Concrete should, after thorough mixing, be rapidly wheeled to the place where it is to be laid, gently tipped (not from a height) into position, and carefully and steadily rammed in layers about 12 inches thick.
For large masses a somewhat slow-setting cement should be used, and the layers should follow one another so that each is laid before the last has had time to set. This leads to a thorough key being formed between the layers, by which horizontal joints are avoided.
It is essential that the layers should be horizontal; if not, the water trickling off will carry the cement with it.
When circumstances require that each layer should be allowed to set separately, it should be carefully prepared to receive the one that is to rest upon it.
A common practice, which in former years was much insisted upon, is to tip the concrete, after mixing, from a height of 10 feet, 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 therefore 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.
This is especially necessary if it has been rammed, for in that case the finer stuff in the concrete works to the top, and 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 he 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.1
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 used for large masses, and if used, not 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 are deposited, the smaller ones in the same way as blocks of stone, those of enormous size by special arrangements which cannot here be described.
Plastic Concrete 2 is a name that has been given to concrete that has been mixed with a very small proportion of water, allowed to set for from 2 to 5 hours, according to the state of the weather, and a little quick-setting cement - such as Roman, Medina, or Orchard - added to it just before it is placed in skips and deposited under water. Concrete deposited in this condition is said to resist the action of the sea and to unite with that previously in position better than concrete deposited in the ordinary liquid condition. On the other hand, it is said that the disturbance of the concrete after it has commenced setting prevents it from ever attaining a proper hardness. The material has not at present been sufficiently used for any decided opinion to be given with regard to its merits.