In mixing concrete various proportions of cement, sand, gravel, and water are employed, depending upon the purpose for which the concrete is to be used. The ideal mixture is one in which all the spaces or voids between the grains of sand are filled with the cement and all the voids in the gravel are filled with the cement-sand mortar. This perfection is seldom attained, because the voids in each lot of gravel and sand vary slightly, and in order to be absolutely safe a little more sand and cement than will just fill the voids are used. The strongest concrete is not required in every structure, and, in many instances, the cost of it would be unwarranted. For important work involving large quantities of materials of unknown qualities, tests should be made to determine the best proportions. Such tests, being rather complicated, are made usually in a laboratory, and are not practical for the user of small quantities of concrete. Various proportions have been tested by experienced engineers to determine which, under average conditions, will develop the greatest strength, best resist wear, and assure greatest impermeability or water-tightness. The mixtures given below have been found to meet the requirements indicated, and having been adopted as arbitrary standards, are recommended for use in farm concrete work. The amount of water required is discussed under "Consistency." ARBITRARY MIXTURES. =Rich mixture.=--Used for concrete subject to high stresses or where exceptional water-tightness and resistance to abrasion are desired: 1:1-1/2:3; i. e., 1 part cement, 1-1/2 parts sand, and 3 parts gravel. =Standard mixture.=--Used generally for reinforced concrete and water-tight work: 1:2:4; i. e., 1 part cement, 2 parts sand, and 4 parts gravel. =Medium mixture.=--Used for plain concrete of moderate strength: 1:3:5; i. e., 1 part cement, 3 parts sand, and 5 parts gravel. Leaner mixtures are sometimes used after a test has proved them to be suitable for the work at hand. It will be noticed that always in indicating the proportions the first number refers to the cement, the second to the sand, and the third to the gravel. The three materials must be measured by volume, using the same unit. The cubic foot is a convenient measure, because a sack of cement, weighing 94 pounds, is considered to contain 1 cubic foot. When the coarse aggregate (gravel, etc.) is omitted the mixture is generally spoken of as mortar and the proportions are indicated thus, 1:2, meaning 1 part cement and 2 parts sand. Mortar is used for plastering, stucco, top coats of floors, and for laying masonry. QUANTITIES OF MATERIALS REQUIRED. More concrete can be made from given volumes of aggregates if the gravel used is graded from fine to coarse than if the particles are too nearly of one size, because the small stones help to fill the voids between the larger ones and less sand-cement mortar is required. The extra mortar thus adds to the volume of the concrete.