Concrete Construction: Methods and Costs

The tearing down of the forms was done entirely by laborers at a cost of 61 cts. per cu. yd.

On concrete work it is also advisable to keep the cost of forms per thousand feet board measure, so as to have such data for estimating on new work. The cost per M. ft. on this job was:

The concrete was mixed by hand, water being carried in buckets from the creek. Ten to twelve men were worked in the gang under a foreman, and the concrete was wheeled from the mixing board to the forms in wheelbarrows. The mixture was made wet enough to run. The cost per cubic yard for the girders in detail was as follows:

The cost of labor for the floor was:

This gives a total cost per cubic yard for the concrete in the girders in the completed bridge as follows:

The cost per cubic yard for the floor was:

Included with this is an item for general expense, being expenses of the contractor in bidding on the work, car fare, and other items of expense in looking after the contract.

It will be noticed that a record is here given of three different mixtures and that the labor cost of mixing and placing increases with the richness of the mixture. This is because it takes a greater number of batches to the cubic yard. Record has also been given of cost of preparing the mixing board and other work necessary to start and clean up each day; also when stock piles could not be arranged close to the mixing board, of the cost of handling the materials. These items, it will be noticed, are large enough to be considered in estimating on new work. The cost of sweeping and cleaning out the forms has also been listed, as this work is extremely important.

The cost of the reinforcing steel is given in with the materials, but the labor of handling it and placing it in the forms is listed under labor. This naturally varies with the amount of steel needed, and with the Kahn bar it will vary from 10 cts. to 75 cts. per cubic yard, as the prongs of the bar must be bent into proper position and at times straightened, when bent in shipment. This cost seems large, but it is done with the ordinary labor, while with round rods a large amount of blacksmith work has to be done and a smith and his helper frequently must place them. The patent bars are all lettered and numbered as structural steel is, and can be placed under the direction of the foreman.

One striking lesson can be learned from the forming. It will be noticed that the cost for common labor for handling and helping to erect the forms was much larger in Example I than in Example II, although the bridge was higher in the latter instance. This was caused by the heavy timber that was used, and equaled an extra cost nearly 50 per cent. of the price of new lumber. It certainly speaks volumes against the use of unnecessarily heavy timber for concrete forms.

In bridge work the height of the floor above the stream to some extent governs the cost of the forms. This is made so by the extra lumber needed as props or falsework to support the forming, and also by the fact that men at some height above the ground do not work as quickly or as readily as they do nearer the ground. For high and long spans a derrick is sometimes needed for the work of placing the centering.

On these jobs the concrete was made so wet that with the proper tamping and cutting of the concrete in the forms the surfaces were so smooth that no plastering was needed.

MOLDING SLABS FOR GIRDER BRIDGES.—The bridges carry railway tracks across intersecting streets; the slabs rest on two abutments and three rows of columns so that there are two 24¼-ft. spans over the street roadway and one 10¾-ft. span over each sidewalk. The larger slabs were 24 ft. 3 ins. long, 33 ins. thick and 7 ft. wide; each contained 16¾ cu. yds. of concrete and weighed 36¾ tons. The smaller slabs were 10 ft. 9 ins. long, 17 ins. thick and 7 ft. wide; each contained 3.65 cu. yds. of concrete and weighed 7.8 tons. The weights were found by actual weighing. They make the weight of the reinforced slab between 160 and 162 lbs. per cu. ft. The concrete was generally 1 part cement and 4 parts pit gravel. The reinforcement consisted of corrugated bars. The method of molding was as follows:

A cinder fill yard was leveled off and tamped, then the forms were set up on both sides of two lines of railway track arranged as shown by Fig. 155. The exact construction of the forms for one of the larger slabs is shown by Fig. 156. The side and end pieces were so arranged as to be easily taken down and erected for repeated use. About 100 floors were used and they had to be leveled up each time used as the lifting of the hardened slab disarranged them. The side and end pieces were removed in about a week or ten days, but the slabs stood on the floor 90 days, being wetted each day for two weeks after molding.

The plant for mixing and handling the concrete was mounted on cars. A flat car had a rotary drum mixer mounted on a platform at its forward end. Beneath the mixer was a hopper provided with a deflector which directed the concrete to right or left as desired. Under the hopper were the ends of two inclined chutes extending out sidewise beyond the car—one to the right and one to the left—and over the slab molds on each side. Above the mixer was another platform containing a charging hopper, and from the rear of this platform an incline ran down to the rear end of the car and then down to the track rails. A car loaded with cement and gravel in the proper proportions was hauled up the incline by cable operated by the mixer engine, until it came over the topmost hopper into which it was dumped. This hopper directed the charge into the mixer below; the mixer discharged its batch into the hopper beneath from which it flowed right or left as desired into one of the chutes and thence into the mold. The chutes reached nearly the full length of the molds and discharged as desired over the ends into the far end of the mold or through a trap over the end of the mold nearest the car.

