Concrete Construction Part 57

Labor 1.5 cts --- Total 4.0 cts.

While some of the cost items are apparently high when compared with the cost of similar work in other places, it should be remembered that the isolated locality and the local conditions were unfavorable for low cost. Owing to the isolated location of the reservoir with respect to large markets and also to local sources of supply the cost of material and labor was quite high. All construction material, except some of the stone for crus.h.i.+ng, had to be hauled over a mountain road from 3 to 4 miles to the top of the hill selected for the reservoir site. Labor was scarce and commanded a wage of $2.50 per day for ordinary work; the laborers mixing concrete were paid $2.75 per day. Another source of much relative expense was the high cost of lumber and carpenter work on the forms. On account of the thinness of the walls and roof, the cost of lumber and labor required per cubic yard of concrete was considerable. A part of the lumber was used the second time in forms, but it was found impracticable to delay the work by waiting for the concrete to harden before beginning the new portions of the walls. This lumber was sold after the completion of the work, but the salvage was inconsiderable, amounting to less than 10 per cent. of the original cost.

[Ill.u.s.tration: Fig. 278.--Reservoir Forms. Bloomington, Ill.]

~CIRCULAR RESERVOIR, BLOOMINGTON, ILL.~--An open circular reinforced concrete reservoir was constructed in 1905-6 for the water-works of Bloomington, Ill. This reservoir is 300 ft. in diameter, 15 ft. deep at the circular wall and 25 ft. deep at the center of the spherical bottom.

The wall construction is shown clearly by Fig. 278, and the floor is a 6-in. spherical slab reinforced by a mat of -in. round rods placed 6 ins. on centers in both directions. The wall reinforcement is corrugated bars. Neither the wall nor the bottom has expansion joints.

_Concrete._--The specifications required not less than 1 part Portland cement to 2 parts sand and 5 parts clean gravel, and stipulated that there should always be more than enough cement to fill the voids in the sand more than enough mortar to fill the voids in the gravel. The proportions were varied, depending on the character of the available material and on the location the concrete was to occupy. The stipulations regarding the minimum quant.i.ties of cement and mortar were, however, always at least fulfilled. A 1-3-4 mixture of cement, broken stone and gravel was largely used in the footing and wall. The gravel was fine and contained 40 to 50% of sand; the broken stone was the crusher-run, with the dust screened out, and the maximum-sized pieces not larger than those which would pa.s.s a 2-in. screen. The mortar facing on the front face of the wall was made of 1 part cement to 4 parts fine gravel, containing sand. Some gravel from the excavation was used in the concrete for the floor. This gravel was so fine that about one-quarter of it was replaced with broken stone and the mixture made 1-6. Both faces of the wall were painted with a 1-1 mixture of cement and sand; the inner face was also painted with a 1-1 mixture of waterproof Star Stettin Portland cement and sand. The sidewalk finish on the surface of the floor consisted of 1-1 mortar.

_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 quant.i.ties. 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 26-in. posts, s.p.a.ced one one-hundredth of the circ.u.mference 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 circ.u.mference 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 44-in. horizontal stringers, which rested on the ground. The upper part of the form was then securely fastened to the 26-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 -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 pa.s.sed 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 a.s.sistance 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 3 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 -in. sidewalk finish surface were laid.

[Ill.u.s.tration: Fig. 279.--Standpipe at Haverhill, Ma.s.s.]

The -in. rods in the bottom are 6-ins. on centers in both directions.

They were in 12 and 16-ft. lengths and were partly woven together in mats before being placed. The rods in one direction were all laid out and woven with four or five of those in the other direction, the joints being tied with small wire. The remaining cross rods were laid after the mat had been placed. The mats were overlapped 1 ft. This method of placing proved economical and efficient, giving at the same time something permanent on which to lay the remaining concrete.

~STANDPIPE AT ATTLEBOROUGH, Ma.s.s.~--The stand pipe was 50 ft. in diameter and 106 ft. high inside, with walls 18 ins. thick at the bottom and 8 ins. thick at the top. Figure 279 shows the general arrangement of the reinforcement. Round bars of 0.4 carbon steel were used; the bars came in 56-ft. lengths, so that three lengths with laps of 30 ins., made a complete ring around the tank. The concrete was a 1-2-4 mixture of to 1-in. broken stone with screenings used as portion of sand.

