Chapter 12. Concrete Work
The New York Rapid Transit Railway Extensions · Engineering News, 1914
The total amount of concrete to be used on the whole of the subway construction is, of course, quite large, but speaking generally there is little of the work where there are large masses, or where a great deal is required at one time, so that there are no elaborate plants for turning it out in large quantities. Three general methods are used, a central mixing plant of comparatively small capacity, the material being hauled in motor trucks to the point of delivery into the forms; a portable or movable mixer at the site; and hand mixing on the planking of the roadway immediately over the work. This latter seems an anomaly in these days of the very general use of machinery but in reality on account of the relatively small amount of material usually required to fill a considerable space in the forms, this latter method in many cases seems to be quite as economical and efficacious as any of the others.
Concreting Plants. The few specific plants and methods described below are fairly typical. On Sec. 8, 10 and 11, Lexington Ave., the concrete was all mixed dry in two l-yd. batch mixers, at the 96th St. Dock on the East River, from whence it was hauled in wagons holding about 3 cu.yd. to the point at which it was to be used. The use of horse-drawn wagons prevented the addition of the water before hauling on account of the length of time required to make the trip. It was usually dumped on the street decking, water added, and the mixture shoveled into chutes directly into the work in the cut-and-cover sections, or delivered down the shafts into 1-yd. cars for the tunnels. These cars were hauled to the point where the material was to be used and there dumped directly when the material was placed in the floors or footings of the walls, or hauled up a short section of movable inclined track to a platform at about the level of the springing line, where they were dumped and the material shoveled into the forms for the arches and sidewalls.
In one case in the tunnels on Sec. 9, a timber platform was suspended at about the springing-line level from eye-bolts, built into the concrete arch. The concrete was delivered into cars at the level of this platform at the shaft, and pushed along a track on it to the point where it was to be used. This required the permanent use of a considerable quantity of timber, however, and after a length of some 300 or 400 ft. had been built this way, a small traveler with an elevator was built to hoist the cars from subgrade level to the springing line. This was in a section where the rock required no support so that the whole section was clear. The platform of the traveler at the springing line reached across both tunnels and was located in the clear excavation ahead of the forms. The footings of the sidewalls were built first and kept ahead, and tracks for both the traveler and the forms were laid on them. The track in one tunnel was used for concrete cars, and in the other for the muck cars. The concrete cars were brought in under the traveler, hoisted by the elevator to the upper platform, from which both arches could be reached. The general layout is shown in the sketch, Fig. 83.
Fig. 83. Layout of plant for placing concrete in double-track tunnels, Lexington Avenue.
Wooden and steel forms have both been used on all four of these sections; the general opinion seems to be that where there is a length of say over 300 or 400 ft. of lining all of the same section, steel forms of the Blaw type are cheaper and quite satisfactory. The Blaw forms keep their shape quite well under ordinary conditions; but they could not be used to advantage in the sections where continuous support of the ground by posts was necessary, and, of course, not at all in sections where posts had to be left in while the lining was placed and only removed after the arch had taken the weight.
On Sec. 9 motor trucks were used for the concrete, thus permitting the complete mixing with water at the dock, which was possible by reason of the rapid means of transit these trucks provided.
On Sec. 12 a 0.75-yd. batch mixer was used, set up along the side of the street and moved along as the work progressed, the concrete being mixed at the point where used. Blaw forms were used, but on account of the continually varying dimensions of structure, were found to be not as easily adaptable as on work where they could be used over and over again without change.
