Concrete Construction: Methods and Costs

by Halbert Powers Gillette; Charles Shattuck Hill

Mixing and Handling.—The concrete mixing plant was set up outside of the site of the reservoir along a side track from the railroad. The concrete materials were delivered on the side track, except some gravel from the excavation that was used. A Foote portable continuous mixer was used in making the concrete for the wall footings and the wall. It was mounted so it could discharge into dump cars on a service track laid on the ground. A double hopper was built up over the mixer, one compartment for sand and one for broken stone. The end of a service track leading from the side track was laid on an inclined trestle up to a floor level with the top of this double hopper, the materials being hauled in dump cars from the side track to the hopper. The service track from the mixer extended entirely around the wall, and 10 ft. from it, on the embankment made there with earth from the trench for the wall-footing. The concrete was dumped from the cars on the service track to portable shoveling platforms near the point where work on the wall was in progress. It was shoveled by hand from these platforms to place in the forms as the presence of the reinforcement bars in the narrow forms precluded dumping in large quantities. The footing was built without forms up to the right-angle joint between it and the base of the wall at the front, and to the top of the 45° slope on its rear face. A layer of concrete 2.5 in. thick was first placed in the completed trench. The reinforcement bars near the bottom were then laid on this green concrete, the vertical bars near the front face of the wall usually being erected at the same time. The concrete in the toe of the footing and in the footing proper up to the top layer of reinforcement was then laid. After the top layer of reinforcing bars had been laid, the footing was completed, except for a top layer about 2 ins. thick at the base of the front face of the wall and 15 ins. thick at the toe of the footing. This left a strip of surface about 6 ft. wide, sloping at about 1 in 6 from the wall toward the center of the reservoir, and furnished the widest and best possible bond for the joint which had to be made when floor was laid.

Location and Construction of Forms and Wall.—The design of the wall of the reservoir, although simple in itself, required unusually accurate work in the location and construction of the forms for it. The location was made with very little difficulty, however, by an arrangement devised by the contractor which enabled the foreman, without the aid of an engineer, to set the necessary grade and reference stakes. A post, 10 ins. in diameter, was set very accurately and firmly in the ground at the center of the reservoir. This post was sawed off squarely on top so that the line of collimation of an engineer's transit set on it without a tripod would be exactly at the grade of the top of the completed wall of the reservoir. A 200-ft. steel tape was used to measure the radial distance from a nail in the center post to the posts of the back, or outside forms for the wall. In the form for the back face of the wall 2×6-in. posts, spaced one one-hundredth of the circumference apart, were set considerably in advance of any concrete work, and were made the basis of all measurement in building the forms. The forms as originally planned are shown in Fig. 278.

The wall, when started, was built continuously in both directions from the starting point. The back forms consisted of planks for lagging nailed to vertical posts, which were accurately set and firmly braced. The front forms were made in lengths equal to one one-hundredth of the circumference of the reservoir, and when set up were fastened to the back forms. Twenty-one of these front form sections were built and all set up at once. Concrete was filled in between the front and back forms, starting at the central form, and was rammed in inclined layers, sloping, at about 1 on 6, both ways towards the end forms. This method was adopted in order that the concrete might be laid continuously and without joints. The lagging of 1-in. boards on the vertical portion of the sections was nailed to the vertical posts, and was carried up just ahead of the concrete filling.

When the concrete had reached the top of the central one of the 21 sections of the forms, and the concrete in that section had set sufficiently, the section was broken up and removed, leaving two sets of 10 sections of the forms. Subsequently the other forms could be removed in turn as desired without being broken up. As the filling-in proceeded between the two sets of 10 forms each, the form in each set nearest the starting point was removed, carried forward, and put in place at the other end of its set of forms. Twelve men were required to take down and transport one of the front form sections.

In setting up a front form, its inner toe was firmly supported by a stake driven into the ground and by the horizontal board, nailed transversely under the bottom 4×4-in. horizontal stringers, which rested on the ground. The upper part of the form was then securely fastened to the 2×6-in. posts of the back forms by temporary wooden connecting strips, which were removed as the concrete filling was carried up. The sections of the front forms were also securely tied to each other.

A facing of gravel concrete, rich in cement and with no pebbles larger than ½-in. was placed on the front face of the wall, extending from the back edges of the vertical reinforcing bars to the surface. A sheet-iron plate, about 8 ins. wide by 5 ft. long, was placed vertically just back of those bars. The concrete was shoveled in loose to the top of these iron plates, and then the mortar was poured in between the latter and the front face forms from buckets. The iron plates were next drawn by handles attached to them, and the mortar and concrete tamped together before either had set. In making joints, the old concrete surfaces were always brushed and wet down, and, if necessary slushed with a grout of neat cement before new concrete was laid on them.

Construction of Floor.—The excavation over the site of the reservoir floor was brought accurately to grade 6 ins. below the surface of the finished concrete by hand after the scoop-bucket excavator had passed over. In making the excavation the levels were given on radial lines drawn from the ends of the 10-ft. sections of the wall to the center. A rod, on which the elevations of the sub-grade at every 10 ft. from the wall to the center of the reservoir were clearly marked, was used in connection with a transit on the center post in locating the elevations of different points in the reservoir floor. By using this method the elevations required were easily found by the foreman in charge without the assistance of an engineer. When the work approached the center, the post was removed and the transit was placed on a portable pedestal which was set on points of known elevation on the finished concrete.

The slanting surface left on the top of the footing inside the wall formed, together with the projecting reinforcement rods, an excellent bond between the concrete of the wall and that of the floor, when the latter was laid. A circular strip of the floor, 16 ft. wide, was put down next to the wall first, and the remainder of the floor was laid according to the progress of the excavation. The lower 3½ ins, of the concrete was usually first spread out over an area 12 or 16 ft. square, then the reinforcement was placed, and after that the top 2 ins. of concrete and a ½-in. sidewalk finish surface were laid.

Fig. 279.—Standpipe at Haverhill, Mass.