The floor was built first, and on it was erected a tower to a height of 60 ft. and a derrick with a 40-ft. boom was set on its top. The derrick was operated by an engine on the ground which also had a revolving gear attached. When the work had reached the top of this tower, the tower was raised to 110 ft. in height and the derrick s.h.i.+fted to the new elevation. The forms were convex and concave sections 7 ft. high and about 11 ft. long. The concave or outside forms were made in 16 panels, with horizontal ribs and vertical lagging; two complete rings of panels were used. The panels were joined into a ring by clamps across the joints, this clamping action and the friction of the concrete holding them in place. The inside forms consisted of vertical ribs carrying horizontal lagging put in place a piece at a time as the filling proceeded. They were supported by staging from the derrick tower. The remaining plant comprised a Sturtevant roll jaw crusher feeding to screens which discharged fines below -in. into one bin, medium stone into another bin and coa.r.s.e stone into a third bin. These bins fed to the measuring hopper of a Smith mixer, which discharged into the derrick bucket.

The mode of procedure was as follows: The reinforcing rings were erected to a height of 7 ft. The bars were bent by being pulled through a tire binder and around a curved templet by a steam engine. The bending gave some trouble, due, it was thought, to the stiffness of the high carbon steel. Vertical channels 4 ins. deep were set with webs in radial planes or across wall at four points in the circ.u.mference. The f.l.a.n.g.es of these channels were punched exactly to the vertical s.p.a.cing of the reinforcing rings. Through the punched holes were pa.s.sed short bars on the opposite ends of which the reinforcing rings were supported and wired. The three sections of rod of which each ring was composed, were lapped 30 ins. and connected by Crosby clips. Considerable difficulty was had in holding the reinforcing rings in line by the method employed; it is stated by the engineer that a greater number than four channels would have been much better.

The reinforcement being in place, an inside and an outside ring of forms was erected. Concreting was then carried on simultaneously from four points on the circ.u.mference and a ring 7 ft. high was concreted in one operation. Several facts were brought out in the concreting; careful and conscientious spading was necessary to get a smooth dense surface; a too wet mixture allowed the stone to settle and segregate; care was necessary in this thin wall containing two rings of bars to keep the stone from wedging among and around the bars and thus causing voids. The engineer states that for this reason the subst.i.tution of mortar for concrete in tank walls is worth considering. He estimates that in this work, costing $35,000, that the use of a 1-2 mortar in place of the 1-2-4 concrete would have increased the cost by $2,300, a 1-2 mortar by $1,500, and a 1-3 mortar by $750. It was also found that there was danger from a movement of the reinforcement in the concrete and of the forms in placing the concrete.

When a ring of wall 7 ft. high had been concreted, the reinforcement was placed as before described for another ring. The two rings of forms below those just filled were removed from the wall, hoisted up and set in place on top. These two operations of placing reinforcement and setting forms for another ring of wall took three days, so that the top surface of the wall to which new concrete was to be added, had become hard. This hard surface was very thoroughly washed and then coated with neat cement immediately before depositing the fresh concrete. Water was admitted to the tank as the work progressed, being kept about 20 ft.

below the work in progress. Numerous small leaks developed, but only two were large enough for the water to squirt beyond the face of the wall.

These leaks appeared to grow smaller as time went on. To do away with them entirely, the inside wall was plastered. The first coat of plaster was not successful in stopping the leaks, so the standpipe was emptied and replastered, five coats being used in the lower 20 ft. This did not serve so resort was had to a Sylvester wash. A boiling hot solution of 12 ozs. to the gallon of water of pure olive oil castile soap was applied to the dry wall. In 24 hours this was followed with a 2 ozs. to the gallon solution of alum applied at normal temperature. Four coats of each solution were applied, which reduced the leakage to a small amount.

To do away with all leakage another four-coat application of Sylvester wash was used.

