FIRE-RESISTING PROPERTIES Reinforced concrete ranks with the best fire proof materials, and it is this quality, perhaps more than any other, which is responsible for the enormous increase in its use for factories and other industrial structures.
Intense heat injures the surface of the con crete; but concrete is so good a non-conductor that, if sufficiently thick, it provides ample pro tection for the steel reinforcement, and the interior of the mass is unaffected, even in unusually severe fires.
For efficient fire protection in slabs, under ordinary conditions, the lower surface of the steel rods should be at least inch above the bottom of the slab. In beams, girders, and col umns, a thickness of to inches of con crete outside of the steel, varying with the size and importance of the member, and the liability to severe treatment, is in general sufficient. In columns, whose size is governed by the loads to be sustained, an excess of sectional area should be provided, as illustrated later.
One of the advantages of concrete construc tion as a fireproof material is that the design may be adapted to the local conditions. For ex ample, in an isolated machine shop where scarcely any inflammable materials are stored, it is a waste of money to provide a thick mass of concrete simply to resist fire. On the other hand, for a factory or warehouse storing a product capable of producing not merely a hot fire—a hot, short fire will not damage seriously —but an intense heat of long duration, special provision may be made by using an excess area of concrete perhaps two or three inches thick.
Actual fires are the best test of a material. One of the most severe on record occurred in the Pacific Coast Borax Refinery at Bayonne, N. J., and the concrete there, as well as in the Balti more and San Francisco fires, made an excellent record.
The best fire-resistance materials for con crete are first-class Portland cement with quartz sand and broken trap rock. Limestone aggre gate will not stand the heat so well as trap, while the particles of gravel are more easily loosened by extreme heat. Neither of these ma
terials, however, if of good quality, need be re jected for building construction, unless the de mands are especially exacting and the liability to fire great. Cinders make a good aggregate for fire resistance, but the concrete made with them is not strong enough for reinforced con crete construction, except in slabs of short span or in partition walls.
The fire resistance of concrete increases with age, as the water held in the pores is taken up chemically and is evaporated.
A recommendation in one of the papers read by Mr. W. M. Bailey before the National Cement Users' Association, stated that if the structural part of the building is of reinforced concrete, not only should the structural steel be protected from fire by covering to a sufficient depth, but also about an inch of additional concrete should be placed on the various members of the struc ture, to protect the structural concrete itself. For example: suppose the design calls for a 12-inch by 12-inch concrete column to carry safely its load. It should be made 14 inches by 14 inches, the extra 1 inch on all faces acting merely as a fire-resisting covering for the con crete inside. In designing buildings of rein forced concrete, it should be borne in mind that concrete is injured by intense heat. When the temperature of concrete reaches about 1,000° F., its surface becomes decomposed, and the water taken up by the cement in hardening is driven off. This process uses a large amount of heat, and is extremely slow after the first inch of the outer layer is affected; this outer coat really forms an obstruction through which the heat must pass before each successive layer beyond can be affected. Tests show that a point 2 inches from the surface will stand an outside temperature of 1,500 degrees to 2,000 degrees, with a rise of only 500 to 700 degrees.