For small diameters built with traveling forms, a comparatively dry concrete is essential, but when the centers are left in place until the concrete has set, a wet mixture is preferable, as it is more easily placed and worked around the reinforcement in the thin shells. Mixers are commonly specified even for small work, because of their generally more uniform and homogeneous product. Portable mixers hauled along the bank and discharging into the forms through chutes, furnish a cheap and rapid arrangement where the section being built has a considerable yardage. The examples given in succeeding sections present various methods of mixing and placing concrete in conduit work.
Fig. 252.—Traveling Form for Pinto Creek Conduit.
REINFORCED CONDUIT, SALT RIVER IRRIGATION WORKS, ARIZONA.—The pipe had the cross-section shown by Fig. 251, and formed a syphon carrying water under the bed of a creek. The concrete was a 1-2½-4 fine gravel mixture, mixed by hand on boards 150 ft. apart along the line. The shell was reinforced as shown.
The forms consisted of an outside form constructed as shown by Fig. 251, by inserting 2½-in.×5½ ft. lagging strips in the metal ribs. The inside form was designed to permit continuous work by moving the form ahead as the concreting progressed. It consisted as shown by Fig. 252, of an invert form on which an arch form was carried on rollers. The invert form was pulled along by cable from a horsepower whim set ahead, being steered, aligned and kept to grade by being slid on a light wooden track. It had the form of a long half cylinder, with its forward end beveled off to form a scoop-like snout. The arch center consisted of semi-circular rings 2 ft. long, set one at a time as the work required. Each ring, when set, was flange-bolted to the one behind, and each was hinged at three points on the circumference to make it collapsible. In operation, the invert form was intended to be pulled ahead and the arch rings to be placed one after another in practically a continuous process. So that the arch rings might continue supported after the invert form was drawn out from under them, invert plates similar to the arch plates were inserted one after another in place of the shell of the invert form. The plan provided very nicely for continuous work, but continuous work was found impracticable for all but about 2,500 ft. of the 6,000 ft. of conduit built. The reason for this seems to have been at least in a great measure, the slow setting cement made at the cement works established by the Government, at Roosevelt. In building the first 300 ft. of conduit, a commercial cement was used and a progress of 120 lin. ft. of pipe per 24 hours was easily made. This work was done in June. Later, but still in warm weather, using the Government cement and 70 ft. of arch plates, not more than 70 ft. of pipe could be completed in 24 hours; if the plates were taken down sooner, patches of concrete fell out or peeled off with them. As the weather grew colder, this difficulty increased, until finally, the idea of continuous work was abandoned and for some 3,500 ft. of conduit only one 8-hour shift per day was worked. In December and January the plates had to remain in place three days, so that the progress was only 24 ft. per day; in warm weather this rate was increased to 40 ft. per day.
Costs were kept on two sections of one of the lines and the figures shown in the accompanying table were obtained.
A gang consisted of a foreman at $175 per month, a sub-foreman at $3.50 per day, and the following laborers at $2.50 per day: one bending the reinforcement rings; two placing the reinforcement; four taking down, moving and erecting the stationary plates; four placing the concrete and outside lagging; two wheeling concrete; six mixing concrete; one wheeling sand and gravel; one watering the finished pipe; four laying track for the steering apparatus, moving the superstructure and hangers, mixing boards, runways, etc.; one pointing and finishing inside the pipe; and one on the whim and doing miscellaneous work. The labor was principally Mexican, and only fairly efficient.
It is important to note that the costs given in the table are labor costs only of mixing and placing concrete and moving forms; they do not include engineering, first cost of forms, concrete materials, reinforcement or grading.
| Wages Per Day. | May, '06. 714 Lin. Ft. Cost. | July, '06. 1,009 Lin. Ft. Cost. | Cost Per Lin. Ft. | Per Cu. Yd. | |
| 4 men Laying track for steering alligator | $5.00 | $71.48 | $43.98 | $0.0670 | $0.16 |
| 4 men Moving and erecting superstructure | 5.00 | 299.94 | 358.44 | 0.3821 | 0.93 |
| 4 men Moving plates | 10.00 | 202.50 | 253.44 | 0.2646 | 0.65 |
| Repairs to alligator | 58.50 | 2.50 | 0.0354 | 0.08 | |
| 1 man Bending rings | 2.50 | 32.87 | 59.87 | 0.0538 | 0.13 |
| 2 men Placing reinforcement | 5.00 | 126.94 | 138.13 | 0.1538 | 0.38 |
| 12 men Mixing and placing concrete | 30.00 | 709.68 | 949.74 | 0.9631 | 2.34 |
| 1 man Watering finished pipe. | 2.50 | 45.00 | 78.27 | 0.0716 | 0.17 |
| 1 man Painting and brush-coating inside | 2.50 | 96.50 | 117.37 | 0.1241 | 0.31 |
| Blacksmith's work | 30.00 | 25.00 | 0.0319 | 0.08 | |
| 1 man Whim | 2.50 | 23.87 | 28.75 | 0.0306 | 0.07 |
| 1 man Screening and hauling sand and gravel | 2.50 | 183.13 | 300.00 | 0.2804 | 0.68 |
| ———— | ———— | ——— | —— | ||
| Total | $1,880.41 | $2,335.49 | $2.4584 | $5.98 |
CONDUITS, TORRESDALE FILTERS, PHILADELPHIA, PA.—At the Torresdale plant of Philadelphia filtration system the clear water conduits are reinforced concrete. The following description is composed from information furnished the authors in 1904 by the Bureau of Filtration, Mr. John W. Hill, then chief engineer. The lengths of the several conduits are as follows: 576 ft. of 7½-ft., 782 ft. of 8-ft., 1,050 ft. of 9-ft., and 1,430 ft. of 10-ft. horseshoe conduit. All sizes of conduit have the same cross-sectional form—the cross-section of the 9-ft. conduit is shown by Fig. 253, and all are reinforced by expanded metal arranged as indicated. The concrete is a 1-3-5, ¾-in. stone mixture. The conduits were first designed with circular sections, but before construction had been begun on these plans, experience had been obtained in building a circular sewer that made a change to the horseshoe section appear desirable. In the circular sewer work, great difficulty had been found in properly placing and ramming the concrete in the lower quarters of the circular section.