28 REINFORCED CONCRETE CONSTRUCTION 



stages of the tests, by an upper set of auxiliary rods. Failure 

 finally occurred by the pulling out of the lower rods, as intended. 

 Tests indicate that the bond per square inch of surface is not 

 affected by the size of rod. A study of the results of many 

 experiments leads to the conclusion that for ordinary round 

 or square bars, not too smooth, the adhesive strength may be 

 taken at from 200 to 300 Ib. per square inch. After bond is once 

 broken, plain rods give a frictional resistance of about two- 

 thirds of the original bond resistance; in other words, the rein- 

 forcement may be moved slightly with reference to the concrete, 

 as by a sharp blow, and still leave about two-thirds of the bond 

 strength effective. 



2X1 "7\ 



FIG. 7. 



A rough surface gives a higher adhesive value than a smooth 

 surface, consequently a thin film of rust on the reinforcing metal 

 should not cause its rejection. Loose or scaly rust should not be 

 allowed, however. Bars in this state of corrosion may be used, 

 if they are first cleansed with a stiff wire brush or given a pickling 

 bath of sulphuric acid solution (consisting of 1 part acid to 6 

 parts of water) , and then dipped in clean water. Oiling and paint- 

 ing of reinforcing steel should not be permitted, as the adhesion 

 is greatly reduced thereby. Round bars show the greatest ad- 

 hesion flat bars the least. 



Let f s be the working tensile strength of the steel, a s the area 

 of bar, o the circumference of bar, d the diameter or thickness 

 of bar, u the working unit bond strength, and x the required 

 length of embedment (or grip) for the above values of f s and u. 

 Then, to develop the strength of the steel, using either round or 

 square bars, 



, nd* , 

 xou=a s f 8 =~f 8 



f s d 

 or x=~ 