To the rear of the mixer car came a cement car provided with a platform overhanging its forward end. Two hoppers were set in this platform each holding a charge for one batch. Coupled behind the cement cars came three or four gravel cars. These were gondola cars and plank runways were laid along their top outer edges making a continuous runway for wheelbarrows on each side from rear of train to front of cement car. The sand and gravel were wheeled to the two measuring hoppers and the cement was handed up from the car below and added, the charge was then discharged into the dump car below and the car was hauled up the incline to the mixer as already described. Two measuring hoppers were used so that one was being filled while the other was emptied, thus making the work continuous.

The molding gang consisted of 33 laborers, two foremen and one engineman. This gang averaged 7 of the large slabs per 10-hour day and at times made as many as 9 slabs. When molding small slabs an average of 12 were made per day. This record includes all delays, moving train, switching gravel cars on and off, building runways, etc. The distribution of the men was about as follows:

This gang mixed and placed concrete for 7 blocks or 117¼ cu. yds. of concrete per day. Assuming an average wage of $2 per day the cost of labor mixing and placing was 61.4 cts. per cu. yd. or $10.28 per slab. It is stated that the slabs cost $11.80 per cu. yd. on storage pile. This includes labor and materials (concrete and steel); molds; loading into cars with locomotive crane, hauling cars to storage yard and unloading with crane into storage piles, and inspection, incidentals, etc. To load the slabs into cars from storage piles, transport them to the work and place them in position is stated to have cost $2 per cu. yd. The slabs were placed by means of a locomotive crane being swung from the flat cars directly into place.

METHOD AND COST OF CONSTRUCTING CONNECTICUT AVE. BRIDGE, WASHINGTON, D. C.—The Connecticut Ave. Bridge at Washington, D. C., consists of nine 150-ft. spans and two 82-ft. spans, one at each end, all full centered arches of mass concrete trimmed with tool-dressed concrete blocks. Figure 157 is a part sectional plan and elevation of the bridge, showing both the main and spandrel arch construction. This bridge is one of the largest concrete arch bridges in the world, being 1,341 ft. long and 52 ft. wide, and containing 80,000 cu. yds. of concrete. Its total cost was $850,000 or $638.85 per lin. ft., or $10.63 per cu. yd. of masonry. It was built by contract, with Mr. W. J. Douglas as engineer in charge of construction. The account of the methods and cost of construction given here has been prepared from information obtained from Mr. Douglas and by personal visits to the work during construction.

General Arrangement of the Plant.—The quarry from which the crushed stone for concrete was obtained was located in the side of the gorge at a point about 400 ft. from the bridge. Incidentally, it may be added, the fact that the contractor had an option on this quarry gave him an advantage of some $30,000 over the other bidders. The stone from the quarry was hoisted about 50 ft. by derricks and deposited in cars which traveled on an incline to a Gates gyratory crusher, into which they dumped automatically. The stone from the crusher dropped into a 600-cu. yd. bin under the bottom of which was a tunnel large enough for a dump car and provided with top gates by which the stone above could be dropped into the cars. The cars were hauled by cable to the mixer storage bin and there discharged. Sand was brought in by wagons and dumped onto a platform about 50 ft. higher than the bottom of the main stone bin. A tunnel exactly similar to that under the stone bin was carried under the sand storage platform. The sand car was hauled from this tunnel by cable to the mixer storage bin using the same cable as was used for the stone cars, the cable being shifted by hand as was desired. Cement was delivered to the mixer platform from the crest of the bluff by means of a bag chute.

The mixer used was one of the Hains gravity type. It had four drops and was provided with four mixing hoppers at the top. The concrete was made quite wet. The proportions of sand and water were varied to suit the stone according to its wetness and the percentage of dust carried by it. The head mixer regulated the proportions and his work was checked by the government inspector. From the bottom hopper the mixed concrete dropped into a skip mounted on a car.

To distribute the skip cars along the work a trestle was built close alongside the bridge and at about springing line level. This trestle had a down grade of about 2 per cent. from the mixer. Derricks mounted along the centering and on the block molding platform lifted the skips from the cars and deposited them where the concrete was wanted. The skip cars were large enough for three skips but only two were carried so that the derricks could save time by depositing an empty skip in the vacant space and take a loaded skip away with one full swing of the boom. Altogether nine derricks were used in the bridge, four having 70-ft. booms and five having 90-ft. booms. These derricks were jacked up as the work progressed.

Forms and Centers.—The forms for wall and pier work consisted of 1-in. lagging held in place by studs about 2 ft. on centers and they in turn supported by wales which were connected through the walls by bolts, the outer portions of which were removed when the forms were taken down.

