In considering concrete foundations we must recognize that the human factor plays a greater part than where steel or timber is used, for these products come to us with known qualities while concrete has but one ingredient (cement) of the three that is known and we must depend upon a proper selection of sand and rock and the method of mixing and placing it to the judgment of some one who may or may not know the experiences of others.
The failures in concrete structures
The failures in concrete structures are seldom due to miscalculations of compressive stresses but to the results of vibratory forces and it is imperative that the human factor be elert in taking every precaution to make a good concrete and to form strong bonds between the machinery and the concrete and the concrete and the bedrock.
Some foundations fail from lack of care in selecting sand and rock. To insure against this contingency blocks of concrete should be made of the materials it is intended to use and subjected to physical tests.
Small sizes of concrete mixers run by gasolene or steam engines are so readily obtained at small cost that it will pay to use a mixer even for a small job, but no doubt many millwrights will prefer to mix by hand where but a small amount of concrete is.
Cement in concrete
For convenience the cement in concrete is often spoken of as the matrix while the sand, gravel or crushed stone is designed the aggregate. The cement should be kept in dampproof storage until ready to be used, and then used immediately, as rain or dampness causes it to set prematurely and renders it, if not useless, at least undesirable for good work.
Concrete should always be mixed fresh; that which has been mixed over Jhr. has an initial set and should be rejected. The sand should be hard, and angular and when it contains over 5 per cent, fine dirt is should be washed or screened.
The broken stone should be hard and sound and should be washed before using to remove all dirt or dust, so that the cement may adhere to the surfaces. If a particularly rich mixture is required for finishing the top of a structure the cement should be mixed with sand in the proportion of 2 or 3 of sand to 1 of cement.
Cement mixing process
Cement will not readily take up any more water than is necessary for mixing (16 per cent, by weight is necessary for thorough combination) ; the excess will come to the top, and does no harm unless carried to the extreme, causing a separation of the cement from the aggregate, thereby weakening the mixture.
A good way to judge of the right proportion of water is to fill a mold with the mixed concrete, when, upon striking the surface of the mass with the flat of a shovel, it will quake like jelly and water will appear at the point struck.
Wet concrete takes longer to set and eventually becomes harder than when the water is only in quantity sufficient to create adhesion. Concrete of this latter description should be well rammed until water comes to the surface.
While setting, concrete should be kept wet, for this retards the setting and, as stated in regard to wet concrete it produces a more lasting structure. It is not necessary for any piece of work to be of the same proportional composition throughout, for the richness of a concrete should be proportioned to the strains it must bear; for example, the base of a foundation being usually larger than the top may be made of a poorer mixture.