SPECIFICATIONS FOR PORTLAND CEMENT. 619 



(In case quick-setting cement is desired, the following tensile strength 

 shall be substituted for the above: 



Neat briquettes : After seven days, 400 Ibs.; after twenty-eight days, 

 480 Ibs. 



Briquettes of 1 part cement to 3 parts standard sand: After seven 

 days, 120 Ibs. ; after twenty-eight days, 180 Ibs.) 



(12) The highest result from each set of briquettes made at any 

 one time is to be considered the governing test. Any cement not show- 

 ing an increase of strength in the twenty-eight-day tests over the seven- 

 day tests will be rejected. 



(13) When making briquettes neat cement will be mixed with 20 

 per cent of water by weight, and sand and cement with 12 per cent 

 of water by weight. After being thoroughly mixed and worked for 

 five minutes, the cement or mortar will be placed in the briquette mold 

 in four equal layers, and each layer rammed and compressed by thirty 

 blows of a soft brass or copper rammer three quarters of an inch in 

 diameter (or seven tenths of an inch square, with rounded corners), 

 weighing 1 Ib. It is to be allowed to drop on the mixture from a height 

 of about half an inch. When the ramming has been completed, the 

 surplus cement shall be struck off and the final layer smoothed with 

 a trowel held almost horizontal and drawn back with sufficient pressure 

 to make its edge follow the surface of the mold. 



(14) The above are to be considered the minimum requirements. 

 Unless a cement has been recently used on work under this office, bidders 

 will deliver a sample barrel for test before the opening of bids. If this 

 sample shows higher tests than those given above, the average of tests 

 made on subsequent shipments must come up to those found with the 

 sample. 



(15) A cement may be rejected in case it fails to meet any of the 

 above requirements. An agent of the contractor may be present at 

 the making of the tests, or, in case of the failure of any of them, they 

 may be repeated in his presence. If the contractor so desires, the en- 

 gineer officer in charge may, if he deem it to the interest of the United 

 States, have any or all of the tests made or repeated at some recognized 

 standard testing laboratory in the manner herein specified. All ex- 

 penses of such tests to be paid by the contractor. All such tests shall 

 be made on samples furnished by the engineer officer from cement ac- 

 tually delivered to him. 



