This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
No general rule can be given for laying out a plant for concrete work. Every job is generally a problem by itself, and usually requires a careful analysis to secure the most economical results. Since it is much easier and cheaper to handle the cement, sand, and stone before they are mixed, the mixing should be done as near the point of installation as possible. All facilities for handling these materials, charging the mixer, and distributing the concrete after it is mixed, must be secured and maintained. The charging and distributing are often done by wheelbarrows or carts; and economy of operation depends largely upon system and reguof operation. Simple cycles of operations, the maintenance of proper runways, together with clock-like regularity, are necessary for economy. To shorten the distance of wheeling the concrete, it is very often found, on large buildings, that it is more economical to have two medium-sized plants located some distance apart, than to have one large plant. In city work, where it is usually impossible to locate the hoist outside of the building, it is constructed in the elevator shaft or light well. In purchasing a new plant, care must be exercised in selecting machinery that will not only be satisfactory for the first job, but that will fulfil the general needs of the purchaser on other work. All parts of the plant, as well as all parts of any one machine, should be easily duplicated from stock, so that there will not be any great delay from any breakdown or worn-out parts.
The design of a plant for handling the material and concrete, and the selection of a mixer, depend upon local conditions, the amount of concrete to be mixed per day, and the total amount required on the contract. It is very evident that on large jobs it pays to invest a large sum in machinery to reduce the number of men and horses; but if not over 50 cubic yards are to be deposited per day, the cost of the machinery is a big item, and hand labor is generally cheaper. The interest on the plant must be charged against the number of cubic yards of concrete; that is, the interest on the plant for a year must be charged to the number of cubic yards of concrete laid in a year. The depreciation of the plant is found by taking the cost of the entire plant when new, and then appraising it after the contract is finished, and dividing the difference by the total cubic yards of concrete laid. This will give the depreciation per cubic yard of concrete manufactured.