DURABILITY OF CONCRETE 90. Destructive Agencies.—Well-constructed concrete, under the conditions usually met in ordinary work, is practically an inde structible material, but under special conditions, when subject to the action of agencies peculiar to particular classes of work, con crete may yield like any other material. The cracking of concrete through contraction, as explained in Section 85, may be of injury to a structure, hut the body of concrete itself is not destroyed or disintegrated by cracking.
Concrete has sometimes seemed to be injured by the action of certain chemical agencies, such as oils, salts of sea water, alkalies, and acids. Destruction by electrolysis and by fire have also some times occurred.
Effect of oils have no ill effects upon concrete, and have sometimes been used for the purpose of rendering the surface less pervious, and to prevent dust upon surfaces subject to abrasion. Some animal fats and vegetable oils seem to cause dis integration in concrete. When such oils at high temperatures come into contact with concrete, a combination of lime from the cement with acids contained in the oils produces compounds which expand when crystallizing in the pores. In manufacturing plants, where animal or vegetable oil may come into contact with concrete, the effect of such contact should be carefully investigated.
Effect of containing acids should not come into contact with concrete before it has well hardened. Hard concrete may resist the action of such solutions unless they are in rather concentrated form. The effect of any destructive agency of this character will be much greater for porous concrete into which the liquid may readily penetrate than for dense concrete.
Electrd some instances, injury to concrete containing steel reinforcement has resulted from the leakage of electric cur rents through the mass. The Joint Committee on Concrete in its 1917 report make the following statements concerning electrolysis: Electrolysis.—The experimental data available on this subject seem to show that while reinforced concrete structures may, under certain conditions, he injured by the flow of electric current in either direction between the reinforcing material and the concrete, such injury is generally to he expected only where voltages are considerably higher than those which usually occur in concrete structures in practice. If the iron he positive, trouble may manifest itself by corrosion of the
iron accompanied by cracking of the concrete, and, if the iron be negative, there may be a softening of the concrete near the surface of the iron, resulting in a destruction of the bond. The former, or anode effect, decreases much more rapidly than the voltage, and almost if not quite disappears at voltages that are most likely to he encountered in practice. The cathode effect, on the other hand, takes place even under very low voltages, and is therefore more important from a practical standpoint than that of the anode.
Structures containing salt or calcium chloride, even in very small quantities, are very much more susceptible to the effects of electric currents than normal concrete, the anode effect progressing much more rapidly in the presence of chlorine, and the cathode effect being greatly increased by the presence of an alkali metal.
There is great weight of evidence to show that normal reinforced concrete structures free from salt are in very little danger under most practical conditions, while non-reinforced concrete structures are practically immune from troubles.
91. Effect of Sea Water upon Concrete.—There have been numerous instances of failure of concrete subject to the action of sea water, the causes of which are not fully determined. The results of experiments seem to indicate that salts contained in sea water act upon nearly all cements to which the water has free access, producing compounds which expand, disrupting the mass of mortar, or which soften the mortar and cause disintegration. This action is probably due to sulphates in the sea water, which are decomposed in contact with the free lime of the cement,, the sulphuric acid com bining with the lime. A considerable deposit of magnesia also commonly occurs in cement mortar when exposed to sea water,' indicating that the sulphate of magnesia may be the source of the trouble.