CONCRETE 19 



the compressive. Tests seem to show that reasonable values for 

 ultimate strength in direct tension at the end of one month are 

 about as follows: 



1:2:4 mixture. . . . 160-200 Ib. per square inch 

 1:3:6 mixture. . . . 100-125 Ib. per square inch 



The tensile strength of concrete at the place of greatest stress, 

 that is, at the fiber most remote from the neutral axis, limits the 

 strength of unreinforced concrete beams. Transverse tests of 

 plain concrete should therefore show about the same relative 

 results as tensile tests, and, in fact, they are quite as significant in 

 this connection. The value of the ratio of modulus of rupture 

 to tensile strength will ordinarily range from 1.8 to 2.0. 



12. Shearing Strength. A true shearing failure can only 

 occur when the external forces producing the failure act in 

 opposite directions and close together. The actual strength of 

 concrete in direct shear is greater than was formerly supposed 

 because, in the earlier experiments, shear was confused with the 

 diagonal tension which occurs in the web of a beam. Diagonal 

 tension is explained at length in Chapter IV. At the present 

 time it is sufficient for the student to realize that, in this 

 article, shear is considered to be the strength of the material 

 against a sliding failure, when tested as a rivet or bolt would be 

 tested for shear. 



Very few tests have been made to determine the shearing 

 strength of concrete since it is difficult to arrange an experiment 

 to determine this strength without involving some bending 

 stress. Perhaps the best set of tests was made at the Massachu- 

 setts Institute of Technology in 1904 and 1905 under the direction 

 of Prof. Charles M. Spofford. Three grades of concrete were used 

 1:2:4, 1:3:5, and 1:3:6 and the specimens were stored in 

 both air and water from 24 to 32 days. The specimens tested 

 were cylinders 5 in. in diameter and 18 in. long, and the end 

 thirds were held rigidly by cast-iron yokes. The pressure was 

 applied through a cast-iron, half-cylinder bearing which fitted 

 between the two yokes and caused the shearing of the concrete 

 across two planes. Six extra cylinders of the same concrete 

 were tested at the same time for compression. The tests gave a 

 shearing strength ranging in general from 60 to 80 per cent of 

 the compressive strength of the concrete. 



13. Contraction and Expansion. The contraction and expan- 



