4 BULLETIN 552. U. 8= DEPARTMENT OE AGE I C EXT US E = 
The cost of the many forms necessary in the manufacture of wet-mix 
pipe precludes the nse of this method except for the highest class of 
work. The time of setting is greatly hastened by the nse of steam. 
The greatest carrying capacity is attained with pipes made in this 
manner. One maker in California has succeeded hi using a mixture 
so damp that coating is unnecessary, yet the forms are removed 
immediately, as though the pipe were made with a dry mixture. 
Before setting, this pipe slumps down slightly, contributing to dis- 
tortion of shape and thickening of the shell. The "pull" of the 
forms being removed before setting leaves a rough interior surface. 
Pipes made with a very wet mixture must of necessity be left in 
the forms until well set. Oiling or soaping the forms is necessary 
to prevent sticking, and makes for a smooth interior surface if the 
mixture is well worked when put into the forms. Also, if the forms 
are rigidly true to shape, the resulting pipe is not distorted. 
In the assembling of precast units there must of necessity be a 
joint every few feet. The standard length of small pipes is from 2 
to 3 feet, while large ones are made in sections up to S or 10 feet long. 
In the early days extreme care was not taken to make smooth joints, 
but m present-day construction of the best workmanship on lines 
with pipe of such a size that a man may work inside, the joints 
can hardly be distinguished from the rest of the pipe, except by the 
color. On smaller pipes the joints are carefully wiped ; so that all 
excess mortar is removed.. 
At first thought it would seem that a pipe oi monolithic construc- 
tion should present an imbroken surface, but in actual practice it is 
a difficult matter to move the forms forward so that shoulders or 
offsets are not formed in the pipe at the ends of the sections. (See 
PL II. ng : 2 = ) This is especially true on curves. These shoulders 
may be much improved by touching up the interior after removal 
of the forms, but in many cases this does not appear to have been 
done. In retouching, the general idea should be to avoid sudden 
enlargements or contractions and all shoulders should be tapered to 
minimize loss of head. 
Wood forms are of course cheaper than steel, but the resulting 
surface is very inferior unless most carefully made, as any crack 
between the boards is clearly defined in the concrete. This may be 
bettered by a surface coat, but it is doubtful if any coat is as smooth 
as the surface first left by a well-oiled, rigid steel form, if the concrete 
is carefully worked into close contact at all points. Coating wooden 
forms with sheet metal of course gives approximately the same 
surf ace as steel forms, 
