This formula is rational and perfectly general. Other experimenters may find it desirable to use constants slightly different from the 1.1 and the 0.9, for fine sands swell more than coarse sands, and hold more water.
The reader must bear in mind that when the voids in the sand exceed the cement paste, and when the available voids in the gravel (or stone) exceed the mortar, the formula becomes:
| 27 | |
| N | = ——— |
| ng |
These formulas give the amounts of cement in mortars and concretes compacted in place. Tables X to XIII are based upon the foregoing theory, and will be found to check satisfactorily with actual tests.
In using these tables remember that the proportion of cement to sand is by volume, and not by weight. If the specifications state that a barrel of cement shall be considered to hold 4 cu. ft., for example, and that the mortar shall be 1 part cement to 2 parts sand, then 2 barrel of cement is mixed with 8 cu. ft. of sand, regardless of what is the actual size of the barrel, and regardless of how much cement paste can be made with a barrel of cement. If the specifications fail to state what the size of a barrel will be, then the contractor is left to guess.
Table X.—Barrels of Portland Cement per Cubic Yard of Mortar.
(Voids in sand being 35%, and 1 bbl. cement yielding 3.65 cu. ft. of cement paste.)
| Proportion of Cement to Sand | 1 to 1 | 1 to 1½ | 1 to 2 | 1 to 2½ | 1 to 3 | 1 to 4 |
| Bbls. | Bbls. | Bbls. | Bbls. | Bbls. | Bbls. | |
| Barrel specified to be 3.5 cu. ft. | 4.22 | 3.49 | 2.97 | 2.57 | 2.28 | 1.76 |
| " " " 3.8 " | 4.09 | 3.33 | 2.81 | 2.45 | 2.16 | 1.62 |
| " " " 4.0 " | 4.00 | 3.24 | 2.73 | 2.36 | 2.08 | 1.54 |
| " " " 4.4 " | 3.81 | 3.07 | 2.57 | 2.27 | 2.00 | 1.40 |
| Cu. yds. sand per cu. yd. mortar | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | 1.0 |