Selection of System

Corrosion of Steel.

An important feature in all concrete steel systems is the effect of the concrete on the steel. Some authorities have held that, on account of its alkaline nature, the presence of Portland cement in concrete is sufficient to prevent any corrosion of the steel. Observations of actual structures, and tests specially conducted, have shown, however, that under cer tain conditions steel will rust when imbedded in Portland cement concrete, while under certain other conditions it will not rust in such an environment. It has been held by some, for example, that this rusting will not occur unless sulphur is present in the concrete.

Professor Norton of the Massachusetts Institute of Technology has conducted a series of tests to observe the conditions under which steel in concrete will corrode. A number of mixtures of concrete were used, consisting of standard brands of cement and of both cinders and stone. The cinders showed very little sulphur present, and the concretes were distinctly alkaline. The metal imbedded was in the form of steel rods, sheet steel, and expanded metal. The results showed that when neat cement was used no corrosion occurred. It was also demonstrated that when corro .

sion occurred in either the cinder or stone concrete, it was coinci dent with cracks or voids in the concrete which allowed the moisture and carbon dioxide to penetrate. If the concrete was mixed wet, so as to form a watery cement coating over all the steel, this coating protected the metal even when cracks and voids were present.

Professor Norton announced the further conclusion that when rusting occurred in cinder concrete it was due to the iron oxide or rust in the cinders, which acted as a carrier of the moisture and carbon dioxide, and it was not due to the presence of sulphur. Also, that if cinder concrete was well rammed when wet, and was -free from voids, it was about as effective as stone concrete in preventing rust.

His conclusion as regards the part played by rust in itself aiding the further corroding action by assuming the role of carrier for the active agents, shows the importance of having the steel free from rust when it is imbedded in the concrete.

The above observations and conclusions are of the utmost importance as establishing the conditions under which, in both stone and cinder concretes, steel may reasonably be expected not to corrode, and as showing clearly the precautions and methods that should be observed in such construction.

Paints. Paints used for the protection of steel, consist, like all other paints, of a pigment and a vehicle. The pigments

used are generally red lead, iron oxide, carbon, and graphite. The vehicle commonly used is linseed oil ; and generally this is boiled oil, although raw oil is sometimes used.

Observations covering a period of about four years were made by Mr. Henry B. Seaman, Member of the American Society of Civil Engineers, on various kinds of paint exposed to the locomo tive smoke and gases on viaducts over the Manhattan Elevated Railroad in New York City. His report, published in the New York Evening Post, concludes that carbon and graphite paints stand such exposure rather better than others, and the carbon paints somewhat better than the graphite. None was entirely efficient. A detailed paper on paints for steel was prepared by Mr. G. M. Lilley, Associate Member of the American Society of Civil Engineers, and was published in Engineering News, April 24, 1902.

The value of paints as agents in the prevention of rusting of steel depends much upon the conditions under which the painting is done, the quality of the paint, and the treatment of the metal after painting.

The experiments of Professor Norton, already mentioned, have established that the essential thing is a coating of the steel which will not crack or peel off and is non-porous, and that the steel must be clean. The fact that ill many cases paint has been applied over a coating of rust, does not, of course, afford any reason for condemning the use of paint because of its failure in such cases to prevent further corrosion.

If the paint can be applied in such a way as to form for the steel a continuous coating that will not crack, or blister, or peel off, it will probably be a very effective preventative of rust. All paints, however, are more or less porous, and to this extent inefficient.

It is, however, the opinion of the authorities who have given this subject most study, that, while more expensive, a thin coating of Portland cement applied continuously to a clean surface of steel is more effective than paint.

The alkaline character of the cement neutralizes the carbon dioxide which may be present, or which may tend to filter through to the steel. In this regard, therefore, it is probable that a small degree of rust in the steel before it is coated with cement would not be likely to cause further rust, as would be the case if the coat ing were of ordinary paint, since the carbon dioxide present in the rust would be neutralized by the cement.