Concrete Construction

Chapter 37

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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

Lumber at $25 per M. ft. $12.50 Erecting at $25 per M. ft. 25.00 Moving at $5 per M. ft. 5.00 Total per M ft. 42.50

_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 1-2-4 concrete faced with a 1-3 mixture of Dragon Portland cement and bluestone screenings from 3/8-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 princ.i.p.al 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 ma.s.s 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 f.l.a.n.g.es or the f.l.a.n.g.es will crack and break off; blocks molded with a 2 in. f.l.a.n.g.e projecting 1 ins. gave such trouble from cracking on this work that a f.l.a.n.g.e 5 ins. thick was subst.i.tuted. 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 3/8-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:

Cement $ 1.95 Sand 0.35 Stone 1.14 Forms, lumber and making 0.80 Mixing and placing concrete 1.50 Dressing 4.73 Handling and setting 2.00 Superintendence, plant, incidentals at 25 per cent. 3.12 Condemnation at 5 per cent. 0.78 ------ Total cost blocks in place $16.37

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.

_Ma.s.s 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 ma.s.s 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.

=============================================================================== | | Per Superficial Foot of | Per Cubic Foot. | Showing Face.

-------------------------+------+-----+------+-----+------+------+------+------ Brackets under Lamps and | | | | | | | | Rail Posts (Cap and Base)| 344| 16.0| 5,500|$0.27| 10.5 | 3,630|$0.41 | 0.66 Moulding under coping | 770| 5.9| 4,560| 0.30| 3.8 | 2,930| 0.47 | 0.64 Dentils between Moulding | 520| 5.5| 2,860| 0.20| 8.0 | 4,160| 0.14 | 1.45 Coping | 494| 61.2|30,220| 0.12| 35.4 |17,490| 0.21 | 0.58 Pedestal (3 courses) | 162| 27.2| 4,400| 0.15| 14.1 | 2,290| 0.29 | 0.52 Rail Posts (Top and Base)| 296| 7.1| 2,100| 0.50| 17.3 | 5,100| 0.21 | 2.43 Lamp Posts and Parapets | | | | | | | | over Piers (Top and Base)| 248| 22.9| 5,690| 0.17| 26.5 | 6,580| 0.15 | 1.16 -------------------------+------+-----+------+-----+------+------+------+------ Average of above--Totals | 2,834| 19.5|55,330|$0.17| 14.8 |43,190|$0.26 | 0.77 -------------------------+------+-----+------+-----+------+------+------+------

TABLE XX.--SHOWING COST OF Ma.s.s CONCRETE WORK PER CUBIC YARD.

[Transcriber"s note: Table split]

=========================================================================== | | | Cost Delivered | | | | | on Mixer. | | Description. | | +--------+------+-------+ | | | Average | | | | | | | Yardage | | | | | | Propor-| for Days| | | | Total | | tions.| Run. | Cement.| Sand.| Stone.| Materials.| -------------------+--------+---------+--------+------+-------+-----------+ Cla.s.s A, in Piers | 1:2:4 | 150 | 1.65 | 0.39 | 1.08 | 3.12 | Cla.s.s A, in Arches | 1:2:4 | 200 | 1.65 | 0.39 | 1.08 | 3.11 | Cla.s.s B, in Piers | | | | | | | --Solid Work | 1:3:6 | 160 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, in Piers | | | | | | | --Hollow Work | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, in | | | | | | | Spandrel Walls | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, in | | | | | | | Spandrel Arches | 1:3:6 | 200 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, | | | | | | | in Abutments | 1:3:6 | 150 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s C, Filling | | | | | | | over Bridge | 1:3:10 | 145 | 0.90 | 0.31 | 1.30 | 2.51 | -------------------+--------+---------+--------+------+-------+-----------+

