LIME, CEMENT, AND CONCRETE 



251 



155 



which provide for a cross section of one square inch at the middle, 

 with thicker ends for insertion in the jaws of the testing machine! 

 This test requires considerable expertness to get satisfactory results, 

 for the proper mix- 

 ing and tamping into 

 the -molds can only 

 be satisfactorily done 

 by one of consider- 

 able experience. After molding, the briquettes are kept under a damp 

 cloth for about twenty-four hours and then under water until tested. 



Many machines are now made for testing the tensile strength of 

 cement, most of them being light enough to be portable. A new 

 automatic machine, manufactured by the Olsen Testing Machine 

 Company of Philadelphia, is shown in Fig. 156. The machine is 

 operated by first placing the briquette in position and balancing the 

 lie; ti 11 at the top. The load is then applied by allowing the shot to run 

 from the pan on the right end of the beam. The spring balance gives 

 the exact weight of the shot and, consequently, the tensile stress on the 

 briquette at any time during the test. After the briquette is broken 

 the tensile strength in pounds per square inch is recorded on the dial. 



204. Speed of application of load. It has been found that the 

 rapidity with which the load is applied has considerable effect upon 

 the results obtained in making tension tests of cement. The follow- 

 ing table clearly shows this effect.* 



EFFECT OF SPEED OF APPLICATION OF LOAD ON TENSILE 

 STRENGTH OF CEMENT 



Proc. Inst. Civ. Eng., 1883. 



