266 



STRENGTH OF MATERIALS 



gives 1600 lb./in. 2 for the compressive strength of 6-inch cubes 

 thirty days old. A slightly higher value was found for cubes from 

 a different mixture. 



From an investigation of the tensile strength of steel reenforcing 

 bars, the writer referred to above obtained the following values. 



With a 1-3-6 concrete a 1.5 per cent reenforcement of steel, 

 having an elastic limit of 33,000 lb./in. a , and a 1.0 per cent i 

 ment of steel, having an elastic limit of 55,000 lb./in. 2 , has been 

 used without developing the full compressive strength of the concrete.* 

 In this case the percentage is figured oil the area of concrete ahovr 

 the center of the metal reenforcement. This percentage may also be 

 figured on the area of cross section of the beam. 



217. Position of the neutral axis in reenforced concrete beams. 

 In Article 210 it was pointed out that the modulus of elasticity of 

 concrete in compression is not constant. This indicates that in the. 

 case of flexure the position of the neutral axis changes with tin > 

 at first lying near the center, but moving toward the compression side. 

 as the load is increased. In a reenforced concrete beam the neutral 

 axis also undergoes a displacement, due to the non-homogeneity of 

 the cross section, since the moduli of elasticity of steel and concrete 

 are not the same. In this case, if the beam is reenforced only on the 

 tension side, and the metal reenforcement is designed to carry all the 

 tensile stress, the neutral axis usually lies nearer the tension side of 

 the beam than the compression side.f 



From tests made at Purdue University, Professor Hatt found the 

 ratio of the moduli of elasticity of steel in tension to concrete in 



* Proc. Amtr. Sor. for Testing Mutrr'xils. 100.". 



t Se> jirtirl*' by S. E. Sloruin. entitle*! " Rational Formulas for the Strength of a Con- 

 crete Steel Beam," Engineering News, July 30, 1903. 



