FIRE-RESISTING KUILDINGS. 85 



renders reinforcement necessary where tensile strain is applied- 

 When properly embedded in suitable concrete, steel is protected 

 against the destructive influences of damp and heat. Coke 

 breeze and cinders are not favoured for reinforced work as 

 the possible presence of sulphur would act deleteriously upon 

 the steel. 



Portland cement is universally adopted as the matrix of 

 fire resisting concrete, and only very finely ground and medium 

 setting cement should be used for the purpose. 



The coefiicients of expansion for concrete and steel being 

 practicallv the same, a combination of these materials secures 

 homogeneity, and ensures that the two materials will act together, 

 and not throw undue stress upon either. This is most important, 

 for otherwise a rise in temperature, say, by the application of 

 fire, would cause the two materials to separate. 



Success again depends largely upon the adhesion of the 

 concrete to the steel, and this depends, to a great extent, upon 

 the contraction that takes place during the " setting " of the 

 concrete. An adhesive strength of lOO lbs. per square inch is 

 generally recommended, though 250 lbs. at the age of one 

 month is attainable with care. It may be interesting to note that 

 slightly rusted steel bars give better adhesion than clean ones. 



Painting or coating prevents adhesion, and is therefore to 

 be avoided. Practical tests have proved that concrete, with the 

 best aggregates and cement when mixed 4:2:1, possesses a com- 

 prehensive strength of about 1,800 lbs. per square inch at the 

 age of one month, while broken stone concrete, mixed 6:3:1, 

 should give a tensile strength of about 350 lbs. per square inch. 



The effect of heat is to reduce the strength of concrete so 

 that a varying factor of safety has to be decided upon. 



Experiments made by Professor Woolson, in the United 

 States of America, showed that concrete, with an aggregate of 

 trap rock mixed 4:2:1, gave a compressive strength per square 

 inch as follows : — 



Four inch cubes were tised in the tests. 



Fire tests with flours of dift'erent types, as made by the 

 British Fire Prevention Committee, have proved that, as far as 

 the concrete itself is concerned, the following prevented the 

 passage of fire when exposed to temperature varying from 

 1,800° F. to 2.000° F. for several hours: — 



1. Coke breeze and Portland cement mixed 5 :i and 5" thick. 



2. Blast furnace slag, sand and cement, mixed 3:2:1 and 

 5/4" thick. Became red hot. 



3. Broken brick, sand and cement, mixed 3:2:1 and SV^^' 

 thick. 



4. Broken granite, sand and cement, mixed 3:2:1 and S^-^" 

 thick. 



