264 STRENGTH OF MATERIALS 



subjected to the severest exposure. However, the existence of cracks 

 on the tension side of reen forced beams makes the exposure of tin* 

 metal rods possible, and thus adds a new danger to the life of the 

 beam; but the small hairlike cracks that occur after the elastic limit 

 of the concrete has been passed probably have no effect in this respect. 

 When they become large enough to expose the reenforcement, tin- 

 strength of the beam is endangered. 



215. Adhesion of the concrete to the reinforcement. When a reen- 

 forced concrete beam is subjected t<> stress, there is always a tendency 

 to shear horizontally along the reenforcement. This is prevented in 



part by the adhesion between tin- >teel and 

 v - concrete. Failure often occurs, due to this 

 horizontal shear, especially when the beam is 

 over-reenforced, i.e. when the area of cross 

 section of the ive'nt'oivcment j s large as com- 

 pared with the total area of cross section of 

 the beam. When plain round or square rods 

 are used, the adhesion between the Mc.-l and 

 CMii.-ivte furnishes the <nly bond. For 



1 '" ' ' mereial purposes, howe\ forms of 



i.Kah.,tn,sH,,ihar:-j .Mms,,,, [{ ^ are ordinarily used to increase 

 corrugated inn-: "-, ih.-K-iier 



i.uii.i.ar; i.Ransometwisu-.i this l>.nd. Four of these commercial types 



h:ir are illu>trated in Fig. 159. The .Join 



Thacher, and Kansome bars aiv provided with projections and inden- 

 tations to prevent the bar from pulling out of the concrete, while the 

 Kahn bar, by means of the projecting arms that extend upward along 

 the lines of principal Btre&fl in the beam, is also designed to act as 

 a truss. Several other commercial types of bar are also in use. hut, 

 all are provided with projections or indentations of some kind to 

 prevent slipping. 



Many tests have been made to determine the force necessary to 

 pull the various forms of roils from concrete. The following table 

 gives the results of pulling-out tests made by Professor Edgar Mar- 

 burg, of the University of Pennsylvania.* The rods in this case were 

 imbedded centrally in 6 in. x 6 in. concrete prisms 12 in. long, and 

 were tested after thirty days. In most cases, except in that of the 



* Proc. Amer. Soc.for TV rials, 1904. 



