There is an economic question involved in choosing between ordinary concrete and gunite for metal protection, the former being cheaper per cubic unit but requiring a larger volume and, consequently, more metal to sustain its greater weight. The gunite is much more dense than ordinary concrete, and hence affords better protection against moisture. In order to prevent its cracking under changes of temperature, it generally requires coarse wire mesh or expanded metal to hold it together. It adheres so closely to structural steel that, in order to remove it after it has fully set and hardened, chiseling is necessary. Gunite should be at least one inch in thickness. When shot horizontally or vertically upward the covering is strong and uniform; but, when shot vertically downward, sand-pockets are likely to form, unless the operation be carefully watched and all improperly-cemented material instantly removed.

Treatment of Steel that is to be Encased in Concrete
or Gunite

In most cases, sufficient attention is not paid to the covering of metal which is to be buried in concrete; because, if it is given a coat or two or ordinary paint, the concrete may adhere to the said paint all right, but the latter may eventually separate from the metal, thus loosening the whole protection and either lessening or destroying its efficiency. In the old days, when the question was much more simple than it is now (involving, as it did, only the burying of anchor bolts or anchor metal in the concrete), the author solved it for his work by giving the metal a coat of boiled linseed oil at the shops, and by scrubbing it off at site just before placement. That method would be impracticable today on account of the large amounts of steel to be protected; hence one might put on the ordinary shop coat of red-lead paint and take it off after erection by means of a sand blast, thus leaving the clean metal exposed to the concrete or gunite.

Toch Brothers, however, claim that their No. 1087A "R.I.W." paint, which contains no saponifiable oil and, therefore, avoids all chemical action between the concrete and the paint, will make a permanent bond between the steel and the concrete. Some other manufacturers make similar claims for special products of their own.

Protection Against Brine Drippings

One of the most destructive agencies in respect to bridge metal is brine drippings from refrigerator cars; and, as yet, no satisfactory protection against it has been found. The metal most injured is that in the top flanges of stringers and cross-girders; but the webs and bottom flanges thereof and the buck braces suffer also more or less. Of course, the ideal method of protection would  be  to  catch  the  drippings  in  receptacles on the cars and thus