CONCRETE AND STEEL IN COMBINATION 27 



structure, steel may be used with concrete in the form of small 

 rods to reinforce the concrete; or it may consist of structural 

 steel shapes in the form of a core, simply surrounded and held 

 rigidly in place by the concrete most of the load being carried 

 by the steel; or finally, a steel column may be used and the 

 concrete employed merely for the purpose of adding symmetry 

 and providing fire protection. This last case does not come under 

 the head of reinforced concrete and will not be considered in 

 this course. The use of a moderate amount of steel with concrete 

 so as to produce a material with a reliable tensile and bending 

 resistance has opened the way for the use of this combination 

 in a great variety of structures and in special individual forms. 



24. Bond Between Concrete and Steel. Usually the entire 

 stress which is carried by the steel of a reinforced concrete 

 beam is transmitted to the reinforcing bars by the bond or 

 adhesion between the concrete and the steel. Stress is also 

 conveyed to the steel of a reinforced concrete column in like 

 manner. Experience has shown the bond to be reliable and per- 

 manent, and that plain bars may be used in most structures with 

 success. Deformed and twisted rods are used to a large extent 

 in structures where the stress between the steel and concrete 

 exceeds the safe working adhesion of the plain rod. The in- 

 dented surfaces of deformed bars produce a mechanical bond in 

 addition to the adhesion already mentioned. 



The values derived from different experiments for the adhesion 

 in pounds per square inch of contact surface vary quite widely. 

 With plain rods, results vary from about 200 to 750 Ib. per square 

 inch. The quality of the concrete and the manner of making 

 the tests are important factors. 



Tests to determine the strength of bond are usually made 

 either by pulling out a rod that has been embedded in a block 

 of concrete or by forcing the steel out by compression. In either 

 case the concrete is in compression, and there is the tendency 

 to increase the friction by adding to the pressure at the surfaces 

 in contact. In an important series of tests made at The Univer- 

 sity of Wisconsin, test beams were arranged as shown in Fig. 7 

 the reinforcing bars being embedded only a short distance from 

 each end, leaving the middle portion exposed. The stress in the 

 rods was computed from the observed deformations. The 

 conditions were quite similar to those which exist in an ordinary 

 beam, but the beam was prevented from failing in the early 



