CONCRETE AND STEEL IN COMBINATION 35 



26. Behavior of Reinforced Concrete Under Tension. The be- 

 havior of reinforced concrete under tension, and especially when 

 constituting the tensile side of a beam, has been the result of much 

 study by many experimenters. Early tests indicated that the 

 ultimate strength of reinforced concrete is as much as ten times 

 that of plain concrete, but such results were due to the fact that it 

 was found extremely difficult to determine just when the concrete 

 begins to crack. Cracks do not become noticeable, even on very 

 close examination, until a stretching occurs corresponding to a 

 tensile stress much beyond the ultimate strength of the concrete. 

 The steel forces the concrete to elongate uniformly throughout, 

 so that a crack will open up very slowly and will remain invisible 

 for some time. 



A method of detecting minute cracks was accidentally discov- 

 ered in 1901-2 in some experiments made at the University of 

 Wisconsin. It was found that when beams were hardened in 

 water and only partially dried before testing, very fine hair-cracks 

 became noticeable at a moderate load. Before these cracks 

 occurred, however, dark wet lines appeared across the beam, and it 

 was observed that each of these lines was later followed by a very 

 fine crack. That these watermarks were incipient cracks was 

 determined by sawing out a strip of concrete along the outer 

 part of the beam. Careful measurements of extension showed 

 that these streaks, or watermarks, occurred at practically the 

 same deformation at which the concrete ruptured when not 

 reinforced. This same phenomenon has since been observed 

 by many careful experimenters, and the fact is now generally 

 established that concrete, reinforced with steel, does not elongate 

 under tensile stress to any greater extent before cracking than 

 plain concrete. 



A reinforced concrete beam for working loads is usually more 

 heavily stressed on the tension side than the ultimate tensile 

 strength of plain concrete enough steel being usually embedded 

 near the lower face to permit the full allowable compressive 

 strength of the concrete to be utilized. The presence, then, of the 

 cracks above referred to, occurring long before a reinforced con- 

 crete beam has obtained its working load, must seriously affect 

 the tensile strength of the concrete. The formulas now in most 

 general use for the design of reinforced concrete beams neglect 

 entirely the tensile strength of the concrete. 



The important question which arises is how far concrete may 



