124 



Bulletin 86 



effect of water hammer. The evidence of tests indicates that pipe 

 lines made of 1 : 3 mortar will be safe with 20 or 25 feet pressure 

 head, or with 30 feet head if there is no possibility of water hammer 

 and if the full pressure need not be applied until the pipe line is well 

 cured in the trench. In case higher heads must be provided for, the 

 mortar can be mixed in the proportions of 1 : 2>4, the walls can be 

 made thicker, and, if necessary, wire reinforcement can be used. 



Hand-made pipe made of 1 : 3 mortar (or of 1 : 2>4 : 1>4 concrete) 

 withstands pressures about one-half as great as machine-made pipe. 

 Applying the same factors of safety, hand-made pipe can be con- 

 sidered safe under 10 or 12 feet head, or under 15 feet head if there 

 is no danger of water hammer. Several hand-made pipe lines are 

 under heads exceeding 20 feet. The strength of 1 : 4 pipe is about 

 two-thirds as great as that of 1 : 3 pipe. Where the line is under 

 little or no pressure 1 : 4 pipe may prove successful, but the -added 

 security, less breakage in handling, and slight difference in cost will 

 justify the stronger pipe in almost every case. 



EXTERNAL PRESSURE TESTS 



In order to test the resistance of cement pipe to external press- 

 ure, a testing equipment, as shown in Fig. 26, was prepared. The 

 joint of pipe to be tested was bedded on a box of sand so that one- 

 fourth of the perimeter of the center line of the shell was supported 

 in the sand. A bottomless box or frame was then placed over the 

 pipe and filled with sand so as to distribute the pressure similarly 



Fig. 26. — Apparatus for making external-pressure tests. 



over the upper fourth of the pipe. A short piece of plank rested on 

 the sand and on that was a steel knife edge. No support was given 

 to the sides of the pipe. This arrangement of bearing at top and 

 bottom is called the sand bearing; it is known also as the Iowa bear- 

 ing. It has been found by experiment that the breaking loads as 

 found with sand bearings approximate very closely to the actual 



