Concrete Construction for the Home and the Farm

by Atlas Portland Cement Company

Having discovered that the sand you contemplate using is not clean, and provided you cannot readily obtain any that is clean, you may use what you have, provided you wash it in the following manner:—

Build a loose board platform from 10 to 15 feet long, with one end a foot higher than the other. On the lower end and on the sides, nail a board 2 by 6 inches on edge, to hold the sand. Spread the sand over this platform in a layer three or four inches thick, and wash it with a hose. The washing should be started at the high end, and the water allowed to run through the sand and over the 2 by 6-inch piece at the bottom. A small quantity of clay or loam does not injure the sand, but any amount over 5 per cent. does.

Stone or Gravel

This is known as the “coarse aggregate” of concrete. Great care should be used in its selection. The pebbles should be closely inspected to see that there is no clay on their surface. A layer of such clay prevents the “binding” of the cement. If necessary stone or gravel may be washed in the same way as above described for sand. Indeed, it is more easily done than sand, as the water flows through the larger voids in the gravel more readily than through the voids in the sand. Dust may be left in the crushed stone without fear of its interfering with the strength of the cement, but care should be taken to see that such dust is distributed evenly through the whole mass, and when dust is found in stone, slightly less sand should be used than ordinarily.

As to the size of stone or gravel, this must be determined by the form of construction contemplated. For foundations or any large thick structure, use anything from ½ to 2½ inches in diameter. For thin walls use ¼ to 1-inch stone.

The best results are obtained by the use of a mixture of sizes graded from small to large. By this means the spaces or voids between the stones or pebbles are reduced and a more compact concrete is obtained. Moreover, this method makes it possible to get along with less sand and less cement.

Pure Water Necessary in Mixing

Water for concrete should be clean and free from strong acids and alkalies. It may be readily stored in a barrel beside the mixing board and placed on the concrete with a bucket. If you are at all in doubt about the purity of the water that you contemplate using, it would be well to make up a block of concrete as a test, and see whether the cement “sets” properly.

Proportioning the Mixture

That mixture in which all the spaces (called “voids”) between the stone or gravel are filled with sand, and all the spaces between the sand are filled with cement, is the ideal mixture. This mixture is rarely attained, as the voids in each load of gravel and sand vary slightly, and in order to be absolutely safe, it is well to use a little more cement than will just fill the voids.