KEENFOKCED CONCRETE 



265 



plain rods, failure was due to the breaking of the rods or the cracking 

 of the concrete. On account of the projections on some of the rods 

 these can hardly be called adhesion tests, but should more properly 

 be called pulling-out tests. 



As might be expected, the plain rods show the lowest values, since 

 any reduction in cross section of the rod, due to the tensile stress 

 upon it, largely destroys the adhesion of the concrete. Square reen- 

 forcing rods, or those that present sharp angles, are likely to cause 

 initial cracks upon the shrinkage of the concrete. To have the 

 strongest bond a rod should be round, with rounded projections. 



PULLING-OUT TESTS 



216. Area of the metal reenforcement. Since the small hairlike 

 cra.-ks mentioned in Article 213 occur early during the flexure of 

 a reenforced concrete beam, it is evident that in designing little can 

 be allowed for the tensile strength of the concrete. The problem 

 becomes one of opposing the compressive strength of the concrete 

 and the tensile strength of the reenforcement. This means that 

 knowing the safe compressive strength of the concrete and the area 

 of the concrete in compression, sufficient steel must be used to carry 

 safely a tensile load equal to the compressive load on the concrete. 

 Professor Marburg, in the paper referred to in the preceding article, 



