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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 11 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 1- inches would be sufficient. For ordinary slabs, inch is ample protection; but for long-span slabs the fireproofing thickness should be from 3 inches to 1 inch. 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.

West Newbury Massachusetts Concrete Cutting and Core Drilling