The first section of Table 60 gives the amount of materials required for Portland cement mortar when the proportions are stated by weight; the second and third sections refer to proportions by volume of loose sand to packed cement when the size of the cement barrel is assumed at 3.65 cu. ft. and 3.33 cu. ft., respectively. The fourth section gives the materials required when the proportions are given in terms of loose sand to loose cement. Likewise, the first section of Table 61 for natural cement refers to proportions by weight; the second, third and fourth sections, to proportions by volume of loose sand to packed cement when the cement weighs 265 pounds, 280 pounds and 300 pounds, net. per barrel, respectively; while the fifth section refers to proportions of loose sand to loose cement.
As has been shown, the method of stating proportions by weight is the most accurate, but when the sand does not approximate the weight of 100 pounds per cubic foot when shoveled dry into a measure, the sections of the tables referring to weight proportions may require a correction, and it may be simpler to use the sections giving proportions by volume of loose sand to packed cement. The method of stating proportions by volumes of loose sand to loose cement is to be deprecated, but since it is occasionally used, provision is made for it in the tables.
In using those portions of the tables where the proportions are stated by volume, it should be borne in mind that if the sand is damp when used it will weigh less per cubic foot, and hence more, by measure, will be required to make a cubic yard of mortar.