NOIKS II'UN Ari'l.K'AllON OF R IC INKOKCKl) L'oNrUKTK. 09 



protected against high teiupeiatures in such a way as to render it a 

 most suitable niatei-ial for fire-proof constructions. Several cases have 

 come under notice of large buildings of reinforced concrete being ex- 

 posed to serious fires. Quite recently a large motor-car manufactory 

 in America had all the lighter material burnt out, but so little dajnage 

 was done to the main structure that work was ivsunipfl in the factory 

 about two days after the fire had subsided. 



AUoH'abh Sfresaes in Concrete and Steel. - -Ynv the safe designing of 

 structures with reinforced concrete, regulations have been framed in 

 different countries stating what stresses may be allowed in concrete 

 and steel as well as standard tests for Portland cement, the specifi- 

 cations for stone and sand, and inannei- in whicli concrete shall l)e 

 prepareil. 



Following is a table showing those stresses allowed by the Bureau 

 of Building for the Borough of Manhattan, New York. These have 

 been used by the Public Works ])epartme)it of this colony. 



Kind of Stress. Allowed Stress in lbs. 



per s<[. inch. 

 Extreme fibre stress on concrete incompression . . . -500 



Shearing stress in concrete ... ... ... ... .50 



Concrete in direct compression ... ... ... 350 



Tensile stress in steel ... ... ... ... ... 1 6,000 



Shearing stress in steel ... ... ... ... 10,000 



Adhesion of concrete to steel Not greater than the shearing 



strength of the concrete. 



The ratio of the iiiuituli of elasticity of concrete and steel to be 

 taken as 1 to 12. 



In the selection of material the greatest care must be e.\ercised. 

 Stone should be of good quality, properly broken and clean ; sand to 

 be sharp, clean and of the proper grade. The Portland cement should 

 .stand the most rigorous tests. These tests are as follovrs : — 



Gravity. 

 Soundness. 

 Fineness. 

 Time of setting. 

 Tensile strength. 



A certificate of the above should be procured before the cement leaves 

 the factory, and tests made after it arrives upon the works and <luring 

 the progress of the same. 



The steel used for reinforcement must be of the proper ductility 

 and strength, a certificate also being furnished from the manufacturers. 



The proportion of the aggregates composing the concrete are 

 various, governed by the strength required and the sizes of these 

 aggregates. The usual proportions in all good works is 1 of cement, 

 2 sand and 4 of broken stone. 



In mixing concrete a moderately dry mixture well rammed in is 

 found to make the best concrete ; but as it is not always possible to 



