Concrete Construction: Methods and Costs

by Halbert Powers Gillette; Charles Shattuck Hill

BOX CULVERT CONSTRUCTION, C., B. & Q. R. R.—Mr. L. J. Hotchkiss gives the following data. Box sections of the type shown by Fig. 169 are used mostly; they range in size from single 4×4-ft. to double 20×20-ft. and triple 16×20-ft. boxes. These boxes are more simple in design and construction than arches, and for locations requiring piles they are less expensive. The form work is plain and the space occupied is small as compared with arches, so that excavation, sheeting and pumping are less and the culvert can be put through an embankment or under a trestle with less disturbance of the original structure. Finally, less expensive foundations are required.

For small jobs where it does not pay to install a power mixer a hand power mixer mounted on a frame carried by two large wheels has been found at least as efficient as hand mixing; more convenient and easier on the men. The machine is turned by a crank driving a sprocket chain; it is charged at the stock piles and then hauled to the forms to be discharged. Local conditions determine the capacity of power mixer to be used. Difficulties in supplying material or in taking away the concrete may readily reduce the output of a large machine to that of one much smaller, and the small machine is cheaper in first cost and in installation and operation. Where the yardage is sufficient to justify the installation of equipment for handling the materials and output of a large mixer it is found preferable to a small one, as the increase in plant charges is not proportionately so great as the increase in the amount of concrete handled. Again it may occur on a small job that the concrete must be taken a long distance from the mixer, that a large batch can be moved as quickly and as easily as a small one and the time consumed in doing it is sufficient for the charging and turning of a large mixer before the concrete car or bucket returns to it. Here a large mixer, while it may stand idle part of the time, is still economic.

Fig. 169.—Box Culvert and Form, C., B. & Q. R. R.

The plant lay-outs vary with the local conditions, as the following will show. In one case of a culvert located under a high, short trestle the following arrangement of plant was employed: A platform located on each side of the approach embankment about 8 ft. below the ties was built of old bridge timbers. A track was laid on each platform and ran out over a mixer located on the end slope of the embankment. Two mixers, one for each platform, were used. From each mixer a track led out over the culvert form and a track along the top of this form ran the full length of the culvert. Gravel and sand were dumped from cars onto the side platforms and thence shoveled into small bottom dump cars, which were pushed out over the mixer and dumped directly into it. Cars on the short tracks from mixers to culvert form took the mixed concrete and dumped it into the distributing cars traveling along the form. The cars were all hand pushed.

An entirely different lay-out was required in case of a long box culvert located in a flat valley some 600 ft. from the track. A platform was built at the foot of the embankment with its outer edge elevated high enough to clear two tracks carrying 5 cu. yd. dump cars. The sand and gravel was dumped from cars onto the side of the embankment, running down onto the platform so that scraper teams moved it to holes in the platform where it fell into the dump cars. These cars were hauled by cable from the mixer engine and dumped at the foot of an inclined platform leading to a hopper elevated sufficiently to let a 1½ cu. yd. dump car pass under it. A team operating a drag scraper by cable moved the material up the inclined platform into the hopper, whence it fell directly into the car to which cement was added at the same time. The charging car was then pulled by the mixer engine up another incline, at the top of which it dumped into the mixer. The concrete car was hauled up another incline to a track carried on the forms and reaching the full length of the culvert work.

The placing of the reinforcement is given close supervision. When a wet concrete is used it is found necessary to securely fasten the bars in place to prevent them being swept out of place by the rush of the concrete. A method of supporting the invert bars is shown by Fig. 169; 2×2-in. stakes are large enough and they need never be spaced closer than 6 ft. The longitudinal bars are held on the stakes by wire nails bent over and the transverse bars are wired to them at intersections by stove pipe wire. The vertical wall bars are placed by thrusting the ends into the soft footing concrete and nailing them to a horizontal timber at the top; the horizontal wall bars are wired at intersections to the verticals. In the roof slab the stakes are replaced by metal chairs, or by small notched blocks of concrete.

The form construction is shown by Fig. 169. It is not generally made in panels, since, as the work runs, the locations of boxes of the same size are usually so far apart that transportation charges are greater than the saving due to use a second time. No general rule is followed in removing forms, but they can usually be taken down when the concrete is a week old.

The boxes are built in sections separated by vertical joints, one section being a day's work. The vertical joints are plain butt joints; tongue and groove joints give trouble by the tenons cracking off in the planes of the joints. A wet mixture is used and smooth faces obtained by spading.

ARCH CULVERT COSTS, N. C. & ST. L. RY.—The cost of arch culvert construction for the Nashville, Chattanooga & St. Louis Ry. is recorded in a number of cases as follows: