatible with the scale of use (a cable several hundred meters long and several tens of centimeters in diameter is commonplace for a suspension bridge). In the long term, however, it's fair to wonder whether carbon nanotube (NT) cables will not supplant their metallic counterparts.