NATURAL AND ARTIFICIAL CEMENTS. 



255 



Fig. 1 



amount of water to render it plastic. Too small an amount will 

 leave some particles dry; too large an amount will gather in 

 masses, will evaporate, leaving pores, and will give too small 

 results. The test is now commonly made by molding a bri- 

 quette of a form approved by engineers, as shown 

 in Fig. 1, which is drawn of one third actual size. 

 The mold is a clamp of metal exactly one inch in 

 thickness and exactly one inch across at R. This 

 makes the area of the smallest place exactly an 

 inch. The moistened cement is carefully placed in 

 the mold with a spatula and pressed enough to ren- 

 der the whole mass homogeneous. It is left in the 

 mold until it can be removed by opening the mold, 

 and then it is exposed to the air for exactly twenty- 

 four hours, after which it is put into water and 

 allowed to rest there until the test is made. ■ The 

 length of time depends upon the purpose of the 

 test. In order to make certain that all the cement 

 produced is of a uniform quality, seven days is suf- 

 ficient. Such a test is made of every lot shipped 

 by the Milwaukee Cement Company, and probably by all other 

 reliable manufacturers. If the test is to determine the ultimate 

 strength developed or to compare cements from different sources, 



then a series of tests should be made by break- 

 ing " briquettes " made at the same time but left 

 in water for different periods. The reason is, 

 that a quick-setting cement 'will develop its full 

 strength in a short time, and if the test is made 

 at the end of that time it might show a greater 

 tensile strength than another one slow in setting, 

 even when the latter would ultimately have sev- 

 eral times its strength. 



The test can be made in any form of testing 

 machine, though one in which the test is applied 

 by uniformly increasing the strain, as by running 

 shot into a bucket upon the end of a lever, gives 

 the most accurate results ; but the briquette 

 should be held in a clutch that presses accu- 

 rately upon the sides, as shown in Fig. 2. This 

 applies the tension equally, and gives a very ac- 

 curate test. A long series of these were made 

 by Mr. D. J. Whettemore, C. E., at Milwaukee in 

 1874, in which seventeen native cements showed 

 an average tensile strength at the end of seven 

 days of 80 &• pounds. The lowest of these broke at 38 pounds, 

 while the highest sustained 139f pounds. Later tests made in 



Fig. 2. 



