612 CEMENTS, LIMES, AND PLASTERS. 



as practicable; they may be stored in tanks or pans, which should 

 be of non-corrodible material. 



TENSILE STRENGTH. 



67. The tests may be made on any standard machine. A solid 

 metal clip is recommended. This clip is to be used without cushion- 

 ing at the points of contact with the test specimen. The bearing at 

 each point of contact should be \ inch wide, and the distance between 

 the centers of contact on the same clips should be 1^ inches. 



68. Test pieces should be broken as soon as they are removed from 

 the water. Care should be observed in centering the briquette in the 

 testing-machine, as cross-strains, produced by improper centering, tend 

 to lower the breaking strength. The load should not be applied too 

 suddenly, as it may produce vibration, the shock from which often 

 breaks the briquette before the ultimate strength is reached. Care 

 must be taken that the clips and the sides of the briquettes be clean 

 and free from grains of sand or dirt, which would prevent a good bear- 

 ing. The load should be applied at the rate of 600 Ibs. per minute. 

 The average of the briquettes of each sample tested should be taken as 

 the test, excluding any results which are manifestly faulty. 



CONSTANCY OF VOLUME. 



69. Significance. The object is to develop those qualities which 

 tend to destroy the . strength and durability of a cement. As it is 

 highly essential to determine such qualities at once, tests of this char- 

 acter are for the most part made in a very short time, and are known, 

 therefore, as accelerated tests. Failure is revealed by cracking, check- 

 ing, swelling, or disintegration, or all of these phenomena. A cement 

 which remains perfectly sound is said to be of constant volume. 



70. Methods. Tests for constancy of volume are divided into two 

 classes: (1) normal tests, or those made in either air or water main- 

 tained at about 21 C. (70 F.}, and (2) accelerated tests, or those made 

 in air, steam, or water at a temperature of 45 C. (115 F.) and upward. 

 The test pieces should be allowed to remain twenty-four hours in moist 

 air before immersion in water or steam, or preservation in air. 



71. For these tests, pats, about 1\ cm. (2.95 inches) in diameter, \\ 

 cm. (0.49 inch) thick at the center, and tapering to a thin edge, should 

 be made, upon a clean glass plate [about 10 cm. (3.94 inches) square], 

 from cement paste of normal consistency. 



