Concrete

Concrete structures were well known to the Romans, and those of their buildings which still exist, for example the Pantheon, bear witness to the excellence of their work.

Concrete is obtained by binding together with cement an inert fragmentary material known as the aggregate, and consisting of gravel, limestone, granite, broken bricks, or some other similar material. A graded aggregate is better than a uniform one, and the strength of the concrete depends on the strength of the aggregate and the distance of separation of the particles. Concrete structures may be reinforced by steel rods in such a way that tensile stresses are taken as far as possible by the steel, and compression stresses by the concrete. The chief danger in the use of reinforced concrete, especially for ships, lies in the possibility of the penetration of water or saline solutions to the iron, the consequent rusting causing expansion and breaking up of the concrete. The action has been ascribed to an electrolytic effect.

In the use of concrete tanks discrimination should be exercised as to the liquids introduced into them. Acids, especially those which form soluble calcium salts, must be avoided.