Page:Popular Science Monthly Volume 38.djvu/684

666 In speaking of cement testing, Prof. Griffin says:

"No one of these" (meaning tests for compression, tension, torsion, 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 valuable 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 satisfactorily, the others within certain limits must also be satisfactory. 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 available, 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 satisfactory 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 conditions. Owing to the greater amount of pressure machine-made briquettes are subjected to (about one hundred and fifty pounds