Details of the cost of the work are not available. There were 770 cu.

yds. of concrete in the walls and 185 tons of steel bars. Altogether 3,000 Crosby clips, costing $1,100 were used. The cost of the concrete in place was about as follows:

Cement, per cu. yd. of concrete $ 4.80 Sand and stone, per cu. yd. of concrete 3.90 Mixing, per cu. yd. of concrete 0.40 Placing, per cu. yd. of concrete 2.20 Forms, per cu. yd. of concrete 2.65 ------ Total per cu. yd. of concrete $13.95

~GAS HOLDER TANK, DES MOINES, IOWA.~--The tank was 84 ft. in diameter and 21 ft. 5 ins. deep. It had a horizontal floor 16 ins. thick 5 ft. below ground level and a wall 21 ft. high, 18 ins. thick at base and 12 ins.

thick at top under coping and with alternate pilasters and piers around the outside. The concrete for the floor was a 1-2-5 2-in. stone mixture and the concrete for the walls was a 1-2-4 1-in. stone mixture. The floor was constructed first, with a circular channel for the wall footing, and then the wall was constructed.

Piles were driven in the bottom and their heads cut to level and filled around with tamped cinders. Two circ.u.mferential rows of posts were driven around the edge so that a pair of posts, one inner and one outer, came on each radius through a wall pilaster or pier. These posts served primarily to carry the frames for the wall forms and secondarily for holding the forms for the circular wall footing channel as shown by the sketch Fig. 280. The floor concrete was put in in diamond-shaped panels between forms, whose top edges were set to floor level. Each form was designed to make a groove in the edge of the slab so that adjacent slabs would bond with it. The concrete was wheeled to place in barrows, thoroughly tamped, roughly floated to surface and finally given a trowel finish.

[Ill.u.s.tration: Fig. 280.--Forms for Constructing Channel for Wall in Reservoir Floor.]

To construct the walls, the posts before mentioned, were cut off to exact level 6 ins. above the finished floor. A bent for the wall forms was then erected on each radial pair as shown by Fig. 281. The bents were erected by hand and carefully plumbed and lined up, both radially and circ.u.mferentially. The pier and pilaster forms were then erected across wall opposite each bent as shown by Fig. 281. The forms for the wall between pilasters and piers consisted of panels 4 ft. high.

[Ill.u.s.tration: Fig. 281.--Frame for Forms for Circular Reservoir Wall.]

[Ill.u.s.tration: Fig. 282.--Form Panels for Circular Reservoir Wall.]

A panel for the inner face of the wall is shown by Fig. 282, the panel for the outer face was similar in construction but was, of course, concave instead of convex. Enough panels of each kind were made to reach entirely around the tank. The inside panels were bolted at the ends to the uprights of the bents; the outside panels were similarly lag screwed to the uprights of the pier and pilaster forms; Fig. 281 shows the holes for bolts and lag screws. The s.p.a.ces between ends of inside panels in front of the bents was closed by a 6-in. steel plate the full height of the wall; this plate was bolted to the bents and had anchor bolts every 3 ft., reaching into the wall. This anchoring of the plate to the wall permitted the diagonal bracing of the bents to be removed to allow runways to be laid on the cross-pieces, since the plate held firmly the bent post to which it was bolted as indicated by Fig. 283. A complete circle of inside and outside forms was erected and filled, then the forms were raised 3 ft. by block and tackle from cross timbers across wall between bent and pilaster form, and this depth concreted and the forms raised again. The forms were oiled on the faces coming against the concrete. It took about half a day to raise and set a complete circle of forms. The concrete was mixed outside the tank and was wheeled up inclines and dumped onto runways laid on the cross pieces of the bents and then loaded and wheeled to place. The runway was raised to successive horizontals as the work progressed.

[Ill.u.s.tration: Fig. 283.--Sketch Showing Filler for Joint Between Form Panels.]

Only a few general cost figures are available. The labor for mixing and placing concrete was as follows:

For floor, per cu. yd. 3.4 hrs.

For walls, per cu. yd. 5.2 hrs.

For cornice, per cu. yd. 5.4 hrs.

The cost of unloading the reinforcing steel from cars and placing it in the structure was $7 per ton, or 0.35 ct. per lb. The cost of form lumber, framing, erecting and taking down forms was 9 cts. per square foot of wall covered.

