Fig. 272.—Casting Bordenave Pipe at Swansea, England.
Molding.—When the three positions of the steel skeleton were completed, as shown by Fig. 271, they were set on curved wooden curbs made to the exact shape necessary to center them and preserve the correct thickness of cement coating. A collapsible core was lowered into position in the inside, and a two-part sheet steel mold was erected outside; the space between core and mold was then poured with a thin mortar of one part Portland cement to two parts clean river sand. During the process of pouring, the outer steel mold is sharply struck with wooden mallets to facilitate the escape of air bubbles. The mortar was mixed on an elevated traveling platform which is shown in Fig. 272, which also shows a completed pipe, a core being withdrawn, a filled mold and a section of reinforcement set up. The difficult feature of the molding process was found to be the determination of the time for withdrawing the core and removing the exterior mold; the time of setting of the mortar was different in warm and in cool weather and varied with the wetness of the mixture, the brand of cement, etc. By using a single brand of cement that ran very uniform in quality and time of setting it was possible, however, for the workmen, after a little practice, to gage very accurately the correct time for removing the molds. With four sets of molds a gang of eight men would curb 16 pipes per day under favorable conditions, but when the temperature was low it was not possible to make more than six or eight pipes. The pipes were allowed to stand four or five days after the removal of the mold; they could then be removed by a crane and laid in stock until used. It was found advisable to let the pipes age about four weeks before laying; by this time, it is stated, they would stand as much rough usage as cast iron pipe.
Laying.—The pipes were laid much in the same way as cast-iron pipes are laid; they were each 9 ft. 9½ ins. long and weighed each about 12 cwt., and were handled by ordinary tackle. In laying, the pipes were adjusted end to end and the joint enclosed by a temporary steel ring inside which the bitumen seal, Fig. 270, was run and allowed to set when the steel ring was removed. The joint was then encircled by a collar of similar construction to the pipe itself and the space between collar and pipe was poured with cement mortar. About ten lengths of pipe were laid per day by one gang of men, one jointer and his assistant making all the cement and bitumen joints as fast as the gang could lay the pipes.
CHAPTER XXII.
METHODS AND COST OF CONSTRUCTING RESERVOIRS AND TANKS.
Floor, wall and roof work of structurally very simple character sum up the task of the constructor in reservoir and tank construction. The only intricacy involved lies in form design and construction for cylindrical tank work. Several examples of such work are given in this chapter, and in each the construction and handling of the forms are described. To repeat details here would serve no purpose, but one general instruction may be enunciated. No care is too great which ensures rigidity and invariable form, both in the construction of the individual form units and in the assembling of these units into the complete form. This is particularly true of cylindrical tank work and especially high cylindrical tank work where the forms are moved upward as the work progresses. To the designer it may be suggested that any beauty he may gain by giving the walls of his standpipe a batter is paid in the extra cost of form work.