10 BULLETIN 906, IT. 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 
