Origin Of Sands And Pebbles

Both fine and coarse aggregates should be selected with some regard as to the source from which they are derived, that is, the physical properties

\ of the materials. Sand and pebbles have different origins, that is, are of different kinds, depending upon the rock formation from which they originally came. The harder the rock, the better it is in general for concrete work. It will make a stronger concrete and one less subject to wear, if it is to be exposed to such usage as in walks, driveways, stable floors and other pavements.

Sand grains which can easily be crushed by rubbing between the palms of the hands or by slight pressure are not fit for use. These have probably had their origin in some shaly or clay-like rock and will not help to make a strong concrete. Quartz or flint sands, as a general rule, are preferable to all others. They contain less free mineral matter and the individual grains have great strength. Some sands have their origin in hard crystalline limestone. These also are excellent for use in concrete. Sometimes screenings from crushed stone that are clean and free from dust if graded from 1/4 inch downward, are used the same as natural sand when making concrete. Any sand that contains clay, loam, dirt, or similar matter should not be used until such foreign material has been removed by washing-. This can readily be done in a number of ways, and simple equipment for the purpose will be illustrated and described later.

Some flower boxes, vases, pedestals and posts-work of manual training pupils with concrete.

Pebbles Or Broken Stone

In selecting pebbles or broken stone for concrete, precautions similar to those outlined for the selection of sand should be followed. If pebbles are used instead of broken stone, they should be hard and durable in nature and should not be long and flat in shape. Such pieces do not compact solidly, so the concrete in which they might be used would not be so strong nor as dense as when the pebbles are nearly round or egg-shaped. When broken stone is used instead of pebbles, it should have been obtained by crushing some one of the hard, durable rocks such as granite, trap rock or a hard limestone. Some kinds of slag, which is the waste product from smelting iron ore, make a good coarse aggregate when crushed to proper sizes.

In selecting aggregates for concrete, one other thing should be borne in mind; that is, concrete is a wonderful fire-resisting material and its fire-resisting qualities can be greatly improved by selecting a hard, quartz-like, or siliceous sand, and a hard coarse aggregate like trap rock or slag, both of which have in nature, or from the source of their origin, been exposed to high heat, hence will stand such exposure in the concrete.