PLAIN CONCRETE 239 



as in footings, a 1-2^-5 concrete would safely sustain as 

 much as 500 Ib. per sq. in. When reinforced concrete is 

 subjected to compression from loads causing bending, it is 

 customary to figure the safe allowable unit compressive 

 stress in the compression portion of a reinforced-concrete 

 beam at from 500 to 600 or even 750 Ib. per sq. in. 



In tension, concrete has little value; in fact, it cannot be 

 relied on to resist this stress. Generally, the tensile strength 

 of plain concrete is about one-tenth of its compressive 

 strength. 



The modulus of rupture, or the unit value for figuring the 

 transverse strength of plain concrete, is much lower than 

 the modulus of rupture of any of the good building stones. 

 The safe unit bending stress for plain concrete, based on a 

 factor of safety of 4, from values of the modulus of rupture 

 obtained from recent tests made on concrete 30 da. old, is 

 about 110 Ib. This value is for concrete composed of 1 part 

 of cement, 2 parts of sand, and 4 parts of broken stone. 

 With a poorer mixture, as a 1-2-5 concrete, a safe bending 

 stress of about 95 Ib. should be used, while with a 1-3-5 

 mixture, the safe bending stress is barely 70 Ib., and this 

 value shows a corresponding decrease as the mixture becomes 

 leaner. 



The safe unit shearing stress of plain concrete is, in practice, 

 taken at a very low figure when compared with recent tests 

 giving the ultimate shearing resistance of this material. 

 This low figure is probably due to the fact that few tests 

 have been made to determine the value of plain concrete 

 in shear; or perhaps it is due to the unreliability of concrete, 

 as found in practice, to resist this stress. The conservative 

 safe unit shearing stress of plain concrete is taken .at 50 Ib., 

 although the value may be increased for rich mixtures and 

 careful workmanship to 75 Ib. 



The safe grip, or bond, as it is called, of concrete on steel 

 rods or bars with plain surfaces embedded in it, is taken, for 

 purposes of calculation, at 50 Ib. per sq. in. of the surface 

 of the metal in contact with the concrete. 



All the values just mentioned are based on concrete at 

 least 1 mo. old. There is great diversity of opinion regard- 



