This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
127. The various tests which have been conducted - including the involuntary tests made as the result of fires - have shown that the fire-resisting qualities of concrete, and even resistance to a combination of fire and water, are greater than those of any other known type of building construction. Fires and experiments which test buildings of reinforced concrete have proved that where the temperature ranges from 1,400° to 1,900° F., the surface of the concrete may be injured to a depth of 1/2 to 3/4 inch or even of one inch; but the body of the concrete is not affected, and the only repairs required, if any, consist of a coat of plaster.
The theory given by Mr. Spencer B. Newberry is that the fireproofing qualities of Portland cement concrete are clue to the capacity of the concrete to resist fire and prevent its transference to steel by its combined water and porosity. In hardening, concrete takes up 12 to 18 per cent of the water contained in the cement. This water is chemically combined, and not given off at the boiling point. On heating, a part of the water is given off at 500° F., but dehydration does not take place until 900° F, is reached. The mass is kept for a long time at comparatively low temperature by the vaporization of water absorbing heat. A steel beam imbedded in concrete is thus cooled by the volatilization of water in the surrounding concrete.
Resistance to the passage of heat is offered by the porosity of concrete. Air is a poor conductor, and an air space is an efficient protection against conduction. The outside of the concrete may reach a high temperature; but the heat only slowly and imperfectly penetrates the mass, and reaches the steel so gradually that it is carried off by the metal as fast as it is supplied.
Mr. Newberry says: "Porous substances, such as asbestos, mineral wool, etc., are always used as heat-insulating material. For this same reason, cinder concrete, being highly porous, is a much better non-conductor than a dense concrete made of sand and gravel or stone, and has the added advantage of being light.".
Professor Norton, in comparing the actions of cinder and stone concrete in the great Baltimore fire of February, 1904, states that there is but little difference in the two concretes. The burning of bits of coal in poor cinder concrete is often balanced by the splitting of stones in the stone concrete. "However, owing to its density, the stone concrete takes longer to heat through."
Actual fires and tests have shown that 2 inches of concrete will protect an I-beam with good assurance of safety. Small rods in girders are more effectively coated, and 1 1/2 inches of concrete is usually considered sufficient protection, although some city building laws specify 2 inches of concrete. Beams usually have the same thickness of concrete for fireproofing purposes as the main girders, although perhaps 1 to 11/2 inches would be sufficient. For ordinary slabs, \ inch is ample protection; but for long-span slabs the fireproofing thickness should be from 3/4 inch to 1 1/2 inches. Columns should have at least 2 inches of concrete outside of the steel; often 3 inches is specified.
Engineers and architects, who made reports on the Baltimore fire of February, 1904, generally state that reinforced concrete construction stood very well - much better than terra-cotta. Professor Norton, in his report to the Insurance Engineering Experiment Station, says:
"Where concrete floor-arches and concrete-steel construction received the full force of the fire, it appears to have stood well, distinctly better than the terra-cotta. The reasons, I believe, are these: First, because the concrete and steel expand at sensibly the same rate, and hence, when heated, do not subject each other to stress; but terra-cotta usually expands about twice as fast with increase in temperature as steel, and hence the partitions and floor-arches soon become too large to be contained by the steel members which under ordinary temperature properly enclose them."
Under the direction of Prof. Francis C. Van Dyck, a test was made on December 26, 1905, on stone and cinder reinforced concrete, according to the standard fire and water tests of the New York Building Department. A building was constructed 16 feet by 25 feet, with a wall through the middle. The roof consisted of the two floors to be tested. One floor was a reinforced cinder concrete slab and steel I-beam construction; and the other was a stone concrete slab and beam construction. The floors were designed for a safe load of 150 pounds per square foot, with a factor of safety of four.
The object of the test was to ascertain the result of applying to these floors, first, a temperature of about 1,700° F. during four hours. a load of 150 lbs. per square foot being upon them; and second, a stream of water forced upon them while still at about the temperature above stated. A column was placed in the chamber roofed by the rock concrete, and it was tested the same way.
The fuel used was seasoned pine wood, and the stoking was looked after by a man experienced in a pottery; hence a very even fire was maintained, except at first, on the cinder concrete side, where the blaze began in one corner and spread rather slowly for some time.
The water was supplied from a pump at which 90 lbs. pressure was maintained, and was delivered through 200 feet of new cotton hose and a 1 1/8-inch nozzle. Each side was drenched with water while at full temperature, apparently; and the water was thrown as uniformly as possible over the surface to be tested, for the required time. The floors were then flooded on top, and again treated underneath.
Inasmuch as the floors and the column were the only parts submitted for tests, the slight cracking and pitting of the walls and partition need not be detailed.
The column was practically intact, except that a few small pieces of the concrete were washed out where struck by the stream at close range. The metal, however, remained completely covered. On the rock concrete side, the beams showed naked metal up to within about 7 inches of the ends on one beam, and about 2 feet from the ends on the other beam. The reinforcing bars were denuded over an area of about 30 square feet near the center; but no cracks developed, and the water poured on top seemed to come down only through the pipe set in for the pyrometer.
On the cinder concrete side, the beams lost only a little of the edges of the covering, not showing the metal at all. There were no cracks on this side either, and the water came down through the pyrometer tube as on the other side. The metal in the slab was bared over an area of about 24 square feet near the center.
During the firing, both chambers were occasionally examined, and no cracking or flaking-off of the concrete could be detected. Hence the water did all the damage that was apparent at the end.
During the test the floors supported the load they were designed to carry; and on the following day the loads were increased to 600 pounds per square foot.
The following is taken from Professor Van Dyck's report:
"The maximum deflection of the stone concrete before the application of water, was 2 15/16 inches; after application of water, 3 3/16 inches; with normal temperature and original load, 3 1/16 inches; deflection after load of 600 pounds was added, 3 13/16 inches.
"The maximum deflection of the cinder concrete before the application of water, was 6 1/16 inches; after application of water, 6 V inches; with normal temperature and original load, 5 11/16 inches; deflection after a load of 600 pounds was added, 6 inches. These measurements were taken at the center of the roof of each chamber."