79 



Before the rod can come into operation during a flexure of tlie beam, 

 tliere must be the necessary adhesion between the concrete and the rod to 

 transfer the stress to the rod. and bring the latter into action. This ad- 

 hesion varies from oOO i)ouuds to 500 pounds per square inch of the sur- 

 face of the rod. and under favorable conditions is sufficient to develop the 

 strength of the steel in the concrete. The adhesion seems to be more of a 

 mechanical action than chemical, and is due to the entrance of the fine 

 cement into the microscopic pits on the surface of the smooth rods. Many 

 designers use artificially deformed bars, such as corrugated bars and 

 twisted steel bars, to increase this adhesion. 



In this way a beam is reinforced so that both the concrete in compres- 

 sion and the steel in tension may be worked to their full value. Any one 

 who has seen a plain concrete beam broken in a testing machine, and then 

 has witnessed a tost of a reinforced concrete beam, will be first of all 

 struck by the apparently greatly increased flexibility of the reinforced con- 

 crete beam, which deflects ten times as much as the plain beam before 

 showing any visible cracks, and when the load is removed the elasticitj' of 

 the steel draws the beam back nearly to its original shape. It is probable, 

 however, that this process of bending the reinforced concrete beam early 

 develops very minute flaws in the concrete which are invisible to the 

 naked eye, so that it is not safe to count upon a tensile strength of the 

 concrete in computing the total resisting strength of the beam. Designers 

 compute the resisting moment of the beam as based upon the compressional 

 stresses in the concrete and the tensional stress in the steel alone. 



The original tests at Purdue University were arranged to determine: 



1. The increased strength added by a given amount of steel inserted in 

 a plain concrete beam. 



2. The law connecting the strength of the beam with the amount of 

 steel. 



3. The law connecting the strength of the beam with the position of 

 the rods in the beam. 



4. The vahie of gravel in reinforced concrete. 



To determine these relations a series of concrete beams was made of 

 first-class materials with rich mortar. In other words, the beams were 

 carefully made with a combination of one part cement to two parts of 

 sand and four parts of broken stone. The concrete was probably superior 

 to that made in the ordinary process of construction. This was proper be- 

 cause the theoretical laws were being verified, and for that purpose it was 



