515. Transporting To Place Of Deposition

In depositing the concrete in place,.care must be taken not to undo the work of mixing. If the concrete is allowed to fall freely a distance of several feet or to slide down an inclined plane, the stones will be likely to separate from the mass, and the result will be a layer of broken stone followed by a layer of mortar. If the concrete is deposited in a pile, the stone will roll down the outside of the cone. This action is especially bad in concrete that is mixed rather dry. The author has seen a pavement foundation in which the limits of each wheelbarrow load of concrete could be distinguished, the foundation presenting the appearance of the cross-section of a honeycomb, made up of irregular hexagons outlined by broken stone having a deficiency of mortar. In all such cases, if the action cannot be avoided by some other method of dumping, then care must be taken to remix the concrete.

There is one method by which the concrete may be deposited by gravity without separation of the materials. This consists in allowing the material to slide down a tube, but the tube must be kept continually full, the concrete being allowed to run out at the bottom only as fast as it is filled in at the top. This method is only applicable where the mixing is continuous, as in the case of machine mixers.

516. Sometimes it will be found possible to mix the concrete so near to the place of deposition that it may be shoveled directly into place. In mixing by hand this is practicable, as the mixing platforms may usually be easily moved, and this method of deposition is carried out even in street work where the concrete is in thin layers and hence requires much moving of platforms.

Where a machine mixer is used that is so mounted as to be portable, the concrete may be delivered in place by a belt conveyor. Such an arrangement for the building of walls and for foundations of pavements, has already been described in Chapter XIV.

The conditions are usually such, however, as to preclude the possibility of mixing the concrete so close to the work that it may be shoveled into place or handled economically on a conveyor of the style mentioned. The next cheapest method is to use a derrick to handle skips or bottom-dump buckets, provided the work is sufficiently concentrated to have one position of the derrick serve to place a large quantity of concrete. The skips should hold about a cubic yard, and if a batch mixer is used, the skip should hold a batch, whatever that may be.

517. If the concrete is mixed on the same level and within less than two hundred feet of the work, wheelbarrows may be used, but for greater distances, carts, or, what is usually cheaper, cars running on a track, should be employed.

For large masses of concrete a cableway may be employed to advantage, provided there is sufficient use for it to repay the high original Cost of plant. The selection to be made from among these common methods is dependent on economy as in handling other material, the only requirements being that the concrete shall be conveyed to place quickly, and that the materials shall not be allowed to separate as a result of any of the manipulation. In laying large quantities of concrete, the difference between success and failure from a financial standpoint may easily rest in the proper transportation of the materials to and from the mixer.

518. Ramming

The concrete should be deposited in horizontal layers about six inches thick, leveled with a shovel and thoroughly rammed. The length of time ramming should be continued and the vigor with which it should be done depend largely on the degree of plasticity of the concrete. If the concrete is made of such a consistency that when struck a smart blow with the back of a shovel a film of moisture will just show on the surface, it should have vigorous ramming to insure a compact mass. A flushing of water to the surface will then indicate when to cease tamping.

With a little more water there is less danger of the larger stones "bridging" and leaving large voids in the mass, and less work will be required to flush water to the surface. With such a consistency, cutting the mass with a spade before starting, the ramming may assist in expelling air bubbles and preventing voids. With still wetter mixtures ramming becomes difficult, as the concrete will soon begin to quake, after which the ramming should not be long continued as the mass is then semi-fluid, and the stones may gradually work themselves to the bottom of the layer, forcing the mortar to the top.

519. Rammers are frequently made of wood, but those of iron are believed to be better. The weight of a rammer is limited by the capacity for work of the man who wields it. They are usually made to weigh from twenty to forty pounds. If a man lifts and drops a forty-pound rammer with forty square-inch face twenty times a minute, he is doing less good to the concrete than if he dropped a twenty-pound rammer with twenty square-inch face forty times a minute. If the face of the rammer exceeds thirty-six square inches, the result is apt to be a mere patting of the surface of the concrete, unless the rammer is so heavy as to require two men to operate it. Iron rammers with face, say, three by six inches, and weighing twenty to thirty pounds, are believed to be the most efficient. Still thinner rammers than this may be necessary in work involving such detail as for filling in between iron beams, and are desirable for tamping near the face of the mold.

520. Rubble Concrete

In massive work the embedding of stones of "one-man size," or larger, in the concrete is a practice that has long been in vogue. The objection is sometimes made that this interferes with the homogeneity of the wall and that variations in expansion may result in injury to the work. It is thought, however, that in large masses this danger is more theoretical than real, and the author sees no objection to this form of construction for many purposes if properly carried out, and it is frequently permitted in important works. Thin walls, the arch rings of bridges, shallow foundations, etc., should not of course be built in this way, because the stresses to which such structures are subjected should be met by a uniform resistance, to avoid the effects of eccentric or irregular loading. In such structures as dams, lock walls, breakwaters, retaining walls, and in many cases bridge piers and abutments, the work may be considerably cheapened without sacrificing the fitness of the structure. The stones thus embedded should be perfectly sound and should not lie nearer one to another than six inches, nor should they lie nearer than this to the face of the wall. The concrete should be mixed rather wet, and much care taken that each stone is completely surrounded by a compact mass of concrete. The stones should be settled into the concrete already laid far enough to assure their having a full bed. Stones used in this manner are sometimes called "plums".