REINFORCED CONCRETE 319 



Strength and Weight of Plain Concrete. The average ulti- 

 mate strength of concrete in tension, compression, and shear is 

 given in the accompanying table for different proportions of 

 mixture, the aggregate of which is broken stone. Concrete 

 made of gravel is 75% as strong and concrete made with cinders 

 is about 65% as strong. 



As the strength of concrete increases with age, it is necessary 

 for the engineer to know when the concrete will be loaded. 

 It is customary to assume a factor of safety based on the 

 strength of the concrete after 6 mo. The engineer must be 

 careful that the concrete, in the first few months after being 

 laid, is not subjected to too great stresses. For general work, 

 a factor of safety of 5 on concrete 6 mo. old is recommended. 

 This will give the required strength for the first few months, 

 and yet will not be wasteful of material at any time. A factor 

 of safety of 4 on concrete 6 mo. old may be used for steady loads, 

 such as earth fills, water pressure, etc. 



The weight of concrete depends mainly on the kind of aggre- 

 gate used. It averages about 140 to 150 Ib. per cu. ft., for 

 broken-stone and gravel concrete, and 110 to 115 for cinder 

 concrete. ^_____ 



REINFORCED CONCRETE 



FORMULAS FOR RECTANGULAR BEAMS 



Reinforced concrete is concrete in which steel or iron is 

 embedded in order to increase the strength of the former. 



Fundamental Principles. Many theories have been advanced 

 as a basis for the design of reinforced-concrete beams, and 

 it is not yet known which is most nearly correct. The formu- 

 las that follow are based on the so-called straight-line theory, 

 which has been almost universally adopted in the United States 

 and has been recommended by a Joint Committee composed 

 of members of the leading engineering societies of this country. 

 This theory is based on the following assumptions, and principles 

 derived from these assumptions: 



1. A plane section of a beam remains plane after it has been 

 subjected to bending. 

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