Concrete Construction Part 55

_Molding._--When the three positions of the steel skeleton were completed, as shown by Fig. 271, they were set on curved wooden curbs made to the exact shape necessary to center them and preserve the correct thickness of cement coating. A collapsible core was lowered into position in the inside, and a two-part sheet steel mold was erected outside; the s.p.a.ce between core and mold was then poured with a thin mortar of one part Portland cement to two parts clean river sand. During the process of pouring, the outer steel mold is sharply struck with wooden mallets to facilitate the escape of air bubbles. The mortar was mixed on an elevated traveling platform which is shown in Fig. 272, which also shows a completed pipe, a core being withdrawn, a filled mold and a section of reinforcement set up. The difficult feature of the molding process was found to be the determination of the time for withdrawing the core and removing the exterior mold; the time of setting of the mortar was different in warm and in cool weather and varied with the wetness of the mixture, the brand of cement, etc. By using a single brand of cement that ran very uniform in quality and time of setting it was possible, however, for the workmen, after a little practice, to gage very accurately the correct time for removing the molds. With four sets of molds a gang of eight men would curb 16 pipes per day under favorable conditions, but when the temperature was low it was not possible to make more than six or eight pipes. The pipes were allowed to stand four or five days after the removal of the mold; they could then be removed by a crane and laid in stock until used. It was found advisable to let the pipes age about four weeks before laying; by this time, it is stated, they would stand as much rough usage as cast iron pipe.

_Laying_.--The pipes were laid much in the same way as cast-iron pipes are laid; they were each 9 ft. 9 ins. long and weighed each about 12 cwt., and were handled by ordinary tackle. In laying, the pipes were adjusted end to end and the joint enclosed by a temporary steel ring inside which the bitumen seal, Fig. 270, was run and allowed to set when the steel ring was removed. The joint was then encircled by a collar of similar construction to the pipe itself and the s.p.a.ce between collar and pipe was poured with cement mortar. About ten lengths of pipe were laid per day by one gang of men, one jointer and his a.s.sistant making all the cement and bitumen joints as fast as the gang could lay the pipes.

CHAPTER XXII.

METHODS AND COST OF CONSTRUCTING RESERVOIRS AND TANKS.

Floor, wall and roof work of structurally very simple character sum up the task of the constructor in reservoir and tank construction. The only intricacy involved lies in form design and construction for cylindrical tank work. Several examples of such work are given in this chapter, and in each the construction and handling of the forms are described. To repeat details here would serve no purpose, but one general instruction may be enunciated. No care is too great which ensures rigidity and invariable form, both in the construction of the individual form units and in the a.s.sembling of these units into the complete form. This is particularly true of cylindrical tank work and especially high cylindrical tank work where the forms are moved upward as the work progresses. To the designer it may be suggested that any beauty he may gain by giving the walls of his standpipe a batter is paid in the extra cost of form work.

Concreting in tank work is expensive. The reasons are two. The work has to be done in a narrow s.p.a.ce, commonly pretty well filled with a network of steel rods or bars. Again the work has to be done uniformly well, not only for appearance sake but because of the necessity of watertightness.

Making a reservoir watertight is, when all things are said, the one difficult constructional task in tank work and the contractor who accepts the task lightly courts trouble. Exceptionally good concreting is essential in tank work if watertightness is to be secured.

The ill.u.s.tration of these general admonitions will be found in the specific examples of tank and reservoir work which follow.

~SMALL COVERED RESERVOIR.~--The reservoir was designed to hold 75,000 gallons of water for fire purposes. As it is of a type which is certain to be frequently constructed and as we have personal knowledge of the costs recorded we describe the work in some detail. The specifications stipulated that the reservoir must be absolutely watertight and that the roof should be capable of sustaining a load of 300 tons evenly distributed and a live load of 5,000 lbs. on two wheels. Figure 273 shows a plan, Fig. 274 a longitudinal section, Fig. 275 a transverse section and Fig. 276 the column construction.

