280 CEMENTS, LIMES, AND PLASTERS. 



(7) Fineness. At least 80 per cent of the cement must pass through 

 a sieve made of No. 40 wire, Stubb's gauge, having 10,000 openings 

 per square inch. 



(8) Time of Setting. The cement shall not acquire its initial set 

 in less than twenty minutes and must have acquired its final set in four 

 hours. 



(9) The time of setting is to be determined from a pat of neat 

 cement mixed for five minutes with 30 per cent of water by weight 

 and kept under a wet cloth until finally set. The cement is considered 

 to have acquired its initial set when the pat will bear, without being 

 appreciably indented, a wire -f% inch in diameter loaded to weigh 

 | Ib. The final set has been acquired when the pat will bear, without 

 being appreciably indented, a wire ^ mc h in diameter loaded to weigh 

 1 Ib. 



(10) Tensile strength. Briquettes made of neat cement shall 

 develop the following tensile strengths per square inch, after having 

 been kept in air for twenty-four hours under a wet cloth and the balance 

 of the time in water: 



At the end of seven days, 90 Ibs.; at the end of twenty-eight days, 

 200 Ibs. 



Briquettes made of one part cement and one part standard sand 

 by weight shall develop the following tensile strengths per square inch: 



After seven days, 60 Ibs.; after twenty-eight days, 150 Ibs. 



(11) 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. 



(12) The neat cement for briquettes shall be mixed with 30 per 

 cent of water by weight, and the sand and cement with 17 per cent 

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

 five minutes the cement or mortar is to be placed in the briquette mold 

 in four equal layers, each of which is to be 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 cor- 

 ners), weighing 1 Ib. It is to be allowed to drop on the mixture from 

 a height of about half an inch. Upon the completion of the ramming the 

 surplus cement shall be struck off and the last layer smoothed with a 

 trowel held nearly horizontal and drawn back with sufficient pressure 

 to make its edge follow the surface of the mold. 



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

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



