CONCRETE 21 



ruined and may be flaked off by the application of a stream of 

 water from a hose; but fortunately, the depth to which the heat 

 penetrates is very limited. The conclusions to be drawn from 

 experimental tests, and also from observations of the results of 

 large fires, are that sharp corners of beams and columns are more 

 susceptible to attack than wide, flat surfaces, such as slabs. 

 Sufficient protection seems to be afforded the reinforcing metal 

 by a 1 1/2 to 2 1/2 in. covering in beams and columns, and a 3/4 

 to 1 in. covering in slabs. Little difference was observed between 

 stone and cinder concrete the burning of the bits of coal in poor 

 cinder concrete being often balanced by the splitting of the stones 

 in the stone concrete. It seems probable, from the composition 

 of the rock, that hard trap or gravel may be preferable to hard 

 limestone, slate, or conglomerate, as fire-resisting material, 

 although further experiments are needed to determine their 

 relative durability in this respect. Soft limestone as the aggre- 

 gate should never be employed, as under the action of fire and 

 water it disintegrates. 



Prof. C. L. Norton, in his report on the Baltimore fire to the 

 Insurance Engineering Experiment Station, says: " Where con- 

 crete floor arches and concrete-steel construction receive the full 

 force of the fire, it appears to have stood well, distinctly better 

 than the terra-cotta." The reason for this he considers to be 

 due to the fact that terra-cotta expands about twice as much as 

 steel, while concrete expands about the same. 



15. Waterproofing Qualities of Concrete. Most of the tests 

 and observations on the waterproofing qualities of concrete have 

 been made with the idea of determining whether or not embedded 

 steel is subject to corrosion from moisture and other causes. In 

 general, the rusting of iron occurs only under the combined 

 action of moisture and carbon dioxide. A coating of Portland 

 cement has long been known to not only exclude moisture and 

 carbon dioxide, but in hardening to absorb any carbon dioxide 

 which may be present. Concrete as actually deposited around 

 reinforcing steel cannot be said to be as effective as an unbroken 

 coating of cement, but experiments show that this is practically 

 accomplished if the concrete is mixed quite wet, so as to furnish 

 a thin coating on the steel, and if it is free from voids and cracks. 

 The preserving quality of concrete is thus dependent upon its 

 consistency and the proportions employed, but for ordinary 

 reinforced work it may be concluded from the results of tests 



