CONCRETE 



I5i 



is continued, making a tongue and-groove joint as indicated 

 in Figure 145. 



Reinforcement. Where a thin wall of concrete with 

 great strength is desired, steel in the form of rods or special 



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VMS ^RODS 



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Ject/on o/trf-B 



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1 



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Fig. 146. Typical reinforced concrete beam 



design is placed in the forms before casting (Fig. 146). The 

 object of reinforcement is to increase the tensile strength, 

 which is the weak part in cement work. The concrete itself 

 has sufficient strength to resist the compressive strains. 

 When reinforced work is necessary, advice should be 

 secured from an experienced person as to the kind and 

 method of reinforcing to be used. 



Strength of concrete. The strength of concrete is depend- 

 ent upon the mixture, the age, the condition of the materials 

 used , and the method of curing . Tensile strength is neglected 

 and compressive strength is the only thing considered, as 

 concrete structures which are not reinforced are usually so 

 built that the stresses are only compressive. The actual 

 load which is to be carried is much less than the load required 

 to crush the completed concrete. As an example, 1:2:4 

 concrete will safely carry from 600 to 700 pounds per square 

 inch under ordinary conditions, but the actual crushing 