The centers for the five 150-ft. arches were all erected at one time; those for the 82-ft. arches were erected separately. The seven centers required 1,500,000 ft. B. M. of lumber or 1,404 ft. B. M. per lineal foot of bridge between abutments, or 1,640 ft. B. M. per lineal foot of arch span. The centers for the main arch spans are shown in detail by Fig. 158; this drawing shows the sizes of all members and the maximum stresses to which they were subjected from the loading indicated, that is the arch ring concrete. The centers as a rule rested on pile foundations. Four piles to each post were used for the intermediate posts and two piles for the posts in the two rows next the piers. Concrete foundations, however, were put in Rock Creek and on the line of Woodley Lane Bridge where it was impracticable to drive piles. As considerable difficulty was experienced in driving the piles, the ground consisting mostly of rotten rock, it is thought that it would have cost less if the contractor had used concrete footings throughout.

Some of the costs of form work and centering are given. The cost of lumber delivered at the bridge site was about as follows:

The following wages were paid: Foreman carpenter, $3.50; carpenters, $2 to $3; laborers, $1.70, with a few at $1.50. An 8-hour day was worked.

The cost, of formwork is given in summary as follows:

The total cost of the main arch span centers to the District of Columbia was $54,000 or $59 per lineal foot of arch span, or $37.33 per M. ft. B. M. The cost of center erection and demolition was as follows:

The salvage on the centers amounted to $11 per M. ft. B. M.

The spandrel arch centers were each used twice and cost per M. ft. B. M. for

Molding Concrete Blocks.—The bridge is trimmed throughout with molded concrete blocks, comprising belt courses, quoin stones, chain stones, ring stones, brackets and dentils. The blocks were made of a 1-2-4½ concrete faced with a 1-3 mixture of Dragon Portland cement and bluestone screenings from ⅜-in. size to dust. They were cast in wooden molds with collapsible sides held together by iron rods. Each mold was provided with six bottoms so that the molded block could be left standing on the bottom to harden while the side pieces were being used for molding another block. The molding was done on a perfectly level and tight floor on mud sills, the perfect level of the molding platform having been found to be an important factor in securing a uniform casting. The blocks were molded with the principal showing face down and the secondary showing faces vertical. The facing mortar was placed first and then the concrete backing. Care was taken to tamp the concrete so as to force the concrete stone into but not through the facing. Mr. Douglas remarks that the back of the block should always be at the top in molding since the laitance or slime always flushes to the surface making a weak skin which will develop hair cracks. In this work the backs of the blocks were mortised by embedding wooden cubes in the wet concrete and removing them when the concrete had set. These mortises bonded the blocks with the mass concrete backing. The blocks were left to harden for at least 30 days and preferably for 60 days and were then bush hammered on the showing faces, some of the work being done by hand and some with pneumatic tools.

Some precautions necessary in the molding and handling of large concrete blocks were discovered in this work and merit mention. In designing blocks for molding it is necessary to avoid thin flanges or the flanges will crack and break off; blocks molded with a 2¼ in. flange projecting 1¾ ins. gave such trouble from cracking on this work that a flange 5 ins. thick was substituted. Provide for the method of handling the block so that dog or lewis holes will not come in the showing faces. Dog holes can be made with a pick when the concrete is three or four weeks old. When it is not practicable to use dogs, two-pin lewises can be used. The lewis holes should be cast in the block and should be of larger size than for granite; they should not be located too near the mortar faces. In turning blocks it is necessary to provide some sort of cushion for them to turn on or broken arrises will result. When the work will permit, it is desirable to round the arrises to about a ⅜-in. radius.

The following general figures of the cost of block work are available. Foreman cutters were paid $5 per day; foreman concrete workers $3 per day; stonecutters $4 per day; concrete laborers $1.70 per day, and common laborers $1.50 to $1.70 per day. Plain and ornamental blocks cost about the same, the large size of the ornamental blocks bringing down the cost. The following is given as the average cost of block work per cubic yard:

It will be seen that the largest single item in the above summary of costs is the item of dressing. This was done, as stated above, partly by hand and partly by pneumatic tools. Hand tooling cost about twice as much as machine tooling, but its appearance was generally better. The average cost of tooling the several forms of blocks is shown by Table XIX. For 42,190 sq. ft. the average cost was 26 cts. per sq. ft. or $2.34 per sq. yd., or $4.73 per cu. yd. of block work. This tooling was done by stone cutters, and was unusually high in cost.

Mass Concrete Work.—All parts of the bridge except the molded block trim were built of concrete deposited in place. Briefly, the molded blocks were set first and then backed up with the mass concrete deposited in forms and on centers. The only features of this work that call for particular description are those in connection with the main arch ring and the spandrel arch construction.

