Concrete Construction: Methods and Costs

Since the lumber was used three times, $30 ÷ 3 = $10 is the charge against each 1,000 ft. B. M. needed to encase the concrete on a floor. There were nearly 34,000 ft. B. M. per floor, hence the cost of lumber ready for erection was $340 per floor. There were as shown below, 200 cu. yds. of concrete per floor, so that the cost was $340 ÷ 200 = $1.70 per cu. yd. of concrete for forms ready for erection. It took a gang of 5 men 7 days to tear down and carry up the forms for one floor; hence 5 × $2 × 7 = $70 per floor, or practically $2 per M. ft. B. M., or $0.35 per cu. yd. of concrete for taking down and carrying forms two stories. It took a gang of 10 carpenters 7 days to erect these forms, which at $3.50 per day was $245 per floor, or $7 per M. ft. B. M., or $1.20 per cu. yd. of concrete.

Concrete.—The amount of concrete per floor was as follows:

A concrete mixer, a hoist and a gang of 14 men mixed and placed the concrete for a floor in 7 days. At $2 per day for labor this gives 14 × 7 × $2 = $196, or say $1 per cu. yd. for mixing and placing the concrete.

Reinforcement.—In each of the 10×10-in. beams there were 4, 1-in. round rods, 2 straight and 2 bent, and stirrups of ⅛×1-in. straps spaced 5 ins. apart at columns and 15 ins. at the center. In each 5×10-in. beam there was half as much steel as in a 10×10-in. beam. The floor slab reinforcement consisted of ¼-in. rods spaced 5 ins. apart and 2 cross-rods in 7-ft. panel. The column reinforcement consisted of 4 rods averaging 1 in. in diameter. In round numbers the amount of steel required for each floor was, therefore, as follows:

This is equivalent to 142 lbs. of steel per cubic yard of concrete, or about 1 per cent of the total volume of reinforced concrete was steel. The steel in the beams was about 3 per cent. It required a gang of 5 laborers 7 days at $2.25 per day, to bend and place the steel for each floor or $86 for labor on 28,400 lbs. of steel. This is equivalent to 0.3 ct. per lb., or 45 cts. per cu. yd. of concrete.

Summary of Costs.—Summarizing the figures given we have the following cost per cubic yard of concrete in floors and columns:

WALL AND ROOF CONSTRUCTION FOR ONE-STORY CAR BARN.—The barn was 50 ft. wide and 190 ft. long, divided into three rooms by two transverse partitions and covered with a 4-in. roof having a pitch of ½ in. per foot. The main walls were 12 ins. thick and the partition walls 10 ins. thick. The main room 110 ft. long had four car tracks its whole length with pits under each and a 6-in. reinforced concrete floor slab between. The floor girders, one under each rail, were 12 ins. square, each reinforced by three 1¾-in. rods, and were carried on 12×12-in. pillars. The total yardage of concrete was 874 cu. yds. divided as follows:

A 1-2½-5 concrete was used for floors, roofs and girders and a 1-3-6 concrete for foundations and walls. There were 26½ tons of reinforcing steel, or 61 lbs. per cu. yd., or 0.45 per cent. of the volume of the concrete was steel. The wages paid were: Foreman, $2.50; blacksmith, $2; engineer, $1.75; laborers, $1.50; two-horse team and driver, $3.67; one-horse team and driver, $2.92; carpenter, $2.25; carpenters worked 9 hours; all others 10 hours.

Forms.—Carpenters framed and erected forms and common laborers under foreman carpenter took them down. Lagging was all 2-in. stuff and uprights 3×4-in. stuff. Props for roof forms were 18-ft. round timber procured on the job. They were 6 ins. in diameter at the top and cost 50 cts. each, 91 being used. These props are not included in the lumber listed below, but their cost is included in the costs given. No record was kept of the number of times the lumber was used, but as 54,643 ft. B. M. were bought and about twice this much would be needed to enclose the concrete if used only once, we will assume that all lumber was used twice. Including the props there were about 60,000 ft. B. M., or 70 ft. B. M. per cu. yd. of concrete. The cost of the lumber was $1,520.86, and the cost of labor on the forms was $1,660.60, so that the cost of forms was:

If the lumber had been used only once the cost per cubic yard would have been $5.38, and per M. ft. B. M., $41.07.

