A common practice, which until lately was much insisted upon, is to tip the concrete, after mixing, from a height of 10 ft., or more, into the trench where it is to be deposited. This process is now considered objectionable, on the ground that the heavy and light portions separate while falling, and that the concrete is therefore not uniform throughout its mass. Wooden shoots or steeply-inclined troughs are sometimes used, down which the concrete is shot from the place where it is mixed to the site where it is to be used. Such shoots are also objectionable, because the larger stones have a tendency to separate from the soft portions of the concrete. Concrete should, after thorough nixing, be rapidly wheeled to the place where it is to be laid, gently tipped (through a height of not more than 3 ft.) into position, and carefully and steadily rammed in layers about 12 in. thick. Each layer should be left till it is perfectly set before another layer is put upon it. It is essential that the layers should be horizontal; if, not, the water trickling off will carry the cement with it. Each layer, after it is thoroughly set, should be carefully prepared to receive the one that is to rest upon it. Its surface should be carefully swept clean, wetted, and made rough by means of a pick.

This is especially necessary if it has been rammed, for in that case the finer stuff in the concrete works to the top, as also a thin milky exudation, which will, unless removed, prevent the next layer from adhering. The joints between the layers are the most important points to be attended to in concrete. When the proper precautions have not been taken, they are found to be sources of weakness, like veins in rocks, and the mass can easily be split with wedges. When there is not time to allow each layer to set before the concreting is continued, it is better to ram it as quickly as possible, and, before it is set, to add the layers above it. Anything is better than to allow the layers to be disturbed by ramming, by walking over them, or in any other way, after they have commenced to set. Concrete made with a very quick-setting cement should therefore not be rammed at all. When concrete has to be laid under water, care must be taken that it is protected during its passage down to the site of deposit, so that the water does not reach it until it is laid. This protection is afforded sometimes by shoots, by boxes, or by specially contrived iron "skips," which can be opened from above when they have reached the spot where the concrete is to be deposited, so as to leave it there.

Sometimes the concrete is filled into bags and deposited without removing the bags. Concrete is also made into blocks varying in size from 2 to 200 tons. These are allowed to set on shore, and deposited, the smaller ones in the same way as blocks of stone, those of enormous size by special arrangements which cannot here be described.

In the construction of walls or buildings of concrete, the latter has to be kept in place or supported by boards or otherwise, until dry and firm enough to be self supporting. Various kinds of suitable apparatus have been invented and patented, all more or less costly. A strong, simple, and inexpensive set may be made after the plan described and illustrated below. In Fig. 1333, which is a perspective view, the boards a b c d e are each made of 3 planks f 9 in. wide and 1 1/4 in. thick, planed on the inner side. The width of each board is thus 2 ft. 3 in., and the length may be various, - 4 ft., 5 ft., 6 ft. The 3 planks f forming each board are held together by a piece of angle iron, screwed on at each end, which also serves to retain the bolts g by which the boards are secured to the uprights h. The last are formed of a strip of board 2 in. wide by about 6 ft. long, to which is screwed a piece of channel iron () of the same width and length. The iron bolts g hold each pair of uprights at the required distance apart, to suit the thickness of the wall, as well as helping to tie together the boards on each side of the wall, and resisting the pressure of the moist concrete.

As the wall advances in height, the bottom boards c d e can be removed and placed above the next row a b, and so on; and when the wall is sufficiently firm, the uprights can be removed and fixed higher. In addition to the straight boards, there will be needed some angle boards for turning corners. Fig. 1334 is an elevation, and Fig. 1335 a horizontal section of the structure. Fig. 133G is a nearly full sized section showing details.

Laying 1176