68 IvKPonT S.A.A. Advaxcemknt of Science. 



the guidance of thojse who ha\e not made a stud}' of the question, 

 tlie principles of the same may be briefly explained thus : — ■ 



In the combination of concrete and metal for the constructive 

 of beams, floors, Arc, the concrete has the properties of resisting the 

 c<impressive stresses ; wliere tensional stresses take place, metal is put 

 in such a position as to resist these. A certain amount of resistance 

 to tension more or less exists in the concrete, but this is, however, 

 disregarded in practice, as so much depends upon the quality of the 

 concrete. The compressive stresses upon reinforced columns and piles 

 are partly provided for by the use of steel bars running through 

 their entire length, fastened together with horizontal straps placed 

 at intervals. 



To assist in resisting the shear in the concrete of a beam, metal 

 members are placed either vertically or diagonally at certain points 

 on the length of the beam. Besides the compressise and tensional 

 properties of the separate materials, the bond or adhesion between the 

 concrete and metal plan's an important part, and to this is attributed 

 the power of the combined materials to act as an elastic body. 



The adhesion of concrete to an iron or steel bar has been known 

 to eijual over 700 lbs. for each square inch of surface in contact, 

 and it has been practically demonstrated in a series of tests made at 

 the South African College, that the bond of plain metal bars embedded 

 in concrete was not destroyed until the elastic limit of the metal was 

 reached. 



A number of steel bars of ^•arious shapes have been patented 

 wliich are claimed to furnish a mechanical bond more or less efl'ective, 

 and some of them ha\e been used in buildings and other structures in 

 many parts of the world. 



Regarding the pi'otection of the metal reinf(jrcement against rust, 

 it has been stated by some that a covering of not more than | in. 

 thickness is sufficient for the purpose, but it is usual to allow more 

 than this whenever possible. 



An example of the protection afforded by a thin envelope of 

 concrete surrounding the reinforcement of some water pipes, which 

 had been in use for over fifteen years in the town of Grenoble, sub- 

 jected to an hydraulic pressure of 34'6 lbs. per square inch, may be 

 interesting. A numlxr of lengths of these pipes were removed for 

 examination ;ind broken up, when the reinforcement was found to 

 be perfectly clean and free from rust. The total thickness of the 

 material in the pipes was only Ijj in. Other instances are also known 

 of metal eml)edded in mortar remaining perfect for centuries. 



'J'he eff"ect of changes of temper-ature on reinforced concrete has 

 also been well iuM'stigated, and many proofs are forthcoming showing 

 the small amount of expansion and contraction taking place in this 

 material. The coefficient of expansion in iron is taken as -OOOOOfiT, 

 and that of concrete -0000057, so that with a change of tenqierature 

 of 70 degrees the difference of deformation of the concrete and iron 

 would be -117 in. for a length of 100 ft. 



Bv I'eason of the low conductiv ity of heat in concrete tlie metal is 