Concrete.—A railway track was run the full length of the building upon what was eventually the fourth track of the car barn and a Ransome mixer was set up as close to the track as possible allowing a platform to be built between it and the track. Cars were brought up to this platform and the materials handled by wheelbarrows direct from cars to mixer. Both platform and mixer were moved twice as the work progressed. The concrete was taken by wheelbarrows on runways to the side walls. For the roof it was hoisted by a horse by means of a mast having an arm with a three-quarters swing; the barrows were hoisted direct using a hook for the wheel and two rings for the handles.

The cost of the concrete for materials was:

The cost of labor per cubic yard was:

Summarizing, we have the following cost per cubic yard:

The cost for handling steel, making stirrups, welding, etc., was $8.90 per ton, or 0.45 ct. per lb.

CONSTRUCTING WALL COLUMNS FOR A ONE-STORY MACHINE SHOP.—The building was 53×600 ft.; each side wall consisted of 40 columns of channel section carried on footings of channel section somewhat heavier than that of the column. The columns were spaced 15 ft. on centers and each was 7½ ft. wide so that there were 7½ ft. spaces between columns, which were filled with 3-in. curtain walls extending 7½ ft. above the floor. Figures 228 and 229 show the column and footing construction. Each column contained 125 cu. ft., or 4.63 cu. yds. of 1-3-5 1-in. crushed slag concrete above the footing and the costs given here relate only to the columns above footings. In the 80 columns there were 370 cu. yds. of concrete.

Forms.—A column form is shown by Fig. 230; it contains approximately 1,000 ft. B. M. of lumber. Ten of these forms were used, so that 10,000 ft. B. M. of form lumber were required for 370 cu. yds. of concrete, or 27 ft. B. M. per cu. yd. of concrete. Each column had a superficial area excluding ends of about 420 sq. ft., so that 420 × 80 = 33,600 sq. ft. was the superficial area of all the columns and 10,000 ft. B. M. ÷ 33,600 sq. ft. = 0.3 ft. B. M., or, say, ⅓ft. B. M., of form lumber was used per square foot of concrete enclosed. The cost of the forms per 1,000 ft. B. M., and, therefore, per form, was:

This gives us a cost per cubic yard of concrete for materials and labor constructing forms of $480 ÷ 370 = $1.30, and per square foot of outside wall area of $480 ÷ (146 × 80) = 4.1 cts.

The erection and taking down of the forms, owing to the weight of some of the pieces, was done by means of special derricks. The footings were brought to within ½ in. of grade and a tenon form of the exact shape of the channel section of the column was placed on top and filled with grout to a depth of 1 in. These tenons served as guides in setting the column forms, and proved to be much quicker and more accurate than points.

The forms were assembled on the ground and erected by a 35-ft. A-frame derrick mounted on wheels. The construction is shown by Fig. 231. This derrick had a capacity of about 4 tons and carried a Ransome friction crab hoist driven by a 5 h.p. Meitz & Weiss kerosene oil engine. It was the practice to set a number of forms before filling any. This enabled the carpenter gang to be plumbing up the first form while the erecting gang were setting others. The forms had to be very securely guyed and braced to withstand the impact of the falling concrete. Very little trouble was had in keeping them well lined up.

Two gangs were employed in assembling forms and a portion of the men in each gang also shaped and placed the reinforcement and placed and tamped the concrete in the forms so that no exact division of labor is possible. The organization of these gangs and the wages paid were as follows:

These gangs assembled and erected the molds and concreted 80 columns in 22 working days, including 2 days lost on account of cold weather, so that 4 columns were completed per day of 9 hours. We can subdivide the cost as follows:

Charging the 4 men placing reinforcement and rethreading bolts to forming and placing reinforcement alone we can figure the cost of fabrication and erection of reinforcement very closely. There were 160 lbs. of reinforcing steel in each column, hence $8.28 ÷ (160 × 4) = 1.3 cts., was the cost per pound of forming and placing it. This includes handling.

