There are other ratios depending on the proportion of sand. Thus we have:
| Cement | Sand | Aggregate |
|---|---|---|
| 1 | 1 1/2 | 4 1/3 |
| 1 | 2 | 5 |
| 1 | 2 1/2 | 6 |
| 1 | 3 | 7 |
| 1 | 3 1/2 | 7 1/2 |
| 1 | 4 | 8 1/4 |
The cost of concrete may be materially reduced without affecting the strength or efficacy of the work, by a plentiful use of stone “plums” or “burrs.” These are bedded in the fluid concrete during its deposition in situ, but care must be taken to see that they are thoroughly surrounded by mortar and not in contact with each other. Furthermore, if they are of a porous nature, they should be well wetted before use.
The mixing of concrete is important. If done by hand, the materials forming the aggregate will be laid out on a platform and covered by the cement in a thin layer. The whole should be turned over thrice in the dry state, and as many times wet, before depositing, in order to bring about thorough and complete amalgamation. Once mixed, the concrete is to be deposited immediately and allowed to remain undisturbed until the action of setting is finished. Deposition should be effected, wherever possible, without tipping from a height of more than about 6 feet, as in greater falls there is a likelihood of the heavier portions of the aggregate separating from the lighter. In extensive undertakings, concrete is more economically mixed by mechanical appliances.
The water used for mixing may be either salt or fresh, so far as the strength of the concrete is concerned. For surface work above the ground level, salinity in any of the ingredients is objectionable, since it tends to produce efflorescence—an unsightly, floury deposit, difficult to get rid of. The quantity of water required cannot be stated with exactitude; it will depend upon the proportion of the aggregate and its porosity. It is best determined by experiment in each particular case. Without being profuse enough to “drown” the concrete, it should be plentiful enough to act as an efficient intermediary between every particle of the aggregate and every particle of the matrix. Insufficient moisture is, in fact, as deleterious as an excess.
Voids.
The following table gives the per cent of voids and weight per cubic foot of some common concrete materials:
| Per Cent Voids | Wt. per Cu. Ft. | |
|---|---|---|
| Sandusky Bay sand | 32.3 | 111.7 pounds |
| Same through 20-mesh screen | 38.5 | 101.5 pounds |
| Gravel, 1/8 to 1/4 inch | 42.4 | 95.0 pounds |
| Broken limestone, egg-size | 47.0 | 87.4 pounds |
| Limestone screenings, dust to 1/2 inch | 26.0 | 122.2 pounds |
It will be noted that screening the sand through a 20-mesh sieve, and thus taking out the coarse grains, considerably increased the voids and reduced the weight; thus decidedly injuring the sand for making concrete.
The following figures show how weight can be increased and voids reduced by mixing fine and coarse material: