Naturally mixed bank sand and gravel are sometimes found in the right proportions for making concrete. Generally, however, there is far too much sand for the gravel, and great care should be exercised in using this class of material. Unless the mixture runs very even throughout the bank, and is found to be made up of one part sand to two parts gravel, it is better to screen the sand out of the gravel and prepare the materials in the usual way.
Herewith is a table showing the quantities for a natural mixture of bank sand and gravel. The quantities can be found in the same way as in [Table I, on page 11].
TABLE II.
Showing the Quantities of Materials and the Resulting Amount of Concrete for Two-bag Batch, Using Natural Mixture of Bank Sand and Gravel.
| Kind of Concrete Mixture. | Proportions by Parts. | Two-bag Batch for Natural Mixture of Bank Sand and Gravel. | |||||
|---|---|---|---|---|---|---|---|
| C e m e n t. | Natural Mixture of Sand and Gravel. | Materials. | C o n c r e t e. | Size of Measuring Boxes. | Water in Gallons for Medium Wet Mixture. | ||
| C e m e n t. | Natural Mixture of Sand and Gravel. | Mixture of Sand and Gravel. | |||||
| Bags. | Cu. ft. | Cu. ft. | Gallons. | ||||
| 1:2:4 Concrete | 1 | 4 | 2 | 7½ | 8½ | 2′×4′ 11½″ | 10 |
| 1:2½:5 Concrete | 1 | 5 | 2 | 9½ | 10 | 2′×5′ 11½″ | 12½ |
There are three kinds of mixtures, in general, on concrete work:—
1st.—Very Wet Mixture.—Concrete wet enough to be mushy and run off the shovel when handling, used for thin walls or for thin sections, etc.
2d.—Medium Mixture.—Concrete just wet enough to make it jelly-like, used for foundations, floors, etc. To better describe this mixture it may be said that a man should sink ankle deep if he were to step on top of the pile.
3d.—Dry Mixture.—Concrete like damp earth, used for foundations, etc., where it is important to have the concrete “set” up as quickly as possible.
The difference between the mixtures is, that the dryer the mixture the quicker will the concrete “set up”—but in the long run, when carefully mixed and “placed,” the results from any of the above mixtures will be identical. It may be said, however, that a dry mixture is the harder to handle, must be protected with greater care from the sun or from drying too quickly; and lastly, is likely—unless used by most experienced hands—to show voids or stone pockets in the face of the work when the “Forms” are removed. The less the voids in the stone or gravel, the greater will be the volume of the concrete. In general, the amount of concrete will be greater in each instance than is shown in the table—especially when gravel is used.