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.
Quality Of Resultant Material Cannot Be Seen Before It Is Embedded In The Work. Unlike steel construction, in which the strength of all material may be tested, and in which the workmanship may be inspected at intervals, in the case of armoured concrete the quality of the work will not be evident until it is thoroughly embedded in the structure, unless indeed the various parts are moulded in advance and are afterwards set in position, as is sometimes done. The latter method has the further advantages of requiring fewer moulds, of saving the need for timbering, and consequently giving greater freedom from props between floors, while also the moulding may be done under cover, and the pieces may be set in position when thoroughly set.
Bad Workmanship. Strength is much impaired by faulty workmanship, by insufficient mixing, by using wrong proportions, by allowing concrete to partially set before putting it in place, by frost during setting, by misplacing the reinforcements, by vibration of moulds during setting, and by the use of faulty materials. All these possibilities of wrong-doing go far to render scientific design useless, and unless they can be avoided the use of armoured concrete is not advisable. However, they may in a great measure be overcome by careful and continuous supervision, and this must be looked upon as an absolute necessity, particularly where an attempt at lightness of construction has been made.
Calculations. Strictly scientific calculations in design are practically impracticable. The properties of concrete vary largely with the materials used, with the quantity of water employed in mixing, with the method of mixing, and with the after treatment. It is impossible to arrive at any very definite figures on which to base calculations, while at the same time the disposition of stresses in beams, etc., has by no means been accurately established. If care be used, however, moderately uniform and satisfactory results may be obtained.
Time Taken In Setting. This results in much obstruction of space by the props necessary to support floors and beams, which must be left in position for several weeks while the concrete gathers strength.
Interruptions In Concreting. It is necessary to discontinue concreting in order to stop work for the night, and at other times while further moulds are being fixed. This slightly affects the continuity of the concrete. In the case of beams the joint between old and new work should be arranged if possible to come immediately over the pillars, or at any rate at those points where the concrete is under compression and shearing stress is small.
The Invisibility Of Metal. The metal being completely embedded and hidden in the concrete, the strength of a beam already constructed cannot be ascertained by measurement and calculation.
7. Monotonous Appearance when used for external walls. The chief use of armoured concrete is not in this position; but architectural effect may be produced with the aid of cement rendering and by carefully forming reverse details on the wooden moulds (see page 137, Vol. I.). A strong feeling very rightly exists against the imitation of stonework in this material. Buildings should not be designed to ape massiveness, and all ornamentation beyond the beauty of lines should be confessedly the decoration of a monolithic material.
The uses to which armoured concrete may be advantageously put are extremely varied. Probably one of the most important uses is in the construction of floors. In this position concrete has been largely used, but generally with almost entire neglect of its considerable resistance in compression. Thus economy is evidently to be expected in this application of its use.
Masonry walls are made sufficiently massive to ensure that the various thrusts which must of necessity come upon them will without doubt bring the resultant thrust within the middle third of the thickness; thus avoiding tensional stress, which bricks or stone blocks set in weak mortar are only slightly capable of resisting. In the case of armoured concrete, bending stresses are no longer to be feared, and the thickness of a wall may with safety be considerably reduced. The advantage gained in this direction is still more evident in the case of arches and their abutment, where the weight may often be reduced to a third or a quarter of that necessary in stone.
As compared with steel construction, the use of armoured concrete is particularly advantageous in its complete protection of the metal, in the absence of vibration, and in the freedom of condensation upon its surface, besides its economy in cost of construction.
It must be borne in mind that the many advantages in the use of armoured concrete can only be attained by careful design and by still more careful supervision.