[Ill.u.s.tration: Fig. 273.--Sectional Plan of 75,000-Gallon Reservoir.]

_Quant.i.ties of Work._--The excavation called for the removal of 579 cu.

yds. of earth. There were 83 cu. yds. of concrete in the structure, although the plans called for less, the additional amount being used in increasing the two 4-in. walls to 6-in. and increasing the bottom and top, on one end, so as to give perfect drainage. The yardage was divided as follows:

Cu. yds.

Footings 3.5 Columns 6.8 Sides 22.6 Girders 11.0 Top 20.0 Floor 19.1 ---- Total 83.0

[Ill.u.s.tration: Fig. 274.--Longitudinal Section of 75,000-Gallon Reservoir.]

A manhole had to be put in the top and a sump in the bottom. Several pipes also had to be placed in the concrete. None of these details is shown on the plan. The structure had to be waterproofed.

_Excavation._--The excavation was made with pick and shovel and the material hauled away in carts, the distance to the dump being 700 ft.

The top was shoveled directly into the carts, while the rest was handled two and three times. When the reservoir was finished dirt had to be filled in around the sides and puddled.

_Wages._--The following rates of wages were paid on the job:

Foreman $3.00 Carpenter 3.50 Carts and driver 3.50 Laborers 1.50

The carpenters worked 8 hours a day and were paid time and a half for overtime. The rest worked ten hours per day and were paid regular rates for overtime.

[Ill.u.s.tration: Fig. 275.--Transverse Section of 75,000-Gallon Reservoir.]

_Forms._--Carpenters framed and erected the forms, but laborers did all the carrying for them. Laborers also tore down the forms. For the girders and columns 2-in. boards were used, but for the sides 1-in.

boards with 34-in. scantlings were used. The props for supporting the girder and top forms were 34. Except for columns and girders and some props, all the forming was used three times. The lumber cost:

400 ft. B. M. at $24 $ 9.60 8,000 ft. B. M. at $18 144.00 ------- Total $153.60

[Ill.u.s.tration: Fig. 276.--Column Construction for 75,000-Gallon Reservoir.]

This makes an average price per 1,000 ft. of about $18.30, which price we shall use in giving costs.

The cost of framing and erecting the forms was $167.27 for the sides, columns, girders and top. In the forms for the sides, forming was only used on one side of the concrete for two sides, the earth bank being used for the other side of the forms, but on the other two sides the banks had caved in, and forming was used on both sides of the wall. The cost per cubic yard for forms was:

Lumber $2.54 Framing and erecting 2.77 Tearing down .54 ----- Total $5.85

This cost is for the yardage of 60.4 on which forms were actually used.

For the total yardage in the tank the cost was:

Lumber $1.85 Framing and erecting 2.01 Tearing down .40 ----- Total $4.26

The common labor cost of a.s.sisting to erect the forms was 15 per cent of the total. Nothing is allowed for foreman, for the contractor acted as his own foreman.

The cost of forms per 1,000 ft. for the amount of lumber purchased was:

Lumber $18.30 Framing and erecting 19.90 Tearing down 4.00 ------ Total $42.20

As the lumber was used three times, the cost per thousand for all work and materials on the forms would be just one-third of this--namely: $14.06.

Since the framing, erecting and tearing down cost $19.90 plus $4, or $23.90 per M. ft. B. M. purchased, and since the lumber was used three times, the labor cost nearly $8 per M. each time that the lumber was used. It will be noted that 8,400 ft. B. M. were required for the 83 cu.

yds. of concrete, or a trifle more than 100 ft. B. M. per cubic yard.

It will be of interest to see the labor costs of forms for the various parts of the structure.

For the sides the cost of framing and erecting the forms was $4.19 per cubic yard. Of this cost 4 per cent. was for common labor and the rest for carpenters. The tearing down cost 47 cts. per cubic yard. For the columns the erecting was $2.35, of which 1 per cent. was for common labor. The tearing down cost 47 cts. For the girders and top the erecting cost $1.83, of which 35 per cent. was common labor. The tearing down cost 61 cts. per cubic yard. A summary would show:

Girders Sides Columns and per per top per cu. yd. cu. yd. cu. yd.

