| Per Cent Voids | Wt. per Cu. Ft. | |
|---|---|---|
| Pebbles, about 1 inch | 38.7 | 101.2 pounds |
| Sand, 30 to 40 mesh | 35.9 | 105.8 pounds |
| Pebbles plus 38.7 per cent sand, by vol. | 19.2 | 133.5 pounds |
Experiments have shown that the strength of concrete increases greatly with its density; in fact, a slight increase in weight per cubic foot adds very decidedly to the strength.
The gain in strength obtained by adding coarse material to mixtures of cement and sand is shown in the following table of results of experiments made in Germany by R. Dykerhoff. The blocks tested were 2 1/2-inch cubes, 1 day in air and 27 days in water.
| Proportions by Measure. | Per Cent. Cement. | Compression Strength | ||
|---|---|---|---|---|
| Cement. | Sand. | Gravel. | By Volume. | Lbs. per Sq. In. |
| 1 | 2 | — | 33.0 | 2,125 |
| 1 | 2 | 5 | 12.5 | 2,387 |
| 1 | 3 | — | 25.0 | 1,383 |
| 1 | 3 | 6 1/2 | 9.5 | 1,515 |
| 1 | 4 | — | 20.0 | 1,053 |
| 1 | 4 | 8 1/2 | 7.4 | 1,204 |
These figures show how greatly the strength is improved by adding coarse material, even though the proportion of cement is thereby reduced. A mixture of 1 to 12 1/2 of properly proportioned sand and gravel is, in fact, stronger than 1 to 4, and nearly as strong as 1 to 3, of cement and sand only.
In selecting materials for concrete, those should be chosen which give the greatest density. If it is practicable to mix two materials, as sand and gravel, the proportion which gives the greatest density should be determined by experiment, and rigidly adhered to in making concrete, whatever proportion of cement it is decided to use. Well-proportioned dry sand and gravel or sand and broken stone, well shaken down, should weigh at least 125 pounds per cubic foot. Limestone screenings, owing to minute pores in the stone itself, are somewhat lighter, though giving equally strong concrete. They should weigh at least 120 pounds per cubic foot. If the weight is less, there is probably too much fine dust in the mixture.
The density and strength of concrete are also greatly improved by use of a liberal amount of water. Enough water must be used to make the concrete thoroughly soft and plastic, so as to quake strongly when rammed. If mixed too dry it will never harden properly, and will be light, porous, and crumbling.
Thorough mixing of concrete materials is essential, to increase the density and give the cement used a chance to produce its full strength. The cement, sand, and gravel should be intimately mixed dry, then the water added and the mixing continued. If stone or coarse gravel is added, this should be well wetted and thoroughly mixed with the mortar.
Materials For Concrete Building Blocks.
Cinders are sometimes used for block work; they vary greatly in quality, but if clean and of medium coarseness will give fair results. Cinder concrete never develops great strength, owing to the porous character and crushability of the cinders themselves. Cinder blocks may, however, be strong enough for many purposes, and suitable for work in which great strength is not required.