196 



ANNALS NEW YORK ACADEMY OF SCIENCES 



was the difference a significant one, so that it was considered legitimate 

 to average the strengths. 



It will be seen by the table below that, even without actual removal of 

 any cement, the formation of laitance has a weakening action upon 

 cement. 



Table S 

 Compressive Strength of Grouts Mixed tcith Varying Proportions of Water 





Crushing strength, 



Per cent of water 



lbs. per square inch, 

 average of two tests. 





Age, 28 days 



50 



6855 



75 



5900 



100 



4500 



150 



3430 



200 



2960 



500 



1810 



The effect of excess of mixing water is therefore seen to result in 

 decrease of strength as the water increases. Whether the effect is a 

 permanent one was the next question that presented itself. To settle 

 this point, a new series was undertaken, in which a larger number of 

 differing percentages was introduced, and in which the resulting strength 

 at two periods was determined. 



The cement was mixed with the stated percentage of water, and 

 worked for two minutes, the drier mixes upon the table in the usual 

 fashion, and the wetter mixes merely poured into the tubes and shaken. 

 Paper mailing tubes were used, 2 inches by 48 inches, treated with 

 molten paraffin and sealed with paraffined corks, so as to be absolutely 

 tight. To obviate the effect of possible leakage, the whole series was 

 stored in damp sand. 



Cylinders two diameters high were cut from the specimens at the 

 stated periods, each cylinder being cut as nearly as possible the same dis- 

 tance from the bottom, and care was taken to avoid including any of the 

 soft cheesy top portion, the settled laitance. 