Framing and erecting $4.19 $2.35 $1.83 Tearing down .47 .47 .61 ----- ----- ----- Total $4.66 $2.82 $2.44

The greater cost of the columns forms over the girders and top was due to the fact that the columns forms were handled almost exclusively by the carpenters, and also in setting them great care and much time had to be used to get them plumb and in line. The cost of the forms for the sides was about twice as great as that for the top and girders. The reasons for this are evident. The walls had forms on both sides, while the top needed forming only underneath it, the area covered on the forms being about 2,200 sq. ft. as compared to 1,000 sq. ft. The side forms had to be set plumb and kept so. The framing was done ahead, but nearly half of the lumber in the sides was erected as the concrete was being put in place. The forms for the top were all put in place before any concreting was done on the top, and the carpenters discharged. A much larger per cent. of common labor could be used in placing forms for top and girders than on the sides. The props were nearly all put in place by laborers. The extra cost of tearing down the forms for the top was due to the fact that the lumber all had to be handled one piece at a time through a small manhole in the top, and carried about 150 ft. to be piled.

To all the costs for forming should be added 6 cts. per cubic yard for nails, wire and lines used on the forms.

_Concrete._--The mixtures varied for the different members. The cost of materials was as follows:

Cement, 110 bbls. @ $1.12 $123.20 -in. stone, 80 cu. yds., @ $1.86 148.80 Gravel, 3 cu. yds., @ $1.33 4.00 Sand, 42 cu. yds., @ $1.20 50.40

The sides were first put in place, then the center columns were built, following which the bottom was placed. Then the forms were erected for the top and the girders, and these cast. In building the sides, one side and half of the two ends were built at one time, and then forms erected for the other half of the sides. For the sides the mixing was done in the bottom of the reservoir. For the rest of the structure it was done on the ground, the mixing board being along side of the reservoir. The labor cost of the concrete work for the various members and the average per cubic yard was as follows:

Columns and Sides. Footings. Bottom. Girders. Top. Average.

Cubic yards 22.6 10.3 19.1 11 20.0 83.

Preparing and cleaning up $0.166 $0.060 $0.095 $0.065 Handling materials 1.022 .306 $0.070 .198 $0.187 .404 Cleaning out forms .040 .070 .053 .032 Mixing and placing 1.542 .728 .353 .792 1.080 .952 Ramming 1.090 .540 .455 .450 .597 .673 Handling steel .890 .020 .395 .083 .324 ------ ------ ------ ------ ------ ------ Total $4.750 $1.654 $0.878 $2.000 $2.000 $2.450

The total cost of labor was $203.35. The mixing was done entirely by hand. Some plastering was done to the walls after the forms were taken off, and the sides and bottom were washed with a brush with cement and water. The plastering cost $6.60, including a barrel of cement and the washing or grouting, two coats, cost $9.10, including a barrel of cement. This added a cost of 19 cts. per cubic yard to the concrete work, making the total cost per cubic yard $2.65.

It was a mistake to have mixed the concrete for the sides in the bottom of the reservoir, as it made two handlings of the materials and compelled all the concrete to be raised by hand to place it in the forms. This accounts for the high cost of these two items.

The handling of the steel was high for the side walls, as it was all separated and put into piles for the different panels and members in getting it out of the pile for the sides. The rammers not only rammed the concrete but they also bent down the p.r.o.ngs of the steel to get them in place in the narrow forms, and afterwards had to pull out these p.r.o.ngs. This had to be done for every piece of steel used, and readily doubled the cost of ramming. The high cost of ramming the top was caused by the fact that the 6 ins. of concrete had to be placed in three layers and each rammed. The steel handling was high on account of the p.r.o.ngs entangling the pieces with others, making them hard to handle. The cost of handling steel per ton was about $4, or 0.2 ct. per pound. The steel was all handled by common laborers.