Fig. 253.—Section of 9-ft. Conduit, Philadelphia Filter Plant.
Forms.—The forms used for the several sizes of conduit were all of the same general type, but improvements in detail were made as successive sizes were built. The last form to be designed was that for the 9-ft. section and this was the best one; it is shown by Fig. 254. The forms were built in sections from 12 ft. to 13½ ft. long. They were covered with No. 27 galvanized sheet iron, and this covering was found of advantage both in giving a smooth finish and in prolonging the life of the centers. The important feature is the construction in sections which could be set up and broken down by simply inserting and removing the connecting bolts. Three sets of forms were made for each size of conduit.
Fig. 254.—Form for 9-ft. Conduit Philadelphia Filter Plant.
Procedure of Work.—The first operation in building a section of conduit was to set to exact line and grade and the length of the form in advance of the finished work the bulkhead shown by Fig. 255. In this space the invert concrete was deposited and formed to a plane 1 in. below the finished invert bottom. The two bottom sections of the form were then assembled and located by bolting one end to the last preceding form and inserting the other end into the bulkhead. About two tons of pig iron were then placed on the invert form to keep it from floating while the liquid granolithic mixture was being poured into the 1-in. space between the form and the invert concrete. In building up the sides a facing form was used for placing the granolithic finish. This consisted of "boards" of sheet steel ribbed transversely on one side with ¾-in. pipe and on the other side with 1½-in. pipe. Two boards were used on each haunch, slightly lapping in the center, as follows: The board was placed with the small ribs against the form and the larger ribs kept the expanded metal just 3 ins. from the face of the form. A 6-in. depth of concrete was placed between the metal board and the outside form or planks, then 6 ins. of granolithic was poured into the 1-in. space between the center and the board and finally the board was raised 6 ins. and the concrete and granolithic mixture tamped together. With the board in its new position, another layer of concrete and granolithic was placed. Toward the crown the granolithic mixture was made stiff and simply plastered onto the mold. The expanded metal was cut into sheets corresponding to the length of the sides of the form and lapped 6 ins. in all directions; the bulkhead having a slot as shown to permit the metal to project 6 ins. from the face of the concrete in order to tie two sections together and also having a rib which formed a mortise in the face of the shell of concrete to key it to the succeeding section.
Fig. 255.—Bulkhead Form for Conduits, Philadelphia Filter Plant.
All the conduits were built in sections from 12 ft. to 13½ ft. long, and there was very little, if any, difference in the labor required to build a section, in from eight to ten hours, of any of the three sizes. One foreman and 18 men on the top of the trench mixed and handled the concrete and granolithic mortar while one foreman, one carpenter and seven men in the trench set the forms and placed and rammed the concrete for one section in generally eight hours. About one-third of the concrete for the whole work was mixed in a portable cubical mixer of ½ cu. yd. capacity, and the remainder was mixed by hand. Owing to the relatively small amount of concrete used per day, about 20 cu. yds., it was found that there was practically no difference in the cost of machine mixing and of hand mixing. The 9-ft. conduit as an average of the three sizes, contained 20 cu. yds. of concrete, 1,200 sq. ft. of expanded and required 125 bags of cement for a section 13½ ft. long. The cost of the work excluding excavation and profit, but including forms, metal, concrete materials and labor, was about $10.50 per cu. yd.
CONDUIT, JERSEY CITY WATER SUPPLY.—In constructing the 8½-ft. reinforced concrete conduit for the Jersey City water supply, use was made of forms without bottoms. Each form was made of segmental sections 12½ ft. long of wood covered with sheet steel. They were set end to end in the trench, resting on 6-in. concrete cubes which were finally permanently embedded in the invert concrete. In each form there was a scuttle about 2 ft. square at the crown, and the bottom was open between the curves of the invert haunches. The form being set and greased and the reinforcement placed, the concrete was deposited on the outside and forced by means of tamping bars down the curve of the invert haunches until it filled the whole space between the form and the earth and appeared at the edges of the bottom opening in the form. Concrete was then thrown through the scuttle and the invert screeded into shape. The concreting of the sides and crown of the arch was then completed, using outside forms except for about 5 ft. of the crown, the scuttle, of course, being closed by a fitted cover. The centers were left in place about 48 hours. The concrete was a 1 cement 7 sand and run of the crusher 2-in. broken stone mixture, and was made so wet that it would flow down an incline of 1 on 8. The mixing was done in portable Ransome mixers, set on the trench bank alongside the work and discharging by chute into dished shoveling boxes provided with legs to set on the erected forms. Coal scoops were used in shoveling from the box into the forms and were found superior to shovels in keeping the relative proportions of water and solids constant.