129. The concrete should be made of a good quality of Portland or best Rosendale cement, mixed with clean, sharp sand, and a proper amount of aggregates. The proportions vary for different classes of work, but a common ratio is 1 part of cement, 2 parts of sand, and 3 or 4 of crushed stone, or similar material. Any natural stone, gravel, broken brick, etc. may be used, but, whatever it is, it should be uniform in color and of an even grade. If a very close imitation of a natural stone is desired, the same stone should be crushed, and in addition, color should be mixed with the cement, to correspond with that of the stone. The finer the stone is crushed, the nearer will be the resemblance to real stone. For very good work, it is sufficient to break the stone to the size of buckshot or fine gravel.

In the construction of the large Florida hotels, before referred to, the concrete was composed of 1 part Portland cement, 2 parts sand, and 3 parts of coquina, a small shell found in large quantities in the vicinity. These shells are so small that most of them will pass through a 1/2-inch mesh sieve. The color of this concrete is a light gray, hardly darker than Indiana sandstone or Westerly, R. I., granite. When a different effect was required, coloring matter was introduced into the cement. The cost of the concrete in place varied from $5 to $8 per cubic yard, including arches, columns, etc.

130. Machine Mixing

Machine Mixing. Concrete mixing by hand has been described in Masonry, § 7. When, however, large quantities are needed, it is usually mixed by machine. The most common one consists of a long screw, about 18 inches in diameter, enclosed in a slightly inclined iron casing of a little greater diameter; the screw is operated by a crank, turned either by hand or by power. The cement, sand, and stone are introduced through hoppers at the upper end, and the water is added through pipes at intervals along the length, and as the screw revolves, the materials become thoroughly incorporated. The concrete is forced out at the lower end of the machine, falling into the bucket or other conveyor. After the concrete is made, the time between the mixing and the ramming in place should be as short as possible, in order to prevent the cement from becoming set.

To hoist the prepared material, a traveling crane is sometimes used, the concrete being placed in buckets, which are transported to the proper place by the crane. Another plan is to have an elevator built at a central point on the site and carried up to the full height of the intended building, runways being made to each floor, as the work progresses.

131. Laying Concrete

Laying Concrete. When the concrete has been dumped out of the conveyor at the proper place, it is spread out on top of the previous layer, the top of which should be somewhat rough and uneven, so as to make a better bond with the new work. The fresh concrete is then settled, by ramming or otherwise, and the operation is continued until the concrete is thoroughly consolidated, and the cement flushes to the surface, which, as before, should be left rough for the succeeding layer. Too much care cannot be exercised in these operations, as on the compactness and uniformity greatly depends the value of concrete construction.

The making and placing of concrete should be very closely inspected, to insure the specified quality and proportions of cement and other materials, and proper manipulation. The cement used should be frequently tested during the progress of the work. Some simple tests are given in the articles on "Cements," in Masonry, § 7. On large and important work, an inspector should be kept constantly on the ground, to see that the concrete is properly mixed and laid.