Clean Materials Necessary

Attention has been called elsewhere to the necessity for using aggregates that are clean. Foreign matter such as clay is particularly objectionable when in the form of a coating on the particles. Where they are so coated, the cement cannot come in contact with the surface of the particles and therefore cannot bind them together.

Washing Materials Sometimes Necessary

Where the only sand and pebbles obtainable contain foreign matter, it is necessary to wash them before using them in a concrete mixture. This can readily be done by building a trough like shown below. Materials to be washed are shoveled into the trough at the high end, while water is applied at the same end through a hose connected to a pipe. Plenty of water flowing down the trough will cause the materials to be cleaned by tumbling and rolling about. At the lower end a screen is placed to separate the sand from the pebbles. The water carries off the foreign material. Wedge-shaped cleats (riffles) are nailed on the bottom of the trough inside to assist in tumbling the materials as they roll down.

A simple home-made device for washing sand and pebbles and for separating the two from each other.

The foregoing description of aggregates and their physical and other requirements to fit them for use in concrete work applies particularly to the broad applications of concrete in building construction. There is another aspect of the subject of aggregates which pertains particularly to the use of concrete in the making of some small ornamental objects or wares, none of which is to be subjected to such tests of strength as is demanded of concrete in certain portions of buildings. For instance, a concrete window flower box need not have any more strength than necessary to protect it from the action of the elements or from possible breakage due to occasional handling. Therefore the permissible requirements of aggregates may be modified somewhat in applying them to school or home work for some small objects. For instance, it is much easier to secure a smooth surface on a concrete flower box or a vase, if the coarse aggregate is omitted entirely and a slight excess of cement used with sand only-the sand, of course, being properly graded. It is possible to give a very attractive surface finish to concrete work in a number of ways, some of which are controlled entirely by selecting aggregate in which the particles are all of uniform size or nearly so, and then treating this surface in a number of ways which will be described later. Pure, Clean Water Necessary.-The water used in concrete mixtures must be pure and clean. It should not contain acid, oil nor alkali, nor be cloudy, due to the presence of clay, loam, silt or similar matter in suspension. It should be good enough to drink. If it is of such quality, it will be best for concrete.

A simple easily made concrete fern box.