8 BULLETIN 852, U. S. DEPARTMENT OF AGRICULTURE. 
Class 2. C s = 0.310. For modern " dry-mix" concrete pipe and 
monolithic concrete pipe or tunnel linings made over rough wood 
forms. Also for surfaces as left by cement gun process. This 
coefficient should be used for pipes as commonly made at the present 
time in the west coast States; that is, in 2-foot sections, with a dry 
mixture, afterwards washed with cement mortar on the inside, the 
work of manufacture and laying being done by contract under little 
or no inspection. Under favorable conditions, such as clear water 
to be conveyed, carefully made joints and thorough inspection, this 
class may be made to approach class 3, but unless sure of his con- 
ditions the designer should use class 2 (for which Table 6 has been 
prepared). This is especially true for pipes less than 12 inches in 
diameter, because of the difficulty in making smooth joints. 
Class 3. C s = 0.345. For small "wet-mix" pipe in short units; 
for " dry-mix" pipes in long units; for average monolithic pipe made 
on steel forms. These pipes may be evenly washed with cement 
mortar or asphaltum. For small cement-lined iron pipes and for 
concrete pipe madeunder pressure with interior coating of neat cement 
applied by a mechanical "trowel." To be used for pipes of class 
4 when the water contains detritus or the line is to be used to con- 
vey sewage. (See Tables 7 and 8.) 
Class 4- C s = 0.370. For glazed-interior pipe lines; for large 
cement-lined iron pipes; for monolithic pipe lines where joint scars 
and all interior surface irregularities are removed. Particularly ap- 
plicable to jointed lines of units made from wet, well-spaded concrete, 
deposited against oiled steel forms and allowed to set firmly before 
forms are stripped. The glazed surface resulting from this treatment 
is to be untouched with brush or other "wash" process and the units 
are to be so uniform in shape that no shoulders are perceptible to the 
touch when the line is finished. The finished joints are to be practi- 
tically as smooth as the rest of the pipe. (See Table 9.) 
This class covers only the highest grade of workmanship and 
materials. The specifications are rigid, but have been and can be 
attained commercially by an experienced organization. They are 
difficult to attain in a pipe less than 30 inches in diameter — too small 
to permit a man to work comfortably inside, and are probably pro- 
hibitive for sizes less than 12 inches in diameter. A few of the pipes 
upon which experiments were made appear to have coefficients higher 
than 0.370, but the writer wishes to be conservative in recommending 
a coefficient that necessitates a surface so nearly ideal. That is to 
say, a better surface may be attained in construction than should be 
anticipated in design. 
All the above formulas will be taken up again after an analysis of the 
data, and specific recommendations made with regard to each one so 
that the engineer familiar with one type of formula, not desiring to 
change to a new one, may have the best suggestions the available data 
offer in terms familiar to him. 
