OBJECTION TO REINFORCED CONCRETE. The most serious ob jection to the use of reinforced concrete relates to its permanency. Concrete is weak in tension, and consequently when reinforced concrete is subjected to any considerable tension, the concrete is likely to crack, and may permit the entrance of water or acid gases which may corrode and finally destroy the steel. It is certain that steel embedded in ordinary concrete not subject to bending stresses or temperature changes, is protected reasonably well, if not per fectly; but this does not prove that tensile stresses may not open cracks wide enough for the penetration of water or acid gases. However, the danger is not very great, since in the first place the cracks are exceedingly small and hence neither water nor gases in any appreciable quantities is likely to reach the steel; and in the second place, even if the crack extends to or past the reinforce ment, the steel is likely to be protected by the film of cement which usually covers the metal; and in the third place, any acid gas that does enter the cracks is likely to be neutralized by the alkali of the cement.
Obviously this danger is much greater with such structures as dams, retaining walls, and footings, than with buildings and certain classes of bridges; but the danger of the deterioration of the steel in most reinforced concrete structures is not serious, and most engineers believe that reinforced concrete in at least most positions will be indefinitely preserved. Notice that the use of waterproof
concrete or of a waterproof coating would not prevent the cracks, although the incorporation in the concrete of a water-repelling com pound (* 373-75) might prevent the penetration of water. Of course a water-tight shield (¢ 384) would keep water away from the steel.
The quality of the concrete depends upon the materials and the workmanship employed in it, but these are matters easily understood and easily guarded against. However, reinforced concrete work requires greater care than mass concrete work, since the chief field of usefulness of reinforced concrete is in the construc tion of comparatively small units, such as beams, floors and columns, where a single batch made of poor materials or badly mixed might endanger the whole structure.
Formerly there was much discussion as to the correct methods to be employed in computing the stresses in reinforced concrete; but within the past few years experiments have established the prin ciples involved, and now the strength of a reinforced concrete struc ture can be computed about as accurately as that of any other similar construction.