Fig. 259.—Form for South Bend Sewer (Third Stage).
Forms and Concreting.—In constructing the sewer the trench was excavated so as to give a clearance of 1 ft. on each side and was sheeted as shown by Fig. 257. The sewer was built in 12 ft. sections as follows: The bottom of the trench was shaped as nearly as possible to the grade and shape of the base of the sewer. Four braces to each 12 ft. section were then nailed across the trench between the lowest rangers on the trench sheeting. A partial form consisting of a vertical row of lagging was set on each of the outside lines of the sewer barrel as shown by Fig. 257. Each section of this lagging was held by stakes driven into the trench bottom and nailed at their tops to the cross braces as shown by Fig. 258. A template for the invert was then suspended from the cross braces by pieces nailed to the four ribs of the template and to the cross braces as shown by Fig. 257. The concrete was now placed and carried to the top of the template, which was then removed. The side pieces of the reinforcing bars were then set and fastened as shown by Fig. 258. The side forms extending up to the springing lines were then placed. They were held in position by braces nailed to their ribs at the tops and by other braces fitting into notches in the ends of their ribs at the bottom. The concrete was then carried up to the springing lines, the arch centers in two pieces were placed; the arch bar of the reinforcement was placed and the extrados forms erected up to the 45° lines, all as shown by Fig. 259. The placing of the arch concrete completed the sewer barrel. The outside forms and bracing were removed about 24 hours after the completion of the arch and back filling the trench was begun immediately, but the inside forms were left in place for two weeks; they were then removed by the simple process of knocking out the notched braces. By building several lengths of invert first and following in succession by the side wall construction and then by the arch construction, the form erection and the concreting proceeded without interruption by each other. It was also found that, by making bends in the form of polygons with 10 ft. sides instead of in the form of curves, there was a material saving in expensive form work. To overcome the friction of the angles in such bends an additional fall was provided at these places. All concrete was made in a Smith mixer mounted on trucks so that it could be moved along the bank of the trench and discharging into a trough leading to the work.
Labor Force and Cost.—With a gang of 12 men from 24 to 36 ft. of sewer was built per 10-hour day, working only part of the time on actual concreting. The disposition of the force mixing and laying concrete and the wages were as follows:
| Item. | Per day. |
| Six wheelers, at 18.5 cts. per hour | $11.10 |
| One mixer, at 22.5 cts. per hour | 2.25 |
| One dumper, at 18.5 cts. per hour | 1.85 |
| Four placers, at 22.5 cts. per hour | 9.00 |
| ——— | |
| Total | $24.20 |
There were 0.594 cu. yd. of concrete per lineal foot of sewer and its cost is given as follows:
| Item. | Per cu. yd. |
| Cost of gravel | $0.774 |
| Cost of sand | 0.36 |
| Cost of cement | 1.50 |
| Cost of steel reinforcement | 0.84 |
| Cost of labor, mixing and placing concrete | 1.094 |
| Cost of moving forms, templates, etc. | 0.757 |
| Cost of forms, templates, etc. | 0.589 |
| Cost of finishing, plastering, etc. | 0.639 |
| Cost of tools and general expenses | 0.841 |
| ——— | |
| Total | $7.394 |
SEWER INVERT, HAVERHILL, MASS.—In constructing sewers with concrete inverts at Haverhill, Mass., in 1905, use was made of the traveling form or mold shown by Fig. 260. The form consists of an inner and an outer shell, the annular space between which forms the mold; in operation the annular space is filled with concrete, then the outer shell is pulled ahead from underneath, leaving the inner shell in place. A second inner shell is then adjusted to the outer shell in its new position, the annular mold is concreted and the outer shell again pulled ahead. Continued repetition of the operations described completes the invert. The merit of the device lies in the fact that the inner shell is not moved until the concrete has attained some degree of rigidity; when, in such devices, the inner mold is slid ahead on the green concrete it is likely so to "drag" forward the material that a rough and pitted surface results.
Mold Construction.—Referring to the drawings of Fig. 260, A is the outer mold of sheet steel bent to the required shape of the outer surface of the conduit to be constructed. A rib, or angle, B, is riveted to the inside of the mold at its front end and a diaphragm C of plank is securely fastened to the rear side of the rib. The opposite or rear end of the mold is open. Angles D forming tracks are riveted inside the mold a short distance below the edges and reaching their full length. The inner mold comprises a steel shell E curved to the form of the inside of the conduit; inside this steel shell is a reinforcing lagging, and at each end there is a wooden diaphragm F. Passing through both end diaphragms and having its ends flush with the end planes of the mold is a timber G. Rearward projecting lips e are secured to the lagging at the rear end of the mold and on each side of the timber G. The diaphragms F have each two arms f which project horizontally beyond the surface of the inner mold and engage the tracks D; locking dogs H are pivoted to the arms f so as to hook under the track angles D and hold the inner form from rising. Setting on the inner mold is an inverted V-shaped deflector I; its edges are flush with the sides of the mold and its purpose is to facilitate the placing of the concrete. There is also a movable diaphragm K, fitting loosely inside the outer mold A and bearing against the end of the inner mold E. The length of the inner mold E is about one-half that of the outer mold A; as a rule several inner molds are provided with one outer mold.
