MACHINERY FOR CONCRETE WORK 331. Concrete Plant. 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 regu larity of 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 re quired 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.
332. Concrete Mixers. The best concrete mixer is the one that turns out the maximum of thoroughly mixed concrete at the mini mum of cost for power, interest, and maintenance. The type of mixer with a complicated'motion gives better and quicker results than one with a simpler motion. There are two general classes of con crete mixers—continuous mixers and batch mixers. A continuous mixer is one into which the materials are fed constantly, and from which the concrete is discharged constantly. Batch mixers are con structed to receive the cement with its proportionate amount of sand and stone, all at one charge; and, when mixed, it is discharged in a mass. No very distinct line can be drawn between these two classes, for many of these mixers are adapted to either continuous or batch mixing. Generally, batch mixers are preferred, as it is a very diffi cult matter to feed the mixers uniformly un less the materials are mechanically measured.
Continuous mixers usually consist of a long screw or pug mill, that pushes the ma terials along a drum until they are dis charged in a continu ous stream of concrete. Where the mixers are fed with automatic measuring devices, the concrete is not regular, as there is no recipro cating motion of the materials. In a paper recently read before the Association of American Portland Cement Manufac turers, S. B. Newberry says: "For the preparation of concrete for blocks in which thorough mixing and use of an exact and uniform proportion of water are necessary, continuous mixing machines are unsuitable; and batch mixers, in which a measured batch of the material is mixed the required time, and then discharged, are the only type which will be found effective." There are three general types of concrete mixers: gravity mixers, rotary mixers, and paddle mixers.