PORTLAND CEMENT TESTING. 141 



The diagram calls attention to the lack of uniformity in the results 

 attained in each sample. The differences in the tensile strength are not 

 only between the two testers, but also brought out by the same individual : 

 those in the increase of strength with age, the failure on the part of one 

 tester to obtain good breaks in many instances, and the gi'eat variation in 

 the time of setting, are all apparent ; yet both operators worked carefully 

 and in strict accordance with the United States Army specifications. 

 The committee in charge, after thoroughly investigating the methods of 

 tlie two operators, reported as follows : 



"The methods followed by each cement tester differ slightly in certain details, 

 but the differences are not regarded by the committee as important or as in 

 violation of the purport of the specifications; and the results obtained in each 

 laboratory are regarded as fair, equitable and reasonable to both manufacturer 

 and user." " 



Even careful inspection of these charts would probably convince almost 

 anyone that the tests, as they stand, are practically useless; that one of 

 these testers, or even both, were inefficient; or that the cement itself 

 was of a most peculiar quality. However, as will be shown, the whole 

 inconsistency was due to a cause the elimination of which the specifica- 

 tions do not even mention. It appears that the work of the testers for 

 time of setting, ? 8-day neat and sand strength, and specific gravity 

 determinations, was accurate and true to the quality of the cement at 

 the time it was tested. The cause of the great variability so evident in 

 diagrams numbers 1 and 2, -lyas due to the fact that each tester worked 

 the cement after it had undergone various degrees of exposure to aeration, 

 and that the influence of this factor produced very marked changes in 

 the quality of the cement. 



TliE EFFECTS OF AliEATION'. 



Most of the cement specifications now in use devote considerable space 

 to the manner in which the samples should be taken, but they all neglect 

 to state how these samples shall be stored and preserved rmtil tested. As 

 a result, they may be sent to the laboratory in wooden or paper boxes, 

 paper or cloth bags, tin cans, galvanized-iron cans, glass jars, etc. The 

 cloth and paper may be thin or thick, and the cans, jars and boxes may 

 have tightly or loosely fitting covers, or even no covers at all. These 

 samples may be tested as soon as they are received at the laboratory or, 

 owing to the amount of routine work already on hand, they may stand 

 for some days before being worked. As a result of all these conditions 

 the cement may have been, subjected to unequal aeration and its charac- 

 teristics changed accordingly; this change has often been sufficient to 

 alter the resulting tests from satisfactory to unsatisfactory. 



■' Final report of cement investigation committee appointed bj' Executive Order 

 No. GO, 1907. — The Government of the Philippine Islands. 



