CONCRETE AND STEEL IN COMBINATION 37 



considered to be 50) in the steel itself plus the tensile stress 

 necessary to crack the concrete. A high elastic-limit steel is 

 thus advantageous. 



The size and spacing of the cracks will also depend upon the 

 bond strength of the reinforcing rods. The distance between 

 cracks in any given case will be the length required to develop a 

 bond strength equal to the tensile strength of the concrete. Thus, 

 bars with irregular surfaces which provide a mechanical bond 

 with the concrete are in general more effective than smooth bars. 



28. Repetition of Stress. Experiments on cubes of neat cement 

 and concrete for repeated loads have shown that the limit of 

 lOOi 



4000 8000 12000 16000 EOOOO 34000 8000 32000 34000 



Number of Repetitions necessary to produce failure 

 FIG. 11. General curve of the fatigue of concrete. 



permanent elasticity for such loads is from 50 to 60 per cent of the 

 ultimate strength. Experiments on reinforced beams also indi- 

 cate similar results. In the case of beams, the failure under 

 repeated loads appears to be largely a gradual fracture in diag- 

 onal tension, ending with a crushing of the concrete at the upper 

 surface of the beam. 



An account of experiments to determine the effect of repeated 

 stress on concrete is given in Trans. Am. Soc. C. E., Vol. LVIII. 

 The summary is shown in Fig. 11, and it is seen that the number 

 of repetitions required to produce failure varies with the load 

 applied. For example, if the load applied is 60 per cent of the 

 ultimate, 4000 applications cause failure; while if the load 

 applied is 70 per cent, about 1000 applications are required. 



