10 BULLETIN 906, TJ. S. DEPARTMENT OF AGRICULTURE. 



pipe that is made of poor materials, poorly tamped, or that lacks the 

 proper proportion or kind of ingredients. 



It is sometimes difficult to judge the quality of concrete pipe with- 

 out having the necessary apparatus for testing it. Usually, however, 

 an intelligent examination with the application of a few simple tests, 

 will give a fair indication of its worth. Two requirements are neces- 

 sary for the best grades — strength and imperviousness. Pipe may 

 stand high pressures and still be rather porous, but a dense pipe 

 that is nearly impervious can be made if proper precautions are 

 taken in the choice of materials and the mixing and tamping or 

 troweling of the concrete. 



Porosity can be tested by filling a length of pipe with water after 

 one end has been plugged. If the outside of the pipe remains dry 

 after water has been in the pipe for several hours, it is probable that 

 the pipe will show very little seepage when under pressure, as seep- 

 age through the shell of a concrete pipe usually shows up under low 

 pressure and does not increase proportionately with increasing 

 pressures. The porosity of pipe can be estimated by weighing a 

 small piece of concrete pipe before and after soaking in water. 

 Dense pipe should increase very little in weight after being soaked 

 for 10 minutes. 



Hand-tamped pipe is often made with concrete that is far too 

 dry, for the reason that a dry mixture is easier to handle when made 

 by hand and will stand up with less tamping than a wet mixture. 

 Some of the dry mixed, poorly tamped pipe will absorb water rapidly 

 and become saturated throughout in a few minutes when soaked in 

 water. A wet mixture is usually quite impervious, as it must be 

 tamped well in order that the forms may be easily removed. Pipe 

 that is made from a wet mixture of concrete will show a fine webbing 

 on the outside of the pipe, while the wettest will show streaks and 

 coarse webbing. This webbing or streaking shows up plainly after 

 the pipe is cured. Such markings are due to moisture being forced 

 outward against the outside form when the pipe is tamped. This 

 mixture of water and cement is streaked vertically along the outside 

 of the pipe when the form is stripped from the newly made pipe. 



Wet-mixed pipe may not have the maximum strength, but usually 

 it can be depended upon for high-pressure work, and it should be 

 durable and impervious if well cured in the yard. Pipe that is made 

 with the best known proportion of water in the mixture, it being not 

 too dry and at the same time wet enough, will show a fine webbing 

 on the outside of the pipe, and will also be hard and tough, pro- 

 viding always that the materials are of good quality and well 

 tamped, or compressed, and cured. Wet-mixed pipe will usually 

 slump when the forms are removed, which may cause a lopsided or 



