PORTLAND CEMENT TESTING. 



169 



Table XXI. — Variations in tensile strength with varying quantities of water; 



l-day results. 



Sample No. 



Neat. 



1 to 3 



mortar, 



12..5 per 



cent 



water. 



■20 per 



cent 



water. 



22.5 per 

 cent 

 water. 



D4-2 



219 

 182 

 167 

 152 



633 

 614 

 612 

 600 

 603 



213 

 208 

 216 

 200 

 209 



Dl-4 .. ___ 



D4-6_- 



D4-8 . 



D4-10 



178 





The higliest results of four good breaks are recorded in each instance. 



It will be noticed that the sand briquettes (12.5 per cent water equal 

 to 50 per cent calculated on the cement) present higher results than 

 the neat with 20 per cent of water; and that 2.5 per cent additional 

 for the paste increases its tensile strength over 200 per cent. The 

 following table shows a failure in either case : 



Table XXII. — Varying quantities of water used with a failing cement. 



Sample No. 



7 days neat. 



28 days neat. 



20 per 



cent 



water. 



24 per 



cent 



water. 



20 per 

 cent 

 water. 



1 

 24 per 

 cent 

 water. 



L181 



200 

 257 

 262 



404 

 438 

 418 



274 

 350 

 318 



468 

 623 

 451 



L184 



L186 



Twenty-seven per cent of water gave lower results than 24 per cent. 



It is a simple matter to Judge when a cement contains enough water 

 if the method of tamping is used. The surface must be wet when the 

 last layer has been tamped into the mold and of not quite the plasticity 

 described for the pats used in determining the time of setting. A dry 

 surface is positive proof that very low tensile strength will result. The 

 determination of the "normal consistency" can not be used for this 

 purpose as the resulting paste is too slushy for tamping. 



If 20 per cent of water gives too dry a mixture, we add an additional 

 quantity sufficient to bring the water to the surface after tamping. The 

 percentage of water necessary to accomplish this result is included with 

 the report of the tests. The results obtained in this way by our tamper 

 and method of tamping are satisfaetorj^, consistent and true to the quality 

 of the cement. The best result of four good breaks is sufficient for all 

 routine work. 



The United States Army specifications state that the best results are 



