The best method is to wheel the concrete in barrows, imme diately after mixing, to the place where it is to be laid, gently tipping or sliding it into position and at once ramming it.
The ramming should be done before the cement begins to set, and should be continued until the water begins to ooze out upon the upper surface. When this occurs it indicates a sufficient degree of compactness. A gelatinous or quicksand condition of the mass indicates that too much water was used in mixing. Too severe or long-continued pounding injures the strength by forcing the stones to the bottom of the layers and by distributing the incipient "set" of the cement.
The rammers need not be very heavy, 10 to 15 lb. will be suffi cient. Square ones should measure from 6 to 8 in. on a side and round ones from 8 to 12 in. in diameter.
After each layer has been rammed it should be allowed sufficient time to "set", without walking on it or in other ways disturbing it. If successive layers are to be laid the surface of the one already set should be swept clean, wetted, and made rough by means of a pick for the reception of the next layer.
Great care should be observed in joining the work of one day to that of the next. The last layer should be thoroughly compacted and left with a slight excess of mortar. It should be finished with a level surface, and when partially set should be scratched with a pointed stick and covered with planks, canvas, or straw. In the morning, immediately before the application of the next layer, the surface should be swept clean, moistened with water, and painted with a wash of neat cement mixed with water to the consistency of cream. This should be put on in excess and brushed thoroughly back and forth upon the surface so as to insure a close contact therewith.
Depositing Concrete Under Water. In laying concrete under water an essential requisite is that the materials shall not fall from any height through the water, but be deposited in the allotted place in a compact mass; otherwise the cement will be separated from the other ingredients and the strength of the work be seriously impaired. If the concrete is allowed to fall through the water its ingredients will be deposited in a series, the heaviest—the stone—at the bottom, and the lightest—the cement—at the top. A fall of even one foot causes an appreciable separation.
A common method of depositing concrete under water is to place it in a V-shaped box of wood or plate iron, which is lowered to the bottom with a crane. The box is so constructed that on reach ing the bottom a latch operated by a rope reaching to the surface can be drawn out, thus permitting one of the sloping sides to swing open and allow the concrete to fall out. The box is then raised and refilled.
A long box or tube, called a tremie, is also used. It consists of a tube open at top and bottom built in detachable sections, so that the length may be adjusted to the depth of water. The tube is suspended from a crane or movable frame running on a track, by which it is moved about as the work progresses. The upper end is hopper-shaped, and is kept above the water; the lower end rests on the bottom. The tremie is filled in the beginning by placing the lower end in a box with a movable bottom, filling the tube, lowering all to the bottom, and then detaching the bottom of the box. The tube is kept full of concrete by more being thrown in at the top as the mass issues from the bottom.
Concrete is also successfully deposited under water by enclosing it in paper bags and lowering or sliding them down a chute into place. The bags get wet and the pressure of the concrete soon bursts them, thus allowing the concrete to unite into a solid mass. Concrete is also sometimes deposited under water by enclosing it in open-cloth bags, the cement oozing through the meshes sufficiently to unite the whole into a single mass.
Concrete should not be deposited in running water unless pro tected by one or other of the above-described methods; otherwise the cement will be washed out.
Concrete deposited under water should not be rammed, but if necessary may be levelled with a rake or other suitable tool imme diately after being deposited.
When concrete is deposited in water a pulpy, gelatinous fluid is washed from the cement and rises to the surface. This causes the water to assume a milky hue. The French engineers apply the term laitance to this substance. It is more abundant in salt water than in fresh. The theory of its formation is that the immersed concrete gives up to the water, free caustic lime, which precipitates magnesia in a light and spongy form. This precipitate sets very slowly, and sometimes scarcely at all, and its interposition between the layers of concrete forms strata of separation. The proportion of laitance is greatly diminished by using large immrsion-boxes, or a tremie, or paper or cloth bags.
Asphaltic Concrete is composed of asphaltic mortar and broken stone in the proportion of 5 parts of stone to 3 parts of mortar. The stone is heated to a temperature of about 250° F. and added to the hot mortar. The mixing is usually performed in a mechanical mixer.
The material is laid hot and rammed until the surface is smooth. Care is required that the materials are properly heated, that the place where it is to be laid is absolutely dry and that the ramming is done before it chills or becomes set. The rammers should be heated in a portable fire.