Concrete Construction: Methods and Costs

It will be noted that the sand cost nothing as it was dredged from the trench in which the pier was built, and paid for as dredging. The cost of the plant is distributed over this south pier and over the proposed north pier work on the basis of only 20 per cent. salvage value after the completion of both piers. It is said, however, that 80 per cent. is too high an allowance for the probable depreciation.

DAM, RICHMOND, INDIANA.—The dam shown in cross-section in Fig. 89 was built at Richmond, Ind. It was 120 ft. long and was built between the abutments of a dismantled bridge. The concrete was made in the proportion of 1 bbl. Portland cement to 1 cu. yd. of gravel; old iron was used for reinforcement. The foundations were put down by means of a cofferdam which was kept dry by pumping. On completion it was found that there was a tendency to scour in front of the apron and accordingly piling was driven and the intervening space rip-rapped with large stone. Labor was paid as follows per day: Foreman, $3; carpenter, $2.50; cement finisher, $2; laborers, $1.50. The concrete was mixed by hand and wheeled to place in wheelbarrows. The cost of the work was as follows:

DAM AT ROCK ISLAND ARSENAL, ILLINOIS.—The dam was in the shape of an L with one side 192 ft. and the other side 208 ft. long; it consists of a wall 30½ ft. high, 3½ ft. wide at the top and 6½ ft. wide at the bottom with a counterfort every 16 ft., 26 in all. Each counterfort extended back 16 ft. and was 4 ft. thick for a height of 6 ft. and then 3 ft. thick. There were 3,500 cu. yds. of concrete in the work, which was done by day labor under the direction of the U. S. Engineer in charge.

The forms consisted of front and back uprights of 8×10-in. stuff 24 ft. high, connected through the wall by ¾-in. rods which were left in the concrete. The lagging was 2×12-in. plank dressed down 1¾ ins. placed inside the uprights. These forms were built full height in 16-ft. sections with a counterfort coming at the center of each section. Each section contained 95 cu. yds. of concrete and was filled in a day's work. The concrete was a 1-4-7 mixture wet enough to quake when rammed. Run of crusher limestone was used of which 50 per cent. passed a 1-in. sieve, 17 per cent. a No. 3 sieve and 9 per cent. a No. 8 sieve. The concrete was mixed in Cockburn Barrow & Machine Co.'s screw-feed mixer which discharged into 2-in. plank skips 2 ft. wide 5⅓ ft. long and 14 ins. deep, holding ¼ cu. yd. These skips were taken on cars to a derrick crane overhanging the forms and by it hoisted and dumped into the forms. The derrick was moved along a track at the foot of the wall as the work progressed. The concrete was spread and rammed in 6-in. layers. The men were paid $1.50 per 8-hour's work and the work cost including footing, as follows:

DAM AT McCALL FERRY, PA.—The dam was 2,700 ft. long and 48 ft. high of the cross-section shown by Fig. 90 and with its subsidiary works required some 350,000 cu. yds. of concrete. The plant for mixing and placing the concrete was notable chiefly for its size and cost. Parallel to the dam, which extended straight across the river, and just below its toe a service bridge consisting of a series of 40-ft. concrete arch spans was built across the river. This service bridge was 50 ft. wide and carried four standard gage railway tracks besides a traveling crane track of 44 ft. gage. This very heavy construction of a temporary structure was necessitated by the frequency of floods against which only a solid bridge could stand; it was considered cheaper in the long run to provide a bridge which would certainly last through the work than to chance a structure of less cost which would certainly go out with the floods. The concrete service bridge was designed to be destroyed by blasting when the dam had been completed. The method of construction was to build the dam in alternate 40 ft. sections, mixing the concrete on shore, taking it out along the service bridge in buckets on cars and handling the buckets from cars to forms by traveling cranes.

The concrete mixing plant is shown by Fig. 91. Cars loaded with cement, sand and stone were brought in over the tracks carried on the wall tops of the bins and were unloaded respectively into bins A, B and C, of which there were eight sets. Each set supplied material by means of measuring cars to a 1 cu. yd. Smith mixer. Two sets of cars were used for each mixer so that one could be loading while the other was charging. The mixers discharged into 1 cu. yd. buckets set two on a car and eight cars were hauled to the work in train by an 18-ton "dinky." At the work the buckets were picked up by the traveling cranes and the concrete dumped into the forms. Figure 90 shows the construction of the steel forms. Six sets of forms were used. Each set consisted of five frames spaced 10 ft. apart and braced together in the planes parallel to the dam, and each set molded 40 ft. of dam. The lagging consisted of wooden boxes 8½ ft. wide and 10 ft. long. For the vertical face of the dam these boxes were attached by bolts to the vertical post, for the curved face they were bolted to a channel bent to the curve and held by struts from the inclined post of the steel frame.

