ARBITRARY MIXTURES.
Rich mixture.—Used for concrete subject to high stresses or where exceptional water-tightness and resistance to abrasion are desired: 1:11/2:3; i. e., 1 part cement, 11/2 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.
A common mistake to be guarded against is to assume that the volume of concrete produced is equal to the quantity of sand plus the gravel as indicated in the proportion. For instance a 1:2:4 mixture will not produce 6 cubic yards of concrete, if 2 yards of sand and 4 yards of gravel are used, because the sand will lodge in the voids between the pebbles. If 6 cubic yards of concrete are desired it will be necessary to use 2.7 cubic yards of sand and 5.34 cubic yards of gravel.
Table 1 shows the quantity of cement, sand, and gravel required under average conditions for the indicated proportions.
Table 1.—Materials for 1 cubic yard of rammed concrete.
| Proportions. | Cement. | Sand. | Gravel. | ||
| Cement. | Sand. | Gravel. | |||
| Sacks. | Cu. yds. | Cu. yds. | |||
| 1 | 1 | --- | 19.20 | 0.74 | --- |
| 1 | 2 | --- | 13.48 | 1.00 | --- |
| 1 | 21/2 | --- | 11.00 | 1.01 | --- |
| 1 | 3 | --- | 10.40 | 1.16 | --- |
| 1 | 1 | 2 | 10.52 | .39 | 0.78 |
| 1 | 11/2 | 3 | 7.64 | .42 | .85 |
| 1 | 2 | 4 | 6.04 | .45 | .89 |
| 1 | 21/2 | 5 | 4.96 | .46 | .92 |
| 1 | 3 | 5 | 4.64 | .52 | .86 |
| 1 | 3 | 6 | 4.24 | .47 | .94 |
[CONSISTENCY.]
The quantity of water used in mixing has a very great influence on the strength of the concrete. An excess of water weakens the concrete, while an insufficient amount prevents thorough mixing.
Fig. 2.—The result of using too dry a mixture, lack of spading and careless placing; note irregularity of layers and poor bonding.
Therefore, only sufficient water should be used to produce a workable or plastic mixture.
Recent tests have proved that to secure the greatest strength the concrete should be mixed considerably drier than has heretofore been customary. Of course, for thin walls containing closely placed reinforcement, or for water-tightness, a fairly wet mix is necessary. A little experience will show the proper amount of water to use.
A very rough estimate of the quantity of water required in mixing for general work is 4 to 5 gallons to each sack of cement.
Three degrees of consistency (corresponding to different proportions of water) are used in general practice, namely, wet, medium, and dry. In the light of recent investigations it is thought the wet mixture of present-day practice contains too much water. The following definitions are therefore recommended:
Wet mixture. One that does not flow readily and yet can not be piled up. It is recommended for thin sections when reinforcement is closely placed.
Medium mixture. One that is between the wet and dry mixture. This consistency is recommended for general work.
Dry mixture. One about like damp earth. If a handful is squeezed it will retain its shape. This consistency requires thorough ramming to eliminate voids and is used when forms are to be removed immediately, but should not be used where a water-tight job is expected. The porous structure of the concrete in [Figure 2] is due to the fact that it was placed as a dry mixture.