33 8 THE IMPROVEMENT OF RIVERS. 



the Allegheny Mountains this may be 265 pounds.) Three sacks of cement shall have the 

 same weight as i barrel. If the- average net weight, as determined by test weighings, is 

 found to be below 300 pounds (265 pounds) per barrel, the cement may be rejected, or, 

 at the option of the Engineer Officer in charge, the contractor may be required to supply, 

 free of cost to the United States, an additional amount of cement equal to the shortage. 



6. Tests may be made of the fineness, time of setting, and tensile strength of the 

 cement. 



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 ^ inch in diameter loaded to weigh \ pound. The final 

 set has been acquired when the pat will bear, without being appreciably indented, a wire 

 J f inch in diameter loaded to weigh i pound. 



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 pounds; at the end of twenty-eight days, 200 pounds. 

 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 pounds ; after twenty-eight days, 1 50 pounds. 



11. The highest result from each set of briquettes made at any one time is to be 

 considered the governing test. Any cement not showing 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 1 7 per cent of water by weight. After being thor- 

 oughly 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 J of an inch in diameter (or ^ of an inch 

 square with rounded corners), weighing i pound. It is to be allowed to drop on the mix- 

 ture from a height of about $ inch. Upon the completion of the ramming the surplus 

 cement shall be struck off and the last layer smoothed with a trowel held nearly hori- 

 zontal 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, bidders will deliver a sample barrel 

 for test before the opening of the bids. Any cement showing by sample higher tests 

 than those given must maintain the average so shown in subsequent deliveries. 



