A MODERN street consists of a concrete foundation which extends from curb to curb, upon which is laid a wearing surface of asphalt, brick or other material.
The use of these concretes has an instructive history, which might be profitably preceded by a discussion of the uses of mortars and cements in antiquity, did space permit. So far as I have been able to learn, all the different varieties of cementing materials, including ordinary lime mortars, have been experimented with for the construction of concrete foundations. It is therefore proper that these different materials should be briefly described.
Mortar, in the ordinary sense of the term, designates a mixture of lime and sand. The lime is prepared by heating limestone in kilns, until the carbonic acid of the limestone is expelled and oxide of calcium remains, which readily absorbs water and slacks, as it is termed, and in time reabsorbs the carbonic acid that was driven off. The lime is mixed with the water when it is slacked and the carbonic acid is absorbed from the atmosphere. When mortar is made, the lime is first made into a thin paste with water, and sand is added until the mass ceases to be sticky. Such mortar acquires strength slowly. The excess of water first dries out and then the lime by slow absorption of carbonic acid forms thin particles of limestone between the grains of sand, until the mortar becomes a coherent mass. That this process goes on very slowly is shown by the fact that the mortar between the bricks of chimneys centuries old is found to contain a considerable percentage of unchanged lime. This mortar, when first laid, will not bear wetting, and will set only in dry air.
The Romans had learned before the Christian Era that the addition to lime mortar of volcanic ash or pozzuolana would make the mortar set under water and with additional strength. The so-called Roman cement was noted in antiquity for its superior strength when compared with ordinary lime mortar. Where they could not obtain the pozzuolana they used pulverized brick and pottery.
During the middle ages, for more than a thousand years, the art of making hydraulic cement was lost, and, with every other art, the art of making good mortar declined until the beginning of the eighteenth century, when attempts were made to revive the art of making Roman cement, but with only slight success.