=============================================== | Cost of | | Mixing and Placing. | Description. +--------+--------+--------+ | | | Total | | | | Mixing | | | | and | | Mixing.| Placing| Placing| -------------------+--------+--------+--------+ Cla.s.s A, in Piers | 0.09 | 0.21 | 0.30 | | | | | Cla.s.s A, in Arches | 0.05 | 0.28 | 0.33 | Cla.s.s B, in Piers | | | | --Solid Work | 0.09 | 0.18 | 0.27 | Cla.s.s B, in Piers | | | | --Hollow Work | 0.11 | 0.36 | 0.47 | Cla.s.s B, in | | | | Spandrel Walls | 0.11 | 0.40 | 0.51 | Cla.s.s B, in | | | | Spandrel Arches | 0.07 | 0.26 | 0.33 | Cla.s.s B, | | | | in Abutments | 0.11 | 0.24 | 0.35 | Cla.s.s C, Filling | | | | over Bridge | 0.11 | 0.28 | 0.39 | -------------------+--------+--------+--------+

========================================================================= | Cost of Form Work. | | Description. +----------+-------+--------+-----------+-------------+ | | Taking| | Total | Total Cost | | Erecting.| Down | Lumber.| Form Work | per Yard.[G]| -------------------+----------+-------+--------+-----------+-------------+ Cla.s.s A, in Piers | 0.17 | 0.05 | 0.16 | 0.38 | $3.80 | | | | | | | Cla.s.s A, in Arches | 0.08 | 0.03 | 0.10 | 0.21 | 3.66 | Cla.s.s B, in Piers | | | | | | --Solid Work | 0.17 | 0.05 | 0.16 | 0.38 | 3.70 | Cla.s.s B, in Piers | | | | | | --Hollow Work | 0.77 | 0.25 | 0.64 | 1.66 | 5.18 | Cla.s.s B, in | | | | | | Spandrel Walls | 0.85 | 0.28 | 0.73 | 1.86 | 5.42 | Cla.s.s B, in | | | | | | Spandrel Arches | 0.94 | 0.30 | 0.86 | 2.10 | 5.48 | Cla.s.s B, | | | | | | in Abutments | 0.10 | 0.03 | 0.12 | 0.25 | 3.65 | Cla.s.s C, Filling | | | | | | over Bridge | 0.00 | 0.00 | 0.00 | .... | 2.90 | -------------------+----------+-------+--------+-----------+-------------+

[Footnote G: Add 25% to the cost here tabulated for superintendence, plant and incidentals.]

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 ma.s.s concrete backing. The deflection of the centers at the crown was a maximum of 3 ins. and a minimum of 2 ins.

TABLE XXI--Detail Cost of Engineering and Inspection for Different Cla.s.ses of Work.

Engineering. Inspection.

Kind of Work. Total. Unit. Total. Unit.

Cla.s.s A, concrete, 23,500 cu. yds $3,055.00 $0.13 $1,762.50 $0.075 Cla.s.s B, concrete, 36,580 cu. yds 3,658.00 0.10 1,646.10 0.045 Cla.s.s C, concrete, 2,150 cu. yds 107.50 0.05 53.75 0.025 Cla.s.s D, concrete, 6,250 cu. yds 1,875.00 0.30 4,687.50 0.75 1,000 M. ft. B. M. centering 1,000.00 1.00 440.00 0.44 Cement, 73,000 barrels 365.00 0.005 730.00 0.01 Earth filling, 50,000 cu. yds 1,000.00 0.02 500.00 0.01

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 1/8 in. larger in winter than in summer.

The cost of the ma.s.s 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 Sh.o.r.e & 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 s.p.a.ced 6 ins. on centers 2 ins. from both extrados and intrados, and the transverse bars are s.p.a.ced 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, a.s.sistant Chief Engineer, gives the following figures of cost:

Total. Per cu. yd.

Temporary buildings, trestles, etc. $ 752.33 $0.15 Machinery, pipe fittings, etc. 416.34 0.08 Sheet piling and boxing 1,006.12 0.21 Excavation and pumping 1,619.74 0.33 Arch centers and boxing 3,528.92 0.73 --------- ----- Total $7,323.45 $1.50

Concrete masonry: Cement 8,860.55 1.84 Stone 1,788.50 0.36 Sand 240.00 0.05 Drain tile 103.03 0.02 Labor 8,091.41 1.68 ---------- ----- Total concrete $19,083.49 $3.95 Steel reinforcing rods $ 3,028.39 $0.63 Engineering, watching, etc. 508.40 0.11 ---------- ----- Grand total (4,833 cu. yds. concrete) $29,943.73 $6.19

~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 s.p.a.ced 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 pa.s.sed 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 pa.s.sed 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 const.i.tuting a day"s work. The work was done in 1903 and the concrete cost for mixing and placing:

Labor: Per day. Per cu. yd.

