Concrete has long been used for the foundations of structures of all kinds, and for filling in the spandrils of arches or the hearting and backs of walls.

Of late years, as the material has improved, it has been employed for many other purposes, a few only of which can now be mentioned.

The walls of ordinary houses, as well as the more massive walls of engineering structures, are now frequently built in concrete, either in continuous mass or in blocks.

Concrete is also used for walls in the form of slabs fitted into timber quartering; and in hollow blocks, something like those of terra cotta (see p. 126), filled in with inferior material.

This material is also adapted for arches, for stairs, for flooring of different kinds (see Part II.; p. 371), and even for roofs.

It can easily be made in slabs well fitted for paving (see p. 76), and by the use of wooden moulds can readily be cast in the form of window sills, lintels, dressings of all kinds, steps, etc., and can even be used for troughs and cisterns.

Drain pipes and segments of sewers are also sometimes made of concrete. It was thought that the acids in sewers might act upon the cement, but this has been found practically not to be the case.

The different methods of building monolithic walls, of making blocks, and of casting concrete into different forms, cannot here be entered upon.

Beton is a name given by some writers to any concrete made with hydraulic lime or cement.

By others a distinction is made between the two, concrete being the name given when the materials are all mixed together at once, and beton when the mortar is made separately.

Practically, however, the word "concrete" covers any form of artificial conglomerate, except artificial stones, which receive distinct names under various patents (see p. 74).

Coignet's Be'ton Agglomere' is a description of concrete made from a mixture of Portland cement and lime, to which is added a large proportion of sand, no gravel or broken stone being used.

The ingredients are moistened with a minimum quantity of water and pugged in a special mill; after which the mixture is thrown into a framework of the shape the concrete is intended to assume, and rammed in layers about 6 inches deep.

This material has been largely used in making the Paris sewers, and also occasionally in this country.

Some experiments made to contrast Coignet's Beton with Portland cement concrete showed the former to be a weaker material than the other.1

1 M.P.I.C.E., vol. xxxii. Grant's Experiments.

Rock Concrete Tubes with rebated end joints are made at the Bourne Valley Works from the best Portland cement with carefully selected aggregates. These are filled into iron moulds by machinery under heavy percussive action. They are used chiefly as a substitute for brick sewers of from 21 to 36 inches diameter, and are found superior to them in every way.1

Experiments on the Resistance of Concrete to Compression.

The following particulars are extracted from the accounts of the well-known experiments by Mr. J. Grant.2