On Sec. 13 the concrete-mixing plant is located at the dock at the foot of 125th St., the average haul to the work being about half a mile. A 2-yd. batch mixer is used, arranged with the necessary bins, etc., for convenient feeding of the materials, and so as to deliver the mixed concrete about 10 ft. above the ground. Two motor trucks with self-tipping bodies holding 4 yd. each are used for hauling the mixed concrete to the work. The latter is mixed fairly wet at the mixer, and a little more water is added for cleaning as it is dumped; an air pipe is kept in readiness to be used if necessary, to help get it out and clean the wagon. This is just a slight length of 5 or 6 ft. of inch pipe at the end of a hose, capped at the end and with small holes bored in it. It can be pushed into any mass which has shaken down so as to become a little tight and so start it. At the work when a section is ready for concrete, holes are cut in the wooden street decking at suitable intervals and a line of 8-in. conical sheet-metal spouts in 5-ft. sections is hung to lead from the hole in the decking to the forms. The sections are slung one below the other by chains. Over the hole in the decking is placed a wooden hopper 7x13 ft. and 2 ft. deep, into which the truck dumps its load and is away in about two minutes.
The hoppers are on skids and are not connected with the spout or the decking. They are moved by hitching them by a chain to a truck. The trucks make a round trip from the mixer to the place of dumping and return to the mixer in from 15 to 20 min.
On Sec. 15 there is a central mixing plant with a 1-yd. batch mixer. The concrete is carried in 1-yd. double-hinged bottom-dumping buckets, two of which are mounted on a flat-car; two cars are hauled together by a dinkey on 3-ft. gage track. The cars are arranged on the flat-car frame so that they can be dumped through it with out removing them. On this section the work is only partially decked over and the track is laid over the open trench. The cars with the buckets arc run to the point where the work is being done, spotted over the top of a chute and hopper and each bucket dumped in succession into it.
Fig. 86. Details of design of typical span, Queens Boulevard Viaduct.
For the Harlem Tubes, the outer concrete, as has already been described, was mixed by machinery on a lighter and deposited by means of tremies. For the lining of the inside of the tubes, the compressed air or pneumatic system controlled by the Chicago Concrete Placing Co. was tried at the northerly end for a part of the section.
Fig. 84. Pneumatic Concrete Mixing and Conveying Plant for Harlem River Tunnels (Mixer on left, bins on right).
The views in Fig. 84 show the general layout of this plant, which it was expected to use for the lining of half the length of the tubes. It is installed on a platform built over the water. The materials are fed in the proper proportions from the overhead bins into the cylinder of the machine in half-yard batches. Water is added from a pipe leading into the top of the cylinder at the same time as the dry material enters from above, the quantity being carefully measured, as it is of some importance to get the proper consistency. An air valve is then opened, and used as a blower to clean the gasket forming the seat of the top door which is then closed. Air is then admitted from the second valve over the top of the mixture to seal the top door, and apply pressure behind the mass, then the lower air valve opening into the elbow in the pipe just below the discharge at the bottom of the cylinder is opened, to loosen up the material as it feeds into the pipe, and both these last two valves are kept open until the drop in the pressure shows that the batch has been discharged from the end of the pipe.
Normally, very little, if any, trouble has been experienced in discharging the concrete at distances up to 500 ft., for which an air pressure of about 80 lb. is used, and the material has been carried over 800 ft. in the work, but to do this the air pressure had to be increased to 100 lb. and there were some few difficulties which, however, should be overcome in similar work as the men get used to the apparatus.
On leaving the main cylinder the pipe runs horizontally for 25 or 30 ft., then drops vertically 50 ft. or so to the bottom of the tunnels, and then along the floor to the point of discharge. Only the bottom of the tunnels was put in by this method, which for this kind of work probably has few advantages over any other method. In placing the sides and arch, however, which it was expected to be able to accomplish at one operation, the method should have advantages if successful, as it would avoid lifting the cars or buckets up onto a platform and the necessary shoveling into the forms.
As many as 40 batches (20 yd.) have been placed in one hour but the average for an 8-hr. shift is not over 65 to 70 yd., due to delays for shifting forms, shifting the pipe, etc. The work is being prosecuted continuously day and night, three 8-hr. shifts being worked. The average amount of concrete placed is from 135 to 150 yd. per day of 24 hours.