13 men at $1.80 $23.40 $0.78 Engine and mixer 5.00 0.17 1 team 3.00 0.10 1 foreman 3.00 0.10 ------ ----- Totals for labor $34.40 $1.15

Materials: 0.65 bbl. cement at $2 $1.30 0.9 cu. yd. gravel at $0.50 0.45 ----- Total for materials $1.75 Grand total $2.90

~METHODS AND COST OF CONSTRUCTING A FIVE-SPAN ARCH BRIDGE.~--This bridge consisted of five elliptical arch spans of 40, 45, 60, 87 and 44 ft., carried on concrete piers. The arch rings were 12 ins. thick at the crowns and 18 ins. thick 5 ft. from the centers of piers and carried 4-in. spandrel walls; there were 1,000 cu. yds. of concrete in the arches and 600 cu. yds. in the piers. Each arch ring was reinforced by a grillage of longitudinal and transverse rods.

[Ill.u.s.tration: Fig. 159.--End View of Center for Short Elliptical Arch Spans.]

_Forms and Centers._--Figure 159 is an end view of the center arch. It consists of a series of bents, 6 ft. c. to c., the posts of each bent being 5 ft. c. to c. These posts are made of 26-in. Washington fir.

Upon the heads of the posts rest 26-in. stringers, extending from bent to bent. Resting on these stringers are wooden blocks, or wedges, which support a series of cross-stringers, also of 26-in. stuff, s.p.a.ced 2 ft.

c. to c. On top of these cross-stringers rest the sheeting planks, which are 16-in. stuff, dressed on the upper side, and bent to the curve of the arch. This sheeting plank was not tongue and grooved, and a man standing under it, after it is nailed in place, could see daylight through the cracks. It looked as if it would leak like a sieve, and let much of the wet concrete mortar flow through the cracks, but, as a matter of fact, scarcely any escapes. Figure 160 shows a front view of a bent, and indicates the manner of sway bracing it with 14-in. stuff.

Figure 161 shows the outer forms for the parapet wall, or concrete hand railing, and it will be noted that the cross-stringers are allowed to project about 3 ft. so as to furnish a place to fasten the braces which hold the upright studs. The inner forms for the parapet wall are shown in dotted lines. They are not put in place until all the concrete arch is built. Then they are erected and held to the outer forms by wire, and are sway braced to wooden cleats nailed to the top surface of the concrete arch.

[Ill.u.s.tration: Fig. 160.--Front View of Center for Short Elliptical Arch Spans.]

[Ill.u.s.tration: Fig. 161.--Form for Parapet Wall for Arch Bridge.]

For the five spans the total amount of lumber in the centers was in round figures 28 M. ft., distributed about as follows:

Item. Ft. B. M.

16-in. sheeting 5,600 26-in. longitudinal stringers 2,600 26-in. cross stringers 2,600 26-in. posts 4,000 38-in. sills 1,500 14-in. braces 3,000 Outer forms for spandrel walls 4,000 Inner forms for spandrel walls 4,000 ------ Total 27,300

The aggregate span length of the arches was 276 ft., so that a little less than 100 ft. B. M. of lumber was used for centering per lineal foot of span. The superintendent at $5 per day and five carpenters at $3.50 per day erected the five centers in 18 days at a cost of $400, or a trifle more than $14 per M. ft. B. M.; the cost of taking down the centers was $2 per M. ft. B. M., and the lumber for the centers cost $24 per M. ft. B. M. making a grand total of $40 per M. ft. B. M. for materials and labor. As there were 1,000 cu. yds. of concrete in the arches and spandrels, the cost of centers and forms was $1.12 per cu.

yd. This form lumber was, however, after taking down, used again in erecting a reinforced concrete building. a.s.suming that the lumber was used only twice, the cost of centers and forms for these five arches was less than 80 cts. per cu. yd. of concrete.

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