76 BULLETIN 852, U. S. DEPARTMENT OF AGRICULTURE. 
More or less slime may be expected from waters stored in reser- 
voirs containing algae growths or from waters that have been con- 
veyed long distances in openearth channels. (See Mr. Hazen's 
discussion on p. 97). This may also be true, but to a less extent, if 
the open channel is of wood or concrete. Waters of the Southwest are 
less liable to develop retarding growths in covered conduits than 
those of the East and Northwest. Water containing calcium car- 
bonate (CaC0 3 ) will smoothly coat pipe of any material. If sand, 
gravel, or other debris is also present in the water, the cemented ac- 
cretion will be much more harmful, being rough and more rapid. 
After construction a rough pipe may become somewhat smoother 
if conditions are such that a mineral coating is deposited. An 
originally smooth surface will remain about as constructed or depre- 
ciate slightly by mineral or slime deposits. Slimes come quickly or 
not at all. A depreciation of from 10 to 15 per cent in capacity can 
occur within five to seven months, but does not greatly increase after 
that. Thus, to be of material benefit, cleaning must be frequent if 
necessary at all. For this reason periodic cleanings have been 
abandoned in many places. 1 If algse slimes are anticipated, it may 
be best to allow for 15 per cent depreciation of capacity and no 
cleaning. 
A concrete surface is not subject to a progressive roughening 
influence like the tuberculation found in iron and steel pipes, though 
tuberculation of the reinforcement may occur if too near the surface. 
Koots may affect a poorly made or poorly laid pipe, but will not 
influence a hard, well- join ted pipe. 
In considering various kinds of pipes it is not sufficient to compare 
concrete with wood, conorete with steel or iron, concrete with vitrified 
clay, or even concrete with concrete. This is true not only from a 
capacity standpoint, but also from the standpoint of strength, life, 
and operation. One particular class of concrete pipe must be com- 
pared with the pipe of the other materials or with concrete pipes of 
the other classes. Two estimates or bids for a concrete pipe may 
differ 20 per cent, yet from a capacity standpoint alone the higher 
in cost may be the more economical. 
ACKNOWLEDGMENTS. 
The writer desires to acknowledge indebtedness to the various 
engineers and managers of irrigation, municipal, and power systems 
who permitted and aided in tests upon the pipes in their charge; 
also to the Board of Water Supply of New York City and to the 
Ontario Power Commission of Toronto, Canada, for supplying 
hitherto unpublished results of tests made on their conduits. 
Tests made on pipes of the various United States Keclamation 
Service projects were made under a cooperative agreement between 
that service and this bureau. Especial acknowledgment is due the 
officers and men of the Reclamation Service who were unfailingly 
courteous, energetic in getting the best feasible conditions for experi- 
mentation, and in many cases devoted to these tests long hours 
outside their regular work. 
i Water Works Handbook, Flinn, Weston and Bogert, New York, 1916, pp. 290-291. 
