340 THE IMPROVEMENT OF RIVERS. 



8. Specific Gravity. The specific gravity of the cement, as determined from a sample 

 which has been carefully dried, shall be between 2.7 and 2.8. 



g. Soundness. To test the soundness of cement, pats of neat cement mixed for 

 five minutes with 18 per cent of water by weight shall be made on glass, each pat about 

 3 inches in diameter and i inch thick at the center, tapering thence to a thin edge. The 

 pats are to be kept under wet i-loths until finally set, when they are to be placed in fresh 

 water. They should not show distortion or cracks at the end of twenty-eight days. 



10. Time of Setting. The cement shall not acquire its initial set in less than forty- 

 five minutes and shall acquire its final set in ten hours. The pats made to test the 

 soundness may be used in determining the time of setting. The cement is considered 

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

 indented, a wire fa inch in diameter loaded to \ pound weight. The final set has been 

 acquired when the pat will bear, without being appreciably indented, a wire Vf inch 

 in diameter loaded to i pound weight. 



11. Tensile Strength. Briquettes made of neat cement, after being kept in air 

 under a wet cloth for twenty-four hours and the balance of the time in water, shall 

 develop tensile strengths per square inch as follows: 



After seven days, 350 pounds; after twenty-eight days, 500 pounds. 

 Briquettes made of one part cement and three parts standard sand by weight shall 

 develop tensile strength per square inch as follows: 



After seven days, impounds; after twenty -eight days, 2 20 pounds. 



12. 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. 



13. When making briquettes neat cement will be mixed with 18 per cent of water 

 by weight, and sand and cement with 10 per cent of water by weight. After being thor- 

 oughly 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, -f of an inch in diameter or T 7 ff of an inch 

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

 the mixture from a height of about i inch. When the ramming has been completed 

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

 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 require- 

 ments. An agent of the contractor may be present at the making of the tests, or, in 



