16 REINFORCED CONCRETE CONSTRUCTION 



Elevated Railway Company at the Watertown Arsenal in 1899. 

 The test pieces were 12-in. cubes. 



The theoretical angle of rupture in crushing is about 60 degrees 

 with the horizontal, and this theoretical conclusion is borne out 

 by actual tests. For example, cubes of concrete will leave after 

 crushing, pyramids whose surfaces are at an angle of about 60 

 degrees with the base. Thus, it should be clear that to develop 

 simply the normal compressive strength, the height of a speci- 

 men should be at least one and one-half times its least lateral- 

 dimension. 



Conclusive evidence of the increased strength of cubes as 

 compared with cylinders, due to the reason above given, is 

 shown by the United States Government tests at St. Louis, 

 the results of which are discussed in "the U. S. Geological 

 Survey Bulletin No. 344, 1908. Computations from these tests 

 give a ratio of strength of 8 in. Xl6 in. cylinders to 6-in. cubes, 

 at ages of thirteen and twenty-six weeks, as 0.88. This value 

 coincides almost exactly with the empirical formula evolved 

 by Prof. Johnson from results on sandstone and cast-iron prisms. 

 The formula follows: 



strength of cylinder _ 7 fi 'diameter of cylinder 

 strength of cube height of cylinder 



A study of a number of tests on concrete tend to show that this 

 formula may be applied with sufficient accuracy, considering 

 the variability of the material, when comparing all sizes of 

 concrete cylinders and cubes. 



Tests were formerly made on specimens of cube form, but 

 recently the prismatic or cylindrical form of a height of 2 to 3 

 diameters has been more commonly employed. Since there is a 

 greater freedom for shearing action in the cylindrical specimen, 

 it is generally used in studying the results of tests on columns. 

 The cube form, however, is useful for comparison with the 

 compressive strength of concrete in a beam. 



Tests on cylindrical specimens under different conditions show 



