PORTLAND CEMENT TESTING. 161 



each instance during troweling; those outside will be exposed more than 

 the inner ones and the evaporation caused hj contact with the air may 

 cause setting. We would expect a greater variation from this cause in 

 quick setting cements than in slow ones; and our experience has confirmed 

 thisu Slow setting cements give the least variation in tensile strength. 



2. Another cause of variation is the tendency possessed by some cements 

 to enclose air bubbles, thus producing irregular voids. 



3. Unequal hardening of the exterior and the interior of the cement 

 briquettes may cause differences in heat generated during setting and 

 variable water action during submersion. This cause may also produce 

 internal strains and voids. This variability would also be especially 

 marked in quick setting cements. 



4. Irregularity in the intermingling of the crystals during crystal- 

 lization. 



In summarizing the above conditions, only errors which are unavoidable 

 and such as might occur in a batch of four briquettes made and manipu- 

 lated in the same manner and under the same conditions have been 

 assumed, and our endeavor has therefore been,' if possible, to minimize 

 the personal error, and to this end a new type of tamper differing from 

 that specified by the United States Army engineers was adopted. We 

 found it impossible to raise the specified tamper exactly one-half inch 

 at every blow, and at the same time to apply the blow Just where we 

 wanted it. A simple, accurate, easily and quickly manipulated tamper 

 which gives the same force to every blow, and hits the exact spot desired, 

 Avas therefore devised by us. 



A (fig. 5) is a thin, hollow cylinder open at d and closed at e. It weighs 

 about 60 grams. B (fig. 7) is a solid brass rod which weighs -just 1 pound. The 

 end bearing the lug 6 is inserted into the cylinder A, h following the groove a. 

 To manipulate this instrument, the rod B is held near the top with the thumb 

 and forefinger of the right hand, A being held in the same way with the left. 

 The lug & is drawn hard against the angle in the groove a, and the end e is placed 

 on the surface of the cement just where it is desired to have the blow strike. 

 The rod is then dropped and at the same time the hold on A is loosened. A 

 little practice will enable any one to operate this tamper very quickly, and at 

 the same time to deliver an unvarying blow due to the half-inch drop of the one 

 pound rod. The blows can be directed at will and it is not possible to hit tlie 

 edges of the mold. 



The United States Army specifications direct the tester to raise the 

 tamper one-half inch above the surface of the cement. As the paste and 

 mortar are put into the molds in a lumpy condition, no plane surface 

 line is presented, and as we wished to control the force of each blow as 

 much as possible, a surfacer was devised to enable its to have uniform 

 plane. 



