As to the behavior of concrete-steel arches in an actual fire, a board of experts was appointed by the insurance companies to investigate the causes and extent of damage to the fireproof buildings in the Pittsburg, Pa., fire of May 3, 1897. This board stated in their report that they believed that in important structures of this class "the fireproofing should be in itself strong and able to resist severe shocks, and should if possible, be able to prevent the expansion of the steel work "; and continued, "There seems to be but one material that is now known that could be utilized to accomplish these results, and that is first-class concrete. The fire-resisting qualities of properly made concrete have been amply proven to be equal, if not better than fire clay tile, as shown by the tests carried on by the Building Department of the City of New York".
468. In a report on the Baltimore fire, Captain Sewall,2 Corps of Engineers, U. S. A., says concrete "undergoes more or less molecular change in fire; subject to some spalling. Molecular change very slow. Calcined material does not spall off badly, except at exposed square corners. Efficiency on the whole is high. Preferable to commercial hollow tiles for both floor arches or slabs and column and girder coverings. In form of reinforced concrete columns, beams, girders and floor slabs, at least as desirable as steel work protected with the best commercial hollow tiles. Stone concrete spalls worse than any other kind, because the pieces of stone contain air and moisture cavities, and the contents of these rupture the stone when hot. Gravel is stone that has had most of these cavities eliminated by splitting through them, during long ages of exposure to the weather. It is therefore better for fire-resisting concrete than stone. Broken bricks, broken slag, ashes and clinker all make good fire-resisting concrete. Cinders containing much partly burned coal are unsafe, because these particles actually burn out and weaken the concrete. Locomotive cinders kill the cement, besides being combustible. On the whole, cinder concrete is safe only when subjected to the most rigid and intelligent supervision; when made properly, of proper materials, however, it is doubtful whether even brickwork is much superior to it in fire-resisting qualities, and nothing is superior to it in lightness, other things being equal".
1 Trans. A. S. C. E., Vol. xxxix, p. 149.
2 Report to the Chief of Engineers, U. S. A., by Capt. John Stephen Sewall, Corps of Engineers. Published in Engineering News, March 24,1904.
Since air is a poor conductor of heat, the more porous concretes are the better protectors against fire. On this account, as well as because of its lightness, cinder concrete is preferred for fireproofing. Care should be taken that cinders to be used in fireproofing concrete do not contain any appreciable amount of unburned coal; in concrete to be used next to steel members the cinders should also be practically free from iron rust. (See § 473).
The strength of cinder concrete is much inferior to that made with the ordinary aggregates, and there should be no difficulty in making a porous concrete with the latter. In fact, in many other classes of construction it has been seen that great precautions must be taken to avoid porosity. By the use of insufficient mortar to fill the voids in the stone, voids may be left in the concrete, though at the expense of diminishing somewhat the strength of the mixture. In adopting such an expedient one should not lose sight of the fact that in order to preserve the imbedded steel from corrosion, it must be fully covered with the mortar.
470. Broken bricks are excellent for fireproofing concrete. The bricks themselves are fire resistant, porous and light, while the adhesion of cement mortar to bricks is so great that unless a very weak mortar is used, the strength of the concrete is limited only by the strength of the brick employed.
Sandstones, especially those with siliceous cementing material, are also well adapted for this purpose. Limestone, on account of the low temperature at which it is broken up, is not good, though as to just how far a limestone concrete would be disintegrated by the heat of an ordinary building fire has not, so far as the author knows, been fully investigated. It is known, however, that limestone masonry is calcined to a certain depth in a conflagration.
Granite in large pieces is cracked by only a moderate degree of heat, and spalls badly. Just how much danger there might be of a similar action in concrete aggregates of this material is not known, nor whether small pebbles or fine gravel would have this property in the same degree, though it is believed they would not, and this view has been confirmed by observations of the Baltimore ruins.
Before adopting a given aggregate for fireproof work, one should satisfy himself by actual test as to the suitability of the materials available, but such tests should be conducted upon concretes containing the proposed aggregates, rather than upon fragments of the materials not incorporated with mortar.