114 



Bulletin 86 



inner portion of the pipe tends to produce cracks in the outer por- 

 tion. The inner portion, being of rich mortar and dense, has a 

 relatively high percentage of expansion due to saturation. After 

 a crack has started on the outside, internal pressure or other causes 

 may complete the rupture. Test specimens while being tested for 

 percolation and resistance to internal pressure sometimes fail as a 

 result of the penetration of water into the inner wall. 



In case, however, that the first flow into a dry pipe line does not 

 fill the pipe, then the saturation and expansion of the invert creates 



Fig. 23. — Effect of saturation on pipe that had been broken in internal- 

 pressure testing machine. 



a bending moment at the crown and tends to produce a crack which 

 will begin to open from the inside, if the saturation is wholly from 

 the inside, the first effect is tension throughout the section at the 

 crown, and this is changed to a bending moment as the penetration 

 of the water into the invert increases toward the outer surface of 

 the pipe wall. The bending moment reaches its maximum when 

 the saturation of the wall is complete. The expansion of the lower 

 half of the pipe has the same effect as swelling a wooden wedge in 

 the bottom. If the distortion is enough to cause failure, a crack 

 will occur at the top and will show first on the inside at or near the 

 crown. Rupture at the top may be followed by another crack at 

 the bottom. The strain at the top of the pipe will be greatest when 

 the pipe line is about half filled with water. In the case of freshly- 

 laid dry pipe, therefore, it is preferable to run a full head of water 

 the first time, or perhaps a very small stream at first to help cure 



