PORTLAND CEMENT TESTING. 159 



This laboratorj^ uses the same plasticity in its tests for soundness as for 

 setting pats. The cement in this condition is thoroughly wet and pliable, 

 but still stiff enough to retain its shape, therefore it meets all the require- 

 ments of a just test. Uniformity between different testers is also secured, 

 because 1 per cent of water more or less would so change the nature of 

 the paste that it would be rendered either too dry or too liquid. The 

 results obtained depend much upon the skill of the operator. Sudden 

 changes in temperature during the steaming and boiling tests should 

 always be avoided. Moistening the surface of the glass plate with a damp 

 cloth before applying the paste will insure better adhesion to the plate. 

 A ball of the paste should then be applied to this surface and patted 

 down very vigorously into the desired shape. Vigorous patting with the 

 flat of the trowel eliminates any interior cracks, reduces the air bubbles to 

 a minimum and brings the excess water to the surface. Eor soundness, 

 the top of the pat should be arched and the rim troweled to a thin edge 

 as sho^vn in fig. 4. Pats made in this manner will not warp or crack 

 unless the cement is faulty. 



^ L .^ 



K 2" H 



Fig. 4. 



"We have noticed that different testers interpret the results of soundness 

 tests in different ways. Some operators will report as "unsound" a 

 cement that shows the least trace of warping even after air exposure. 

 Cements showing only slight incipient disintegration are often reported 

 as "disintegrated." In like manner "off plate" and "cracked plate" are 

 often attributed to expansion and contraction. Such an interpretation is 

 unjust to the manufacturer, as warping and cracking to some extent under 

 certain conditions are not to be considered dangeroiis. A sound pat com- 

 bined with a broken plate does not necessarily indicate dangerous con- 

 traction or expansion. Every cement expands more or less, and in this 

 case the adhesion between the cement and the glass is very strong. As 

 the glass also has an expansion factor, all such cases should be reported as 

 satisfactory if the pat itself shows no sign of cracking or warping. To 

 insure a 23erfect understanding between the manufacturer, engineer and 

 tester and to avoid unjust or misinterpreted results, specifications should 

 include a descriptive chart of the proper standard interpretation by which 

 the extent, significance, and importance of the various degrees of warping, 

 cracking, disintegrating and shrinking are to be regulated. This labora- 