The discharge pipe is 8 in. diameter, flanged and bolted, made of a very mild but tough steel, which is expected to have a life estimated by a capacity of about 4000 yds. passing through it, the present pipe, however, which has been used for the discharge of almost 2000 yd., is beginning to show some signs of wear. This was particularly noticeable at the bottom of the perpendicular drop, and a new bend made of extra-heavy cast-iron segments, bolted together to make a curve of about 5-ft. radius, was installed and seems to work satisfactorily.
Reinforced-Concrete Elevated Railway. A part of the elevated structure to be built between the Queensborough Bridge Plaza in Long Island City and Corona, is of reinforced concrete. This section, which is 4271 ft. in length, is that which traverses Queens Boulevard, a parkway 200 ft. in width and one of the main arteries of travel from New York City out into Long Island. It was desired to make this an ornamental structure, and the drawing reproduced in the first of this series of articles (Fig. 3) gives an idea of the appearance of the completed structure.
Fig. 85. Views of Queens Boulevard Reinforced-Concrete Viaduct During Construction. (A) Columns completed. (B) Foundation for columns. (C) Columns showing forms. (D) Abutment at New York end.
The views in Fig. 85 show the construction of the column foundations, the columns and the abutment at the New York end. A longitudinal section on the center line of the structure through the end abutment, is shown in Fig. 87, and longitudinal and cross-sections of the main viaduct arches and piers in Fig. 86. The type of stations, of which there are three in this reinforced-concrete section, is shown in the perspective drawing already referred to and the details, and particularly the overhang for the platforms, in Fig. 87.
Fig. 87. Section through station and details of abutment, Queens Boulevard Viaduct.
As will be seen by the photographs, the columns were first built up to a point slightly above the springing line of the arches, then the heavy cross-girders at each bent were placed in position, after which the arches and parapet walls were poured.
A central mixing plant for the concrete was established near the New York end with a 1-yd. batch mixer operated by an electric motor with chain drive. The concrete was distributed from this point by dinkeys and trains of small four-wheel flat-cars, each carrying a single 1-yd. double-hinged, bottom-dumping bucket. A standard-gage rail track was was first laid the whole length of the work between the two rows of columns. This was used for the operation of two 15-ton locomotive cranes, and for the distribution of material. The cranes were used for various purposes in place of derricks, and for handling the forms and the concrete in the buckets, the latter being picked up off the cars and dumped directly into place. The concrete train and dinkeys were operated on the 2-ft. gage track laid in the center of the standard gage, during the construction of the piers, five cars being used in each train.
As soon as the construction of the columns was finished, the cross-girders, which weighed 27 tons each, were placed on top of them, being handled by a derrick car, as they were rather too heavy for the locomotive cranes.
For the construction of the arches, a standard-gage track for the two cranes was laid just outside the westerly row of columns, and a double-track narrow-gage line for the concrete train outside of this. When concreting from 1000 to 2000 ft. away from the mixer, three trains of five cars each were used to distribute the concrete. Clean, rounded gravel of fairly uniform size, graded from 3/8 to 1.5 in., is used for the aggregate, making a mixture which will probably give good results for the proposed hammered finish for the surface.
It will be noted from Fig. 86 that the arches curve both ways, giving somewhat the effect of a dome. The central third of each arch extending right across the structure, together with a part of the two side parapet walls, was first poured, then when that had set, the two sides between the central third and the cross-girders were poured together. The forms are left in place 28 days and it is expected to use each set of forms three or four times.
For pouring the arches, hoppers are erected above them into which the buckets (Fig. 88), lifted off the cars by the cranes, are dumped (Fig. 89). The concrete is then distributed from the hopper by short chutes as required. For the central section one hopper is used, for the end sections two hoppers, one for each; the central section is placed without top forms, but these latter are required for the steeper slopes of the two end sections.
Fig. 88. Concrete Buckets About to be Loaded.
Fig. 89. Lifting Concrete Buckets Into Place, Queens Boulevard Viaduct.
It is proposed to hammer-finish all exposed concrete faces, using a patent-hammer for all plain surfaces and a bush-hammer for recessed panels. The ends of the cross-girders on top of the columns and the longitudinal panels of the parapet walls are to be faced with colored ornamental tiling.