USE OF CONCRETE PIPE IN IRRIGATION. 15 



mixing the sand and cement with lime water. The addition of slaked 

 lime makes the mortar easier to handle. One man who has had con- 

 siderable experience with large sizes of pipe will not use lime in the 

 mortar in sizes over 20 inches, as he claims mortar mixed with lime 

 will tend to crack in the larger sizes. 



Back filling should follow immediately after laying. If the earth 

 used for back filling is moist, the mortar in the joints will cure better. 

 Pipe laid in the trench is usually dry and a moist earth used for 

 back filling will tend to moisten the pipe and cause a slight expansion. 

 If the expansion takes place before the mortar in the joints is set, the 

 pipe will conform to the new condition created by squeezing the 

 mortar more compactly at the joints, instead of shoving the pipe 

 ahead, which may cause failure at curves or destroy diversion boxes 

 by crushing. The temperature in the trench after back filling is 

 usually much lower than the air temperature, especially in the sum- 

 mer. A decrease in temperature causes the newly laid pipe to con- 

 tract, which will probably be counteracted by the pipe absorbing 

 moisture providing the back filling material is moist. In practice it 

 is often difficult to procure enough water to wet the trench or back 

 fill. In this case pipe should not be laid in the hottest weather. It 

 sometimes happens that the pipe will expand enough when saturated 

 after being filled with water to prevent trouble with contraction 

 cracks. Accordingly water should be let into a newly laid pipe line 

 as soon as possible. This subject will be discussed more fully under 

 another heading. 



After the trench is dug. and the pipe to be laid strung along the 

 side of it, one expert pipe layer with two helpers will sometimes lay 

 as much as 1,200 feet of 8- to 12-inch pipe in a day, but 800 feet is 

 a good day's work for an average crew. Four men will lay the pipe 

 and partially backfill the trench. Four men are usually needed to 

 lay pipe from 14 to 18 inches in diameter, and will average 300 to 

 500 feet of this size in a day. Five to seven men will lay from 300 

 to 500 feet of 24-inch pipe in a day. 



There is wide variation in the cost of laying pipe and the quan- 

 tity of mortar used in making the joints. If heavy bands are made at 

 the joints for high pressure lines, it often costs from 25 to 50 per cent 

 more to lay the pipe. One pipe manufacturer estimates that one 

 sack of cement, made into one to one and a half mortar, will la} 7 

 350 feet of 6-inch pipe, 270 feet of 8-inch, 200 feet of 10-inch, 160 feet 

 of 12-inch, and 70 feet of 20-inch pipe. If bell-ended pipe is used, 

 similar to the ordinary clay sewer pipe, more mortar is needed. 



CAUSES OF FAILURE OF CONCRETE PIPE. 



One of the most frequent causes of failure of concrete pipe is a 

 poor grade of pipe. If the pipe is made of poor material, or a very 



