FIRE PROTECTION 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 com bination 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 tem perature ranges from 1,400° to 1,000° F., the surface of the concrete may be injured to a depth of to inch or even of one inch; but the body of the concrete is not affected, and the only repairs re quired, if any, consist of a coat of plaster.
12S. Theory. The theory given by Mr. Spencer B. Newberry is that the fireproofing qualities of Portland cement concrete are due to the capacity of the concrete to resist fire and prevent its trans ference to steel by its combined water and porosity. In hardening, con crete takes up 12 to 1S per cent of the water 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 sur rounding 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.
129. Cinder vs. Stone Concrete. 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 ad vantage 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." • 130. of Concrete Required for Fireproofing. 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 11 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 con crete for fireproofing purposes as the main girders, although perhaps 1 to 11 inches would be sufficient. For ordinary slabs, inch is ample protection; but for long-span slabs the fireproofing thick ness should be from inch to inches. Columns should have at least 2 inches of concrete outside of the steel; often 3 inches is speci fied.
131. The Baltimore Fire. 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 Engi neering 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 con crete 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 tern perature 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." 132. Fire and Water Tests. 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 con structed 16 feet by 25 feet, with a wall through the middle. The roof consisted of the two floors to be fested. One floor was a rein forced 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.