718 -NEW YOElv STATE MUSEUM 



be all exactly of the same size, and have the sides exactly parallel 

 and the ends exactly perpendicular to them. This is only ob- 

 tainable when large cubes are used, and these require great ma- 

 chines to crush them. 



M. Gary states^ that in order to obtain agreement between 

 different compression tests, the specimens should be made by 

 machinery, and gives the following directions. 



Take 400 grams (14 ounces) of cement and 1200 grams (42.2 

 ounces) of dry standard sand, mix thoroughly in a dish, add 160 

 grams (3.6 ounces) of water, and work the resulting mortar 

 thoroughly for five minutes (quicksetting cements are to be 

 worked but one minute). Put 860 grams (30.3 ounces) of this 

 mortar into the cubic mold properly provided with filling cases 

 and fastened to the bed plate. The iron core is placed into the 

 form, and 150 blows are delivered on it by means of the hammer 

 apparatus, with the hammer of 2 kilograms (4^ pounds) weight. 

 The filling cases and core having been removed, the specimen is 

 struck off flush, smoothed and drawn off the bed plate together 

 with the mold. 



For neat cement specimens, mix about 1000 grams (2.2 pounds) 

 cement with the requisite amount of water. The molds should 

 be oiled a little, and can be removed only after the cement has 

 sufficiently hardened, which is usually from 20 to 24 hours after 

 making. 



"While the elasticity of Portland cement decreases after some 

 years, and the tensile strength ceases to grow after a similar 

 period, its compressive resistance increases. 



The machines used for determining the compressive strength 

 are similar to those employed for the crushing of building stones 

 and bricks. The compression test is seldom carried out in the 

 United States. 



Tensile strength 



To carry out this test the cement is mixed with water to the 

 consistency of a stiff paste and formed into briquets, this being 

 done by means of brass molds. 



When the cement alone is used it is spoken of as " neat '' ce- 

 ment. When mixed with sand the term " cement mortar '^ is 



*Tr-ins. Am. soc. civ. eng. 30: 25. 