~GAS HOLDER TANK, NEW YORK CITY.~--The tank for the Central Union Gas Co.'s gas holder at 136th St. and Locust Ave. has an interior diameter of 189 ft. and a depth of 41 ft. 6 ins. The exterior wall is 42 ft. 6 ins. deep, 5 ft. 6 ins. thick at the base and 4 ft. 6 ins. thick at the top; concentric with it and 11 ft. 6 in. away is the interior wall 166 ft. in external diameter and 16 ft. 6 ins. high with a uniform thickness of 2 ft. 6 ins. The bottom of the tank enclosed by the interior wall is a truncated cone whose base is at the level of the wall top. Fig. 284 shows the arrangement.

It was specified that the diameter of this tank should not vary more than 2 ins. and that the exterior wall should not vary more than 1 in.

from the vertical. The main form was a circular drum whose exterior face formed the inner face of the main wall. Its framework consisted of 40 vertical trusses or radial frames 6 ft. deep and 42 ft. high set equidistant around the tank, these trusses being braced together on both edges by circ.u.mferential timbers. Radial horizontal pieces nailed across the radial frames and projecting beyond their faces carried vertical iron guide strips against which the movable panels of lagging were seated. These panels were cylindrical segments 5 ft. high and long enough to span between two radial frames or 14 ft. 11-5/8 ins. The panels were adjusted radially by wedges to give 1/8 in. clearance in respect to inner face of wall; enough of them were made to form a complete circle and they were set with 1-in. clearance between vertical edges of adjacent panels to allow for swelling when wetted.

[Ill.u.s.tration: Fig. 284.--Section of Gas Holder Tank, New York City.]

The concrete bottom of the annular s.p.a.ce between walls was first constructed. On this floor were set 66-in.8-ft. sills for the radial frames; these were located accurately by transit. The radial frames were then set on the sills by a derrick, adjusted to exact radial position by a measuring wire swiveled to the center point of the tank and plumbed by transit. A complete circle of lagging panels was then adjusted to the frames at the bottom of the trench. For concreting, the wall was divided circ.u.mferentially into three sections. These sections were separately concreted to the top of the lagging panels, that is to a height of 5 ft.

After the concrete had set 48 hours the panels were hoisted 4 ft., so that their lower edges still overlapped the concrete 12 ins., and another ring of wall was concreted. This procedure was repeated until the wall was completed. The back of the wall was formed against the side of the trench where possible and in other places against rough board lagging held in position in any convenient way.

For handling the concrete, four equidistant panels of the form framework were converted into double compartment elevator shafts providing for two balanced cars controlled by a sheave provided with a friction brake.

Three mixers supplied concrete to these elevators. Considering a single elevator, two barrows of concrete were wheeled from the mixer onto the car at the top of the elevator frame, the friction brake was released and the loaded car descended to the work hoisting at the same time its twin car loaded with two empty barrows. The elevators distributed to wheeling platforms cantilevered out from the outer face of the framework and located successively 5 ft., 15 ft., 20 ft., etc., above the bottom of the trench. On these platforms the concrete was distributed as required, the maximum wheeling distance being never over one-eighth the circ.u.mference of the tank. The concrete was mixed very wet and deposited in 6-in. layers.

The inner and outer surfaces of the wall were both painted with two coats of stiff cement grout neat, and in addition the inner surface was rubbed smooth by carborundum brick. Regarding this finis.h.i.+ng work Mr.

Howard Bruce, Engineer of Construction, writes:

"The scouring was done on each section of the wall immediately after the forms supporting these sections had been removed. The object was to rub this interior surface with carborundum before the surface of the concrete had taken its final set. By rubbing the concrete at this stage and at the same time applying with a brush a coating of neat cement grout, we believe the face of the concrete was made more or less impermeable, as examination shows the pores of the concrete are very largely filled up. We have no accurate figures as to the cost per square yard of this treatment, but one can readily see that this cost would be insignificant as compared with the possible improvement of the work. The carborundum brick was selected on account of its hardness. I believe practically any stone would answer the same purpose. In addition to filling the pores of the concrete, this treatment gives the surface a good smooth finish."

~LINING A RESERVOIR, QUINCY, Ma.s.s.~--The following methods and costs are given by Mr. C. M. Saville, M. Am. Soc. C. E., for lining the Forbes Hill Reservoir at Quincy, Ma.s.s. This reservoir is 100280 ft. on the floor, with side slopes of 1 on 1.75, and was built by contract in 1900-1901.

[Ill.u.s.tration: Fig. 285.--Section of Reservoir Lining, Quincy, Ma.s.s.]