In construction the footing and the body of the dam to an elevation of 5 ft. above the beginning of the curve were built continuously across the river; above this elevation the dam was built in alternate 40-ft. sections. The strut back to the service bridge shown in the lower right hand corner of Fig. 90, shows the manner of bracing the first 30-ft. section of the inclined post to hold the lagging for the continuous portion. The lagging was added a piece at a time as concreting progressed. The ends of each set of frames for a 40-ft. section were for the isolated sections closed by timber bulkheads carrying box forms to mold grooves into which the concrete of the intermediate sections would bond.

The concrete used was a 1-3-5 mixture, the stone ranging in size from 2 to 5 ins. Rubble stone from one man size to ½ ton were bedded in the concrete. The capacity of the concrete plant was 2,000 cu. yds. per day or about 250 cu. yds. per mixer per 10-hour day.

DAM, CHAUDIERE FALLS, QUEBEC.—The dam was 800 ft. long and from 16 to 20 ft. high, constructed of 1-2-4 concrete with rubble stone embedded. The rubble stones were separated at least 9 ins. horizontally and 12 ins. vertically and were kept 20 ins. from faces. At one point the rubble amounted to 40 per cent. of the volume, but the average for the dam was 25 to 30 per cent. The stone was broken at the work, some by hand, but most by machine, all to pass a 2-in. ring. Hand-broken stone ran very uniform in size and high in voids, often up to 50 per cent. Stone broken by crusher with jaws 2 ins. apart would run 20 to 30 per cent. over 2 ins. in size and give about 45 per cent. voids; with crusher jaws 1½ ins. apart from 98 to 100 per cent. was under 2 ins. in size and contained about 42 per cent. of voids. It was found that if the crushers were kept full all the time the product was much smaller, particularly with Gates gyratory crusher, though a little more than rated power was required when the crusher was thus kept full. This practice secured increased economy in both quantity and quality of product. The concrete was made and placed by means of a movable traveler shown by Fig. 92. Concrete materials were supplied to the charging platform of the traveler by means of a traveling derrick moving on a parallel track. In placing the concrete on the rock bottom it was found necessary in order to secure good bond to scrub the rock with water and brooms and cover it with a bed of 2 ins. of 1-2 mortar. The method of concreting in freezing weather is described in Chapter VII.

CHAPTER XII.

METHODS AND COST OF CONSTRUCTING BRIDGE PIERS AND ABUTMENTS.

The construction of piers and abutments for bridges is best explained by describing individual examples of such work. So far, in America, bridge piers have been nearly always of plain concrete and of form and section differing little from masonry piers; where reinforcement has been used at all it has consisted of a surface network of bars introduced chiefly to ensure monolithic action of the pier under lateral stresses. In Europe cellular piers of reinforced concrete have been much used. Plain concrete abutments differ little in form and volume from masonry abutments. Reinforced concrete abutments are usually of L-section with counterforts bracing the upright slab and bridge seat to the base slab.

Form work for reinforced abutments is somewhat complex; that for plain abutments and piers is of simple character, the only variations from plain stud and sheathing construction being in the forms for moldings and coping and for cut-waters. For piers of moderate height the form is commonly framed complete for the whole pier, but for high piers it is built up as the work progresses by removing the bottom boards and placing them at the top. Opposite forms are held together by wire ties through the concrete. Movable panel forms have been successfully employed, but they rarely cheapen the cost much. Sectional forms, which can be shifted from pier to pier where a number of piers of identical size are to be built, may frequently be used to advantage. An example of such use is given in this chapter.

Derricks are the recognized appliances for hoisting and placing the concrete in pier work; they are the only practicable appliance where the pier is high and particularly where it stands in water and mixing barges are employed. For abutment work and land piers of moderate height derricks and wheelbarrow or cart inclines are both available and where much shifting of the derricks is involved the apparently more crude method compares favorably in cost.

The methods of placing concrete under water for pier foundations are described in Chapter V, and the use of rubble concrete for pier construction is illustrated by several examples in Chapter VI. The following examples of pier and abutment construction cover both large and small work and give a clear idea of current practice.

COST OF CONSTRUCTING RECTANGULAR PIER FOR A RAILWAY BRIDGE.—This pier, Fig. 93, was built in water averaging 5 ft. deep. The cofferdam consisted of triple-lap sheet piling, of the Wakefield pattern, the planks being 2 ins. thick, and spiked together so as to give a cofferdam wall 6 ins thick. The cofferdam enclosed an area 14×20 ft., giving a clearance of 1 ft. all around the base of the concrete pier, and a clearance of 2 ft. between the cofferdam and the outer edge of the nearest pile. The cofferdam sheet piles were 18 ft. long, driven 11 ft. deep into sand, and projecting 2 ft. above the surface of the water.

The concrete base resting on the foundation piles was 12×18 ft. The concrete pier resting on this base was 7×13 ft. at the bottom, and 5×11 ft. at the top. The pier supported deck plate girders. There were 100 cu. yds. of concrete in the pier and base.

The cost of this pier, which is typical of a large class of concrete pier work, has been obtained in such detail that we analyze it in detail, giving the costs of cofferