256 STRENGTH OF MATERIALS 



remainder in water, until tested. The kind of cement as well as its 

 physical properties must be known ; also the kind of sand and stone 

 and the degree of fineness of each. 



When ready for testing, the concrete cubes are placed in the testing 

 machine, bedded with plaster of Paris or thick paper, and tested in 

 compression. The load at first crack and the maximum load are noted. 



The table on the opposite page is a report of a series of tests made 

 at the Watertown Arsenal on Akron Star cement concrete in com- 

 pression.* It will be noticed that the ultimate strength varied from 

 600 lb./in. 2 to 2700 lb./in. 2 . 



The table on page 258 is taken from the same volume as the 

 preceding, and summarizes the results of tests on concrete made 

 from different kinds of cement. Various kinds of broken stone were 

 used, including broken brick, and the ultimate strength ranged from 

 600 lb./in. 2 to 3800 lb./in. 2 . In making comparisons from the tahle 

 as to strength several things must be noted, namely, the kind and 

 strength of the cement, the proportions and eliaracter of the sand 

 and gravel, the treatment after making, and the age when tested : in 

 other words, a complete history of the materials and their treatment 

 should be known. In the following table the cubes tested wer* 

 in air, in a dry, cool place. 



The location and character of the structure will often determine 

 the kind of materials to be used in making the concrete. Thus, on 

 account of convenience, pebbles are sometimes used with the saml 

 in which they are found. This reduces the cost of the concrete, hut 

 usually impairs its strength, as the proportions of sand and stone as 

 they occur in nature are in a likely to be such as to be suitable for 

 concrete. Theoretically, to get the best results the proportions should 

 be such that the cement fills the spaces between the grains of sand, 

 and the mortar fills the spaces between the pieces of stone. 



In any particular case the cost of material, strength of the concrete, 

 and service required of the structure must determine what propor- 

 tions shall be used. 



Problem 174. A concrete cube 12 in. high when tested in compression sustain* <1 

 a load of 324,000 Ib. at first crack, and 44-VJOO Ib. at failure. Find the int. 

 of the compressive stress in lb./in. 2 at first crack and at failure. 



* Watertown Arsenal Report, 1901. 



