666 



THE POPULAR SCIENCE MONTHLY 



In speaking of cement testing, Prof. Griffin says : 

 " No one of these" (meaning tests for compression, tension, tor- 

 sion, and cross-strain) " can be dispensed with, since material that 

 will endure one satisfactorily will utterly fail in another; . . . but 

 for general purposes the test of cement which is the most valu- 

 able is that which determines its tensile strength." 



There are very few cases in practice where any tests other 

 than for tension are made. The statement that " no one of these 

 can be dispensed with," etc., is contradicted by what follows, that 

 the most general test is for tensile strength. 



From the very nature of cement, these necessary qualities are 

 so dependent one upon the other that practice and experiment 

 have shown that, where one of these physical tests is passed satis- 

 factorily, the others within certain limits must also be satisfac- 

 tory. It is due to this fact alone that tests for tensile strength are 

 accepted as standard, as in construction cement should never be 

 subjected to tension or cross-strain, but is usually subjected to 

 compression, or possibly in some cases to torsion ; but because 

 the compressive strength in cement is generally proportional to 

 its tensile strength, tension tests have, on account of the facility 

 and accuracy with which they can be made, been adopted as 

 standard. 



The form of the test briquette given by Prof. Griffin is not 

 that approved by the American Society of Civil Engineers and 



adopted in all standard cement tests. 

 The approved form of briquette is that 

 shown in Fig. 1. 



These briquettes are usually made 

 by hand, as described by Prof. Griffin. 

 But unless a great deal of help is avail- 

 able, the process is much too slow for 

 any very extended series of tests ; the 

 amount of mortar that can be mixed 

 at once is small ; and where different 

 persons are employed it is impossible 

 to obtain briquettes that give satisfac- 

 tory comparative results, owing to the 

 difference in the personal equations of 

 the makers. This was soon found to be 

 the case in the " State University of 

 Iowa " cement tests, and a specially designed machine was built, 

 having a capacity of making over three thousand briquettes per 

 day, being run by two men. This made possible the making of a 

 much greater number of briquettes under practically the same con- 

 ditions. Owing to the greater amount of pressure machine-made 

 briquettes are subjected to (about one hundred and fifty pounds 



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^^T_L ^ 



Fig. 1. Standard Form of Bri- 

 quette, One Inch in Thickness. 



