471. The rusting of steel members in modern buildings and other engineering structures is one of the most serious menaces to their permanence. The introduction of concrete-steel construction has given rise to some discussion, especially among those unfamiliar with the properties of concrete, as to the effect of the concrete upon the steel.
The rusting of iron takes place only in the presence of moisture, air and carbon dioxide. In perfectly dry air, or in perfectly pure water, iron does not rust. Under the proper conditions, however, the iron, water and carbonic acid combine to form ferrous carbonate, which at once combines with oxygen from the air to form ferric oxide, the carbonic acid being liberated to act on a fresh portion of the metal. It is seen that only a very small amount of the carbon dioxide is necessary. If, however, the carbon dioxide or other acid filling the same role, is neutralized by the presence of an alkaline substance, the foregoing reactions cannot take place. As cement is strongly alkaline, it thus furnishes an almost perfect protection against rusting.
To determine the cause of occasional rusting of steel surrounded by cinder concrete, and consequently the proper methods of applying cement mortar or concrete to steel, Prof. Chas. L. Norton, engineer in charge of the Insurance Engineering Experiment Station at Boston, made tests on several hundred briquets in which steel was imbedded in mortars and concretes of various compositions.1 The briquets were subjected to air, steam and carbon dioxide, others to air and steam, to air and carbon dioxide and to the ordinarily dry air of a room. At the end of three weeks it was found that neat Portland cement had furnished a perfect protection in all cases. The corrosion of the steel in other specimens was always at a point where a void existed in the concrete, or where a badly rusted cinder had lain. In every case where the concrete or mortar had been mixed wet, and the surface of the steel had been thus coated with a thin layer of grout, no rust spots occurred.
In the first tests made by Professor Norton the specimens were thoroughly cleaned before being imbedded in the concrete, but later tests indicated that in specimens that had begun to corrode before treatment, the rusting was arrested by the coating of cement mortar or concrete. After from one to three months in tanks holding steam and carbon dioxide, specimens which had' been in all stages of corrosion before being imbedded in the concrete had not suffered any sensible change in weight or size except when the concrete had been poorly applied.
474. The results of these experiments showed that the steel need not necessarily be freed from rust before being imbedded in the concrete; that the concrete to be applied next the steel should be mixed wet, or that the steel should be first coated with grout by dipping or brushing; and it appeared that the rusting sometimes found in cinder concrete is due to the rust in the cinders rather than to the sulphur, and that if proper precautions are taken, cinder concrete is nearly as effective as stone concrete in preventing corrosion. Prof. Norton says,- "In the matter of paints for steel there is a wide difference of opinion. I cannot believe that any of the paints of which I have any knowledge can compare with a wash or painting with cement".
Report III of Insurance Engineering Exp. Station, Boston, Mass.
The conclusions drawn by Booth, Garrett and Blair from a series of tests made for the Roebling Construction Co., were that cinders from anthracite pea coal contained about two-tenths per cent, of sulphur which they considered sufficient to cause corrosion of unprotected ironwork, more or less rapidly, depending on the presence or absence of moisture; but they further concluded that a "full" concrete (one in which the voids in the cinders were entirely filled by mortar of cement and sand) would fully protect the steel.
In a paper read before the Associated Expanded Metal Companies, Prof. S. B. Newberry has this to say concerning cinder concrete: 1 "The fear has sometimes been expressed that cinder concrete would prove injurious to iron, on account of the sulphur contained in the cinders. The amount of this sulphur is, however, extremely small. Not finding any definite figures on this point, I determined the sulphur contained in an average sample of cinders from Pittsburg coal. The coal in its raw state contains rather a high percentage of sulphur, about fifteen per cent. The cinders proved to contain only 0.6 per cent, sulphur. This amount is quite insignificant, and even if all oxidized to sulphuric acid, it would at once be taken up and neutralized in concrete by the cement present, and could by no possibility attack the iron".
While so far as the corrosion of steel is concerned, the above experiments by Prof. Norton show that the rusting is corrected by the concrete, yet it is quite possible that the adhesion of cement to steel may be impaired by a coating of rust. The cleaning of the steel may be accomplished by first brushing with wire brushes to remove all scales, followed by treatment with hot dilute sulphuric acid, and finally applying an alkaline wash such as hot milk of lime to neutralize all traces of the acid. Oxalic acid may be used in place of the sulphuric, and the application of the milk of lime dispensed with, since the acid oxidizes. The crystals of oxalic acid as purchased commercially should be mixed with about seven parts hot water and the solution applied with a brush or sponge. When the adhesion of the mortar or concrete to the steel is of any importance, as it is in all concrete-steel construction where the stresses are divided between the steel and concrete, any of the ordinary oil paints will not only be quite unnecessary, but may be a very serious detriment to the construction.
1 Engineering News, Apr. 24, 1902.
The experiments quoted indicate the importance of having the steel covered with an unbroken coating of cement or cement mortar. To insure this the steel must either be coated with a layer, preferably of neat Portland, by dipping or brushing, or the mortar placed next the steel must be wet enough to insure intimate contact throughout. It may be added also, that the addition of a small amount of thoroughly slaked lime to Portland cement mortar or concrete will not only render the material more alkaline, but will make the mortar more plastic, and thus insure a better coating of the steel. Such small additions have no deleterious effect on the mortar.
477. Practical instances of the preservation of iron by concrete are not wanting. The writer has stored in water, briquets with small iron plates imbedded in Portland cement mortar, and at the end of six months the plates were found moist, but entirely free from corrosion except where they projected beyond the mortar. A concrete-steel water main built on the Monier system at Grenoble, France, was taken out and examined after fifteen years service in damp ground. The metal imbedded in the mortar showed no signs of corrosion, and the mortar could only be detached from it by hammering.
Mr. W. G. Triest1 relates that in breaking up cast-iron, concrete-filled pillars, a wrench was found that had been buried in the concrete for twenty-two years. The wrench had maintained its metallic surface in the concrete, while a part of it that had been imbedded in coal ashes had corroded badly.
Similar instances showing the action of concrete on steel and iron might be multiplied, but it is sufficient to state that the preservation of iron or steel properly imbedded in Portland cement mortar or concrete is now seldom questioned, and the use of cement paint, in place of the ordinary oil paints, as a steel preservative, has been adopted in many places.
1 Trans. A. S. C. E., April, 1894.