The main arch rings were concreted in transverse sections; Fig. 158 shows the size and order of construction of these sections. Back forms were necessary up to an angle of 45° from the spring line after which the concrete was made somewhat drier and back forms were not used. After Sections 1, 2, 3 and 4 had been concreted they were allowed to set and then the struts and back forms were taken out and the intervening sections were concreted. The large Sections 6 and 7 were concreted in five sections each, in order to permit the taking out of the timber struts supporting the sections above. The concrete in all sections was placed in horizontal layers as a rule and it is the judgment of the engineers in charge of this work that this is the preferable method.

Table XIX.—Showing Cost of Tooling Concrete Ornamental Blocks for Connecticut Avenue Bridge.

Table XX.—Showing Cost of Mass Concrete Work per Cubic Yard.

[Transcriber's note: Table split]

Considerable difficulty was experienced in building the large arches with a concrete block facing on account of the fact that the edges of the blocks are liable to chip off when any concentrated pressure is brought on them. In order to permit the ring of blocks to deform as the centering settled under its load, sheet lead was placed in the joints between blocks at the points corresponding with the construction joints between sections of the mass concrete backing. The deflection of the centers at the crown was a maximum of 3¼ ins. and a minimum of 2½ ins.

Table XXI—Detail Cost of Engineering and Inspection for Different Classes of Work.

The centering of the main arches was not struck until the spandrel arches and all the work above the main arches to the bottom of the coping had been completed. The first and third spandrel arch on each side of the piers was made with an expansion joint in the crown. To permit further of the adjustment of the portion of the masonry above the backs of the main arches, the crown of the middle arch of each set of spandrel arches was left unconcreted until the center of the main arches had been struck. It may be noted here that the expansion joints in the first and third arches were carried up through the dentils and coping, and observations show that these joints are about ⅛ in. larger in winter than in summer.

The cost of the mass concrete work is shown in Table XX. These figures are based on the wages already quoted and the following: Foreman riggers, $4.50; riggers, $1.50 to $1.75 and $2; skilled laborers, $2; engineers, $3.50. The detail cost of engineering and inspection is shown in Table XXI.

ARCH BRIDGES, ELKHART, IND.—At the new Elkhart, Ind., yards of the Lake Shore & Michigan Southern Ry. the tracks are carried over a city street by concrete arches 40, 60 and 160 ft. long. These arches all have a span of 30 ft., a height of 13 ft. and a ring thickness at crown of 28 ins. The reinforcement consists of arch and transverse bars; the arch bars are spaced 6 ins. on centers 2½ ins. from both extrados and intrados, and the transverse bars are spaced 24 ins. on centers inside both lines of arch bars. The proportions of the concrete were generally 1 cement, 3 gravel and 6 stone. The gravel was a material dug from the foundations and was about 50 per cent. sand and 50 per cent. gravel, ranging up to the size of pigeons' eggs. The concrete was machine mixed and was mixed very wet.

The work was done by the railway company's forces, and Mr. Samuel Rockwell, Assistant Chief Engineer, gives the following figures of cost:

ARCH BRIDGE, PLAINWELL, MICH.—The following figures of cost of a reinforced concrete arch bridge are given by Mr. P. A. Courtright. The bridge crosses the Kalamazoo River at Plainwell, Mich., and is 446 ft. long over all with seven arches of 54 ft. span and 8 ft. rise. The arch rings were reinforced with 4-in., 6-lb. channels bent to a radius of 70 ft. and spaced 1.9 ft. c. to c. The contract price of the bridge was $19,900.

The concrete was made of Portland cement and a natural mixture of sand and gravel in the proportions of 1-8 for the foundations, 1-6 for arches and spandrel walls and 1-4 for the parapet wall. The proportions were determined by measure; the wagon boxes being built to hold a cubic yard of sand and gravel. A sack of cement was taken as 1 cu. ft. For foundations the pit mixture was used without screening; stones over 4 ins. in diameter being thrown out at the pit or on the mixing board. For the arches and spandrel walls the gravel was passed over a 2-in. mesh screen on the wagon box. The aggregate for the parapet walls was screened to 1 in. largest diameter. The concrete was mixed in a McKelvey continuous mixer which turned the material eight times. The mode of procedure was as follows: The gravel was loaded upon wagons in the pit and hauled to a platform at the intake of the mixer. Half of the cement required in the concrete was then spread over the top of the load in the wagon box and the whole was dumped through the bottom of the wagon box onto the platform and spread with shovels. The remainder of the cement was spread over the mixture and the whole was shoveled by one man to a second man who shoveled it into the mixer. Water was added after the mixture had passed about one-third of the way through the mixer. The mixer delivered the concrete directly into wheelbarrows, by which it was delivered to the work. The concrete was spread in layers from 2 to 4 ins. in thickness and thoroughly rammed with iron tampers; two men were employed tamping for each man shoveling. The arches were concreted in three longitudinal sections, each section constituting a day's work. The work was done in 1903 and the concrete cost for mixing and placing: