"There can be no doubt that the highest quality of concrete can be made only in a factory where every con dition as to quality of material, methods of manufacture, and seasoning can be carefully studied and regulated. Men expert in each division of the process of manufacture can supervise all operations, and, by the use of mechan ical equipments which it is impossible to establish in the field, will be enabled to work at the highest point of efficiency and economy.
"Next in importance to the quality of concrete is the advantage of economical sections into which it may be moulded under shop methods. Every engineer knows that in a rectangular beam there is great waste of material as regards actual areas under stress. For this reason, steel is rolled into what are commonly known as structural shapes; but even in the manufacture of steel, mechanical difficulties prevent the rolling of truly economical sections. Concrete, being a plastic material, presents no such difficulties; and when factory-made, practically any section can be cast in permanent metal moulds by proper appliances for filling same. Also the distribution of reinforcing steel can be developed to a much higher degree, and the position of the steel in the member made absolutely certain. Many attempts have been made in the field to cast concrete members of more economical shape, and numerous systems have been de vised, consisting mostly of the use of hollow tile or similar material for coring or reducing the sections. But under field conditions the saving is but slight, as the hollow cores must be left in the work, and the whole temporarily supported until the concrete is sufficiently hardened to enable it to resist loads.
"As to the economy of shop methods of manufacture over field methods, this has so often been demonstrated in all lines of industry that any discussion is unneces sary. A plant for such a purpose should be located in the larger centers of population and at or near some source of supply of raw material. Facilities for handling raw materials at a minimum cost, and for handling, cast ing, and finishing the completed products, can be perma nently installed; also crushing, screening, and mixing machinery that will automatically regulate the size and proportions of materials which will not only produce uni form and high-quality results, but at a minimum of expense.
"With a proper arrangement of plant it certainly is not unreasonable to expect that structural concrete sec tions can be manufactured in a factory, transported and erected in the field at a cost per cubic yard not much in excess of what it now costs to erect concrete building blocks, plus the cost of steel reinforcement and the fabri cation of same. This in round figures should not exceed fifteen dollars per cubic yard actual measurement. Bear ing in mind that these factory-made sections can be made with a saving of from 33 to 50 per cent of the volume necessary to produce the same results by field methods, due to the fact already stated that the concrete is twice as strong and of more economical section, its cost would be not over ten dollars per cubic yard, as compared with the actual contents of present field-made monolithic con struction.
"The development of the unit-system of structural concrete is sure to result in a most productive field for the manufacturer of cement products, and should grow into a large and important industry. The greater speed at which buildings constructed of factory-made units may be erected, the comparative ease with which altera tions may be made, and the absolute certainty of results due to the fact that members can be tested, are quite apparent advantages. There seems to be little doubt that the majority of failures of monolithic construction are due to the premature removal of forms. It is uneco nomical to leave these forms in place more than three weeks at the most, and the strength of concrete at this age is seldom more than half of what it attains in two months. The use of factory-made materials will allow full loads to be imposed immediately after erection, and as they require no temporary supports all danger of failures is eliminated.
"From an architectural view, field-moulded concrete is objectionable, not only on account of the enormous size of its members, but on account of the difficulty of getting a high-grade finish; factory-made materials can be cast in smaller sections and in forms, or in sand moulds, which leave a high-grade and very satisfactory finish. Members can even be designed and moulded with a view to archi tectural appearance, combining both the structural and architectural features of the building without the expense of additional work of finishing and ornamentation.