In principle, the concrete could even be allowed to develop slight tension, because any cracks would be limited by the tension in the rods. The improvement of the design of the reinforcement and its coupling to the concrete required many years of development, and is a specialist subject. These two pictures show a concrete beam bridge made by pouring concrete around steel reinforcing rods. Erosion has revealed the metal at the sides of the bridge, probably aided by corrosion of the metal. The potential of reinforced concrete was shown dramatically by Robert Maillart, who made art out of technology by recognising and following the logic of the materials.
Composite materials open up possibilities that are denied to the users of isotropic substances.
Straw in bricks, Tufnol, and fibreglass are a few examples. In 1934, Eugene Freysinnet repaired Le Havre station using stressing wires. He went on to develop the art of pre-stressed concrete to a high degree. Actually, it is not uncommon to see old buildings with metal plates and nuts on the outside, showing where rods have been passed right through to hold bulging walls. It used to be a joke that something was made from string and sealing-wax.