170 Bulletin 86 



important factors of the resistance of cement pipe to external 

 pressure. 



18. I'rom the records of tests that have been published, de- 

 signers can draw plans and prepare specifications. Specifications 

 foT important contracts should provide for ample tests of the pipe 

 before it is laid, and for trial tests of the line when completed. 



19. Injury from alkali in water or in soil depends upon the 

 quality and concentration of the alkali and the density of the pipe. 

 Little difificulty from this cause is to be anticipated in Arizona, but 

 very alkaline soils should be analyzed before pipe lines are built in 

 them. 



STRKNCTH AND WATKRTIGHTNESS 



20. Cement pipe gains in strength for at least one month, and 

 probably for three months, after its manufacture. 



21. Tests of cement pipe should be made with the pipe in a 

 wet condition. The strength of dry pipe is abnormally high. 



22. When dry pipe is wetted by immersion, the internal stresses 

 become entirely equalized within a few hours. If the pipe is laid 

 and is wetted from the inside only, it may be many days before the 

 penetration of water causes the maximum internal stresses in the 

 pipe wall. 



23. Machine-made pipe, as ordinarily made, is safe when sub- 

 jected to hydrostatic heads up to 25 feet. The strength of hand- 

 made pipe of good quality is about one-half as great. Wet-cast 

 pipe can be designed to carry water under heads of somewhat over 

 100 feet. 



24. Machine-made cement pipe up to 20 inches in diameter is 

 safe in trenches under ten feet of backfill and the smaller sizes are 

 safe for much greater depth. Hand-made pipe 16 inches in diame- 

 ter, well made and cured, can be used under ten feet of earth. Tests 

 should be made, however, in all important cases, to determine the 

 strength of the pipe. 



25. Machine-made pipe is practically impervious. Good hand- 

 made pi])e washed inside with neat cement, is nearly impervious and 

 becomes entirely so in time. 



PIPIC l-RICTION AND CARRVINc) CAPACITV 



26. I'Viction factors are well known and permit close designing. 

 The value of "n" in Kutter's formula should be taken as .013. 

 Proper allowance should be made for the eft'ect of frequent risers on 

 the line, and on descending grades an allowance should be made for 

 the effect of entrapped air. 



