Forms for conduits and sewers must be strong enough not to give way, or to become deformed, while the concrete is being placed and rammed; and must be rigid enough not to warp from being alternately wet and dry. They must be constructed so that they can readily be put up and taken down, and can be used several times on the same job. The forms must give a smooth and even finish to the interior of the sewer or conduit. This has been accomplished on several jobs by covering the forms with light-weight sheet iron.

These forms are usually built in lengths of 16 feet, with one center at each end, and with three to five (depending on the size of the sewer or conduit) intermediate centers in the lengths of 15 feet.

The segmental ribs are bolted together. The plank for these forms are made of 2 by 4-inch material, surfaced on the outer side, with the edge beveled to the radius of the conduit. The seg-mental ribs are bolted together, and are held in place by wooden ties 2 by 4 inches or 2 by 6 inches. 359. Forms for Torresdale Filters. In constructing the Torresdale filters for supplying Philadelphia with water, several large sewers and conduits were built of concrete and reinforced with expanded metal. In section the sewers were round and the conduits were horseshoe-shaped, with a comparatively flat bottom. The sewers were 6 feet and 8 feet 6 inches in diameter, and the forms were constructed similarly to the forms shown in Fig. 165, except that at the bottom the lower side ribs were connected to the bottom rib by a horizontal joint, and the spacing of the ribs was 2 feet 6 inches, center to center. Fig. 166 shows the form for the 7-foot 6-inch conduit. The centering for the 9-foot and 10-foot conduits was constructed similarly to the 7-foot 6-inch conduit except that the ribs were divided into 7 parts instead of 5 parts as shown in Fig. 166. The spacing of the braces depended on the thickness of the lagging. For lagging 1 inch by 2 1/2 inches, the braces were spaced 18 inches, center to center; and for 2 by 3-inch lagging, the spacing of the bracing was 2 feet 6 inches.

Fig. 165. Center for Round Sewer.

Fig. 165. Center for Round Sewer.

These forms were constructed in lengths of 8 feet. The lagging for the smaller sizes of the conduits was 1 inch by 2 1/2 inches, and for the larger sizes 2 inches by 3 inches, all of which was made of dressed lumber and covered with No. 27 galvanized sheet iron. The bracing of the forms was arranged to permit the centering being taken apart and brought forward through the sections set in front of it. Three sets of these forms were required for each conduit. The specifications required that the centering; be left in place for at least 60 hours after the con-crete had been placed. It was also required that this work should be constructed in monolithic sections - that is, the contractor could build as long a section as he could finish in a day; and that the sections should be securely keyed together.