The stripping of the forms was carried on by another gang using a derrick similar to the first one described, except it could be of lighter construction as it had to handle only the separate parts of each form and not the forms assembled. The derrick shown in Fig. 232 was a 33-ft. A-frame, with wheels at the bottom of each leg. It had a friction crab hoist driven by an electric motor, both of which were fastened to the derrick frame between the shear legs.

The operation of stripping required only four men and the crabman. The outside flat panel was removed first, and left leaning up against the concrete while the inside trough shaped panel was pried loose and lowered onto the ground with its inside face uppermost. The side panels being comparatively light, were stripped without the use of the derrick, and these panels were assembled on the ground with the inside piece. The derrick then picked up the outside panel again, and placed it in its proper place. After the bolts were put in place, the assembled form was moved on rollers to another point in the line of columns where it was again erected. The arrangement of derricks for erecting and stripping forms is shown in Fig. 233.

The gang stripping forms was made up as follows:

This gang of five men stripped 4 columns containing 18.52 cu. yds. of concrete each day, so that the cost of stripping was $11.70 ÷ 18.52 = 62.7 cts. per cu. yd.

Concrete.—The concrete was mixed in a No. 2 Ransome mixer and delivered to the work in Ransome concrete carts. These carts were pushed along a runway which terminated in a slight incline under the derrick so that their contents could be emptied into the derrick buckets.

The concrete was hoisted in an 8-ft. bottom dump bucket, using the derrick described above. It was necessary to stir up the concrete thoroughly with long-handled slicers as it was being deposited in order to prevent segregation. This expedient combined with a wet mixture and tight molds was found to overcome this difficulty very effectually.

The gang mixing and wheeling concrete was made up as follows:

This gang mixed and wheeled concrete for four columns, or 18.52 cu. yds., hence the cost per cubic yard was 82.6 cts.

With cement at $1.60 per bbl., sand at $1 per cu. yd. and slag at $1.10 per cu. yd. the cost of materials per cubic yard of concrete was $3.

Summarizing the above figures we have the following cost per cubic yard of concrete in place:

CONSTRUCTING ONE-STORY WALLS WITH MOVABLE FORMS AND GALLOWS FRAMES.—In constructing the walls for an 85×30-ft. factory building at Old Bridge, N. J., Mr. A. E. Budell made use of movable forms and gallows frames to construct the curtain walls and columns in one piece. Each side wall was built its full height in successive 50-ft. lengths by depositing the concrete between two forms which were moved upward as the concreting progressed. Fig. 234 indicates the mode of procedure. The form was raised and lowered by means of two gallows frames fitted with blocks and tackle. A steel cable, with a trolley affixed, extending from one frame to the other, provided a convenient mode of hoisting material to the form, and the gallows frames took the place of ladders for climbing onto the structure. No scaffolding whatever was used and only one man was required overhead to dump the buckets and tamp the concrete into place.

The two walls were carried up simultaneously, one form being shifted into place and filled while the other was left in place until the concrete was sufficiently hard. It was found that 18 hours was amply sufficient to allow the concrete to set hard, after which the form was removed and lifted to a higher level. Thus the men were continuously engaged in lifting and filling first one form and then the other. The average length of time required to remove, raise and fill one form was 5 to 6 hours. Thus, two forms could be raised and filled almost every day. The construction of the forms and of the gallows frames is shown by Figs. 234 and 235. The cost of one set of forms and gallows frames was as follows:

This sum covered the cost of forms for one side of the building 85 ft. long and containing 150 cu. yds. of concrete, hence the cost of forms was in round figures $1.10 per cu. yd. of concrete. Each cubic yard of concrete required 1,997 ÷ 150 = 13⅓ ft. B. M. of form lumber.

The concrete was a 1-2½-4½ mixture. A careful record for 15 days, showed an average of 2.8 cu. yds. of concrete placed in 6 hours by a gang of 6.3 men. From this we can figure the cost of concrete in place to be about as follows:

Thus the cost of concrete in place was $16.07 ÷ 2.8 = $5.73 per cu. yd. Adding the cost of forms we get $5.73 + $1.10 = $6.83 per cu. yd. as the cost for labor and materials in constructing forms and mixing and placing concrete.

Offsets and molding decorations were easily made, although they were quite numerous on the building in question, at least more so than would ordinarily be the case in mill building construction. The offset of 1 ft. at every column was made very readily by sliding wooden shoulder pieces into place on the inner face of the form, which pieces in turn received 2-in. faced planking, the latter being slid into place from above. Thus the entire system was collapsible and small alterations were easily made whenever the form was shifted. Flat surfaces or offsets could be obtained at will by either removing or setting in the shoulder pieces. Molding effects were made on the front face of the wall by tacking molding strips to the form wherever necessary. The entire work was done with common labor and the finished building presented a smooth, homogeneous surface which required very little dressing.

FLOOR AND ROOF CONSTRUCTION FOR FOUR-STORY GARAGE.—The building was 53×200 ft., and 4 stories high, with provision for 2 additional stories in the design of footings and columns. Two rows of wall columns connected by transverse girders carrying the floor and roof slabs made a comparatively simple construction, except for a mezzanine floor carried on cantilever beams and except for the use of cantilever footings; these two special details are shown by Figs. 236 and 237. The amount of concrete in the building was 1,910 cu. yds., distributed as follows:

The amount of reinforcing metal used was 237 tons, distributed as follows:

This is equivalent to 2 per cent. of steel in 1,910 - 170 = 1,740 cu. yds.

Forms.—The total area of concrete covered by forms (1,740 cu. yds.) was 94,000 sq. ft., distributed as follows:

For the work 50,000 ft. B. M. of old lumber was used and 170,000 ft. B. M. of new lumber was bought, the cost being as follows:

This is equivalent to 126 ft. B. M. per cu. yd. of concrete. New forms were made for each floor except the sides of the girder molds which were re-used so far as they would fit, but the roof forms were made from lumber used for the floors. In all no more than 20 per cent of the form lumber was used a second time. In round figures new lumber was required for 80,000 sq. ft. of concrete; this gives a cost for lumber of 6.4 cts. per sq. ft. The construction of the column and floor forms is shown by Fig. 238. A force of 15 carpenters at $4.40 per day under a foreman at $35 per week erected and tore down forms; the carrying was done by laborers at $1.70 per day working under a foreman at $35 per week; carpenters worked an 8-hour and laborers a 10-hour day. Forms for one floor were framed and erected in 8 to 10 days. The cost of forms for 1,740 cu. yds. and 80,000 sq. ft. of concrete and per M. ft. B. M. was as follows:

The lumber had a considerable salvage value which is not allowed for in the above figures.

Concrete.—The concrete was a Portland cement, ¾-in. trap rock mixture, mixed wet in two Chicago Improved Cube Mixers equipped with charging buckets. The mixers were located on the ground floor, one at the rear and one at the front of the building, both discharging directly to a hoist. With a gang of 30 men at $1.70 per 10-hour day under a foreman at $35 per week a floor was concreted in 2 days, the columns being concreted the first day and the floor being concreted the second day. The labor cost for mixing and placing concrete and for fabricating and setting reinforcement was as follows:

The cost of concreting includes the cost of granolithic surface for the floor slabs. The girder reinforcement was made up into unit frames and the frames were set as a unit, horses set over the molds being used to suspend and lower them into place. The cost of $2.05 per cu. yd. is equivalent to ¾ ct. per lb. Summarizing, we have the following cost for materials and labor on forms and for labor mixing and placing concrete and reinforcement:

This $10 total does not include the cost of the concrete nor of the steel.

CHAPTER XX.

METHOD AND COST OF BUILDING CONSTRUCTION OF SEPARATELY MOLDED MEMBERS.

This chapter deals exclusively with the methods and cost of molding and erecting separately molded wall blocks, girders, columns and slabs. The structural advantages and disadvantages of this type of construction as compared with monolithic construction will not be considered. The data given in succeeding paragraphs show how separate piece work has been done and what it has actually cost to do it in a number of instances.

COLUMN, GIRDER AND SLAB CONSTRUCTION.—European engineers have developed several styles of open web or hollow girder and column shapes, but in America solid columns and girders have been used except in the comparatively few cases where one of the European constructions has been introduced by its American agents.

Warehouses, Brooklyn, N. Y.—In constructing a series of warehouses in Brooklyn, N. Y., the columns and girders were molded in forms on the ground. For molding the columns, forms consisting of two side pieces and one bottom piece, were used, saving 25 per cent. in the amount of lumber required for a column form, and doing away with yokes and bolts, since only simple braces were required to hold the side forms in place. It was found that the side forms could readily be removed in 24 to 48 hours, thus considerably reducing the time that a considerable portion of the form lumber was tied up. It was figured by Mr. E. P. Goodrich, the engineer in charge of this work, that this possible re-use of form lumber reduced the amount required another 50 per cent. as compared with molding in place. Girders were molded like columns in three-sided forms; the saving in form work was somewhat less than in the case of columns, but it was material. In general, Mr. Goodrich states, the cost of hoisting and placing molded concrete members is higher per yard than when the concrete is placed wet. That is in mass before it is hardened.

Factory, Reading, Pa.—In constructing a factory at Reading, Pa., an open or lattice web type of girder invented by Mr. Franz Visintini and extensively used in Austria was adopted; columns were molded in place in the usual manner with bracket tops to form girder seats. The girders were reinforced with three trusses made up of top and bottom chord rods connected by diagonal web rods; one truss was located at the center of the beam and one at each side. The method of molding was as follows: The trusses were made by cutting the chord rods to length and threading the web diagonals and verticals onto them. To permit threading the web pieces were bent, when rods were used, with an eye at each end; when straps were used the ends were punched with holes. The work was very simple and was done mostly by boys in the machine shop of the company for which the building was being erected. The girders were molded two at a time in forms constructed as shown by the sketch. Fig. 239. A form consisted of a center board, two side boards, two end pieces and the proper number of cast iron cores, all clamped together by three yokes. Triangular cast iron plates, A, were screwed to the bottom boards for spacers. The side, center and end boards were then set up and the end clamps were placed. The cast iron hollow cores, B, were then set over the spacers, and the form was ready for pouring. A layer of concrete was placed in the bottom of the mold and the first side truss was placed; the concrete was then brought half way up and the middle truss was placed; concreting was then continued up to the plane of the second side truss which was placed and covered. Cores and forms were all cleaned and greased each time they were used. The cores were removed first by means of a lever device and generally within three or four hours after the concrete was placed. The remainder of the form was taken down in two to four days and the beam removed.

Kilnhouse, New Village, N. J.—In constructing a kiln house for a cement works one story columns with bracket tops and 50-ft. span roof girders were molded on the ground and erected as single pieces. The columns by rough calculation averaged about 2 cu. yds. of concrete and 675 lbs. of reinforcement each or about 337 lbs. of steel per cubic yard. The girders averaged by similar calculation 5 cu. yds. of concrete and 2,260 lbs. of steel, or 452 lbs. per cubic yard of concrete. The average weight of columns was thus not far from 41.3 tons and of girders fully 11 tons.

Several combinations of arrangements were used for molding the columns and girders. For wall columns having one bracket the arrangement shown by Fig. 240 was adopted. The concrete slab molding platform was covered with paper, and on this the two outside and the middle columns were cast in forms. When those columns had set the forms were removed, the intervening spaces were papered and the two remaining columns were cast. Ten columns, five sets of two columns in line, were cast on each base. The remaining columns were cast in combination with girders as shown by Fig. 241. The two outside lines of columns (1) were molded in forms, allowed to stand until set and then stripped. Using a column surmounted by a shallow side form for one side and a full depth side form for the other side molds were fashioned for the two outside girders, Nos. 2 and 3. One full depth side form and the side of girder No. 2 formed the mold for girder No. 4. Girder No. 5 was then molded between girders No. 3 and No. 4.