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."