This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by P. R. Strong)
Concrete has long been used in such positions as in the foundations of walls or in the construction of heavy retaining walls, etc. Its use in such positions is made advisable by the ease with which the component materials may be obtained and put in place, and also on account of the monolithic nature of the resultant mass. The small strength of concrete in tension restricts its application, when used by itself, to positions where the stresses to be resisted are almost entirely compressional.
The strength of concrete in compression may be taken to be roughly ten times as great as that in tension. Thus if a beam of plain concrete be constructed, its strength will depend upon its resistance to tension, which is small, while its high compressive resistance will never be brought into play. By reinforcing with steel or wrought iron those parts which are placed in tension the full compressional resistance of the concrete as well as the full tensile resistance of the metal may be made use of (see Fig. 23).
Probably the matter which most affects the advisability of using any particular form of construction is that of original cost as well as cost of upkeep. Although concrete cannot be disposed quite so advantageously as can steel, for instance, as regards its distance from the neutral axis, and although the weight of the resultant structure is greater, while at the same time much carpentry is necessary for moulds, yet the conjunction of concrete and steel is found, in the majority of cases to which it is applicable, to be cheaper than metal alone, even for bridging spans as large as 150 feet, and in some instances considerably greater. As the span of a structure increases, the weight of concrete militates against its economical use.
The second part of the question of cost, that of maintenance, is particularly small with this form of construction, which all experience gained hitherto goes to show is practically permanent and immune from deleterious effects of atmosphere. In fact, it may be said that the strength of concrete will improve for an indefinite number of years after construction, while the steel is apparently perfectly protected from deterioration.