SAFE STRENGTH OF MATERIALS 91 



hard wire. It is usually safe simply to specify bright Bes- 

 semer or hard open-hearth steel wire. This will ordinarily be 

 safe at 16,000 pounds per square inch. In other words, the 

 No. 3 wire, which is a convenient size for silo work, will 

 safely withstand a pull of about 1000 to 1200 pounds. For 

 reinforcing rods to be used at the door frame, or for other 

 general reinforcing work, corrugated or twisted bars will be 

 found best; or, if smooth rods are used, they should be 

 hooked at the ends so that they will not slip. 



CONCRETE MIXTURES 



The proper proportion for mixing material composing 

 concrete will depend upon the kind of material used and the 

 strength required by the service to which the concrete will be 

 put. Concrete of the strength given in Table III is made by 

 mixing 1 part cement with 2J/ parts of sand and 5 parts of 

 crushed stone, or by mixing 1 part cement with 5 parts of 

 bank gravel which, if sifted out, would give 1 part sand to 2 

 parts stone. 



The theory is that the particles of cement should thor- 

 oughly coat all of the particles of sand and stone, and at the 

 same time fill in all spaces between the particles of sand and 

 gravel which are not already occupied by smaller particles. 



It will be readily seen that if only sand and cement were 

 mixed, it would be necessary, in order to get the same strength 

 as before, to mix 1 part of cement with 2% parts of sand, thus 

 giving only YT, the quantity of concrete otherwise obtained. 

 This indicates the importance of knowing the grade of gravel 

 or other aggregate. 



Testing the Concrete. It is easy to determine, in a prac- 

 tical way, the amount of cement to make a dense concrete 

 from any quality of aggregate that is available. A represen- 



