If we look at the side of the question which consilers only the price of the raw materials in lessening the cost of a structure, we find that the cost of a cement structure can be lessened with respect to the cost of the cement which it contains in two ways: (a) by employing a cheap quality; or (b) by using a small quantity.
A limit as to quality has been mutually agreed upon by a large majority of cement manufacturers and users by the adoption of a standard specification for cement (see under "Materials and Manufacture of Concrete"). As to a minimum quantity, it may again be pointed out that the proper amount depends upon the characters of the other ingredients, upon the uses to which the completed structure is to be put, and upon the strength or quality required of the mixture under the governing conditions.
The cement is the binder holding together the aggregates which make up the major por tion of the concrete. These aggregates may be sand and stone or gravel in varying proportions, depending upon circumstances.
As to the relative costs of different mixtures of the various concrete ingredients, it is evident that the poorer the mixture (that is, the smaller the proportion of cement it contains) the less the cost per cubic yard. However, concrete of least cost may not be profitable under all circumstances.
If we consider the cost of handling the in gredients, the following points are worthy of notice: Make as much use of the force of grav ity as possible. It is a common practice, on large works, to have high trestles and bins for the storage and handling of concreting materials. From these, the materials move downward by gravity through the mixers and into the con veying devices.
Another point is to have the mixing done as near the point of installation of the concrete as possible, since it is easier and cheaper to handle the dry material than the wet mixture. In the case of one large building with structural steel frame, a small electrically-driven portable mixer was used, and moved about in such a manner that it discharged directly at the point where the concrete was to be placed.
The labor item is another place wherein costs may be reduced—not necessarily by employing cheaper help, but by the economical handling of the men on the work. A few extra men who do
nothing but the little odd jobs such as keeping planks in place for the men with the wheelbar rows can often save considerable time and money. The forming of a cycle of operations with each man having his particular part, is an ideal condition.
Where several thousand yards of concrete is to be raised to a considerable height, the use of platform elevators, bucket hoists, or derricks is recommended as a means of reducing the cost. The actual cost per cubic yard of hoisting con crete by each of these methods varies by only a few cents. With regard to other mechanical de vices, it is sometimes found that the interest on their cost, together with their depreciation, more than offsets the cost of labor performed by them. In case of a bonus allowed for quick work, these devices are often used to an advan tage on account of their saving of time.
Every one at all conversant with the costs of reinforced concrete work knows only too well the disproportionate amount due to centering. In very heavy foundation work, this proportion may not be excessive; but in some buildings of reinforced concrete, the labor cost of installing and removing the wooden falsework, together with the cost of the material itself, has made up 50 per cent of the entire expenditure.
The cost of the centering material itself is heavy, and, when wood is considered, is growing heavier almost month by month. This is due to the constant and rapid increase in the prices of timber and lumber of all kinds, which has recently taken place.
The first plan which comes to mind, looking toward the economizing of centering material, is to make repeated use of each piece. This can readily be accomplished where a building is largely a repetition from floor to floor or from bay to bay. But unless the structure is very large, a considerable time is lost through the necessary delays experienced while waiting for the concrete to set before the centers can be removed. This is the case, however, only where the concrete is handled in a wet state.