CONCRETE AND STEEL IN COMBINATION 29 



The initial movement in the case of indented bars is 

 probably due to a slight crushing of the concrete under the 

 projections. The strength mechanically obtained from the 

 indented surfaces, however, depends upon the shearing strength 

 of the concrete. The bars cannot be pulled through the concrete 

 without shearing off an area equal to the total area of the in- 

 dented portion and, in addition, overcoming adhesion along the 

 remaining surface. In tests of such bars, failures have usually 

 occurred by the splitting of the specimen or by the breaking of 

 the bar, but the results indicate that at least 500 to 600 Ib. per 

 square inch in bond may be expected. 



FIG. 8. 



It is a common practice to bend the ends of reinforcing rods 

 into hooks consisting of either curved or right angle bends. 

 Experiments on the efficiency of such hooks have shown that 

 the ultimate bond strength was greatly increased. It was found 

 that with an embedment in concrete in all directions equal to 8 

 diameters of the bar, a hook of 5 diameters may be assumed to 

 develop the elastic limit of the steel. When curved ends are 

 used, they should consist of bends through 180 degrees with a 

 short length of straight rod beyond the bend, as shown in Fig. 8. 

 Short square hooks upon the ends of bars are not of great value. 



25. Ratio of the Moduli of Elasticity. In order to treat the 

 subject clearly, it will be necessary to give a somewhat extended 

 discussion of the term modulus of elasticity. The modulus of 

 elasticity of a material such as steel is the ratio of the unit stress 

 to the corresponding unit deformation, provided the elastic limit 

 of the material is not exceeded. This ratio is usually denoted 

 by E } or 



in which / denotes the stress in pounds per square inch and d the 

 corresponding deformation per unit of length. 



