4 BULLETIN 852, U. S. DEPARTMENT OF AGRICULTURE,, 



The cost of the many forms necessary in the manufacture of wet-mix 

 pipe precludes the use of this method except for the highest class of 

 work. The time of setting is greatly hastened by the use of steam. 

 The greatest carrying capacity is attained with pipes made in this 

 manner. One maker in California has succeeded in 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 8 or 10 feet long. 

 In the early days extreme care was not taken to make smooth joints, 

 but in 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 of monolithic construc- 

 tion should present an unbroken 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, fig,, 2.) This is especially true on curves. These shoulders 

 may be much improved b}^ 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 

 surface as steel forms. 



