Proportioning the Mixture
That mixture in which all the spaces (called “voids”) between the stone or gravel are filled with sand, and all the spaces between the sand are filled with cement, is the ideal mixture. This mixture is rarely attained, as the voids in each load of gravel and sand vary slightly, and in order to be absolutely safe, it is well to use a little more cement than will just fill the voids.
Fig. 1.—Quantities of cement, sand, and gravel in 1: 2: 4 concrete mixture, which means 1 part cement, 2 parts sand, 4 parts crushed stone or gravel, and the resulting quantity of concrete, which is only slightly greater in size than the gravel, the sand and cement filling the voids in the gravel.
Showing the Quantities of Materials and the Resulting
Amount of Concrete for Two-bag Batch.
| Kind of Concrete Mixture. | Proportions by Parts. | Two-bag Batch. | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| C e m e n t. | S a n d. | Stone or Gravel. | Materials. | Size of Measuring Boxes. Inside Measurements. | Water in Gallons for Medium Wet Mixture. | |||||
| Cement. | Sand. | Stone or Gravel. | Concrete | Sand. | Stone or Gravel. | |||||
| Bags. | Cu. ft. | Cu. ft. | Cu. ft. | Gallons. | ||||||
| 1:2:4 Concrete | 1 | 2 | 4 | 2 | 3¾ | 7½ | 8½ | 2′×2′ 11½″ | 2′×4′ 11½″ | 10 |
| 1:2½:5 Concrete | 1 | 2½ | 5 | 5 | 4¾ | 9½ | 10 | 2′×2½′ 11½″ | 2′×5′ 11½″ | 12½ |
As above explained, concrete is composed of a certain amount of cement, a larger amount of sand, and a still larger amount of stone (or gravel). To determine how much of each of these materials to use, we must first consider the type of work we wish to undertake. For ordinary work about the farm (silos, tanks, cisterns, fence posts, well curbs, etc., etc.) use twice as much stone as sand, and twice as much sand as cement. This is called a 1: 2: 4 mixture—meaning that there are in that mixture:
- 1 part of cement,
- 2 parts of sand,
- 4 parts of stone or gravel.
For sidewalks, gutters, etc., a “weaker” mixture is sometimes used, consisting of:
- 1 part of cement,
- 2½ parts of sand,
- 5 parts of stone or gravel.
The proportions should always be measured by volume, and the best way to do the measuring is by the use of a home-made “measuring box,” of any kind of rough boards having straight sides, but with no top or bottom. The size of these measuring boxes is determined by the proportion desired for your mixture. For such boxes you need the following sized lumber:
- 4 pieces 1 inch by 11½ inches by 2 feet rough (ends of sand and stone boxes).
- 2 pieces 1 inch by 11½ inches by 4 feet rough (sides of sand box).
- 2 pieces 1 inch by 11½ inches by 6 feet rough (sides of stone box).
Note: The two pieces 4 feet long and the two pieces 6 feet long have an extra foot in length at each end to be made into a handle, as shown in [Fig. 3.]
For a 1: 2½: 5 mixture, you require the following sized lumber:
- 4 pieces 1 inch by 11½ inches by 2 feet (ends of sand and stone boxes).
- 2 pieces 1 inch by 11½ inches by 4 feet 6 inches (sides of sand box).
- 2 pieces 1 inch by 11½ inches by 7 feet (sides of stone box).
Note: The two pieces 4 feet 6 inches long and the two pieces 7 feet long have an extra foot in length at each end to be made into a handle, as shown in [Fig. 3.]
To illustrate the use of the measuring box, let us once more assume that a 1: 2: 4 mixture is required, and that the amount of finished concrete needed is 8½ cubic feet. By referring to the [table on page 11] it will be noted that two bags of cement are required, also 3¾ cubic feet of sand and 7½ cubic feet of stone or gravel. Under “size of measuring box” it is found that the sand should just fill a box 2 feet by 2 feet by 11½ inches, and that the stone should fill a box 2 feet by 4 feet by 11½ inches. Lay the sand box, or frame, on the mixing platform and fill it. Then raise the box. Empty two bags of cement on the sand and mix as described under “[Mixing],” see pages 14-22. Even off the mixture thus obtained with your shovel, place the stone measuring box on top of the mixture and fill it. Raise the measuring box—and you have the correct amount of stone all ready to be mixed with the cement and sand. It is important to measure both the sand and stone loose in the box—never “pack” them.
For purposes of explanation, size of mixture will be referred to as a “batch” of so many bags of cement. Thus, a “two-bag batch of concrete” would mean one requiring two bags of cement, with the sand and stone proportioned accordingly, as shown above.
For a “four-bag batch of concrete” it would be necessary to multiply the amount of stone and gravel by 2, also multiplying the cubic contents of the measuring box by 2, and using four bags of cement instead of two.
The [table previously referred to] also shows the amount of water for different sized batches, but it is to be noted that the quantity of this ingredient is only approximated. Use the amount indicated in the table for the first batch, and if it proves too wet for the use desired, reduce the amount of water; if too dry, increase the amount of water. Always use a bucket in measuring the amount of water, as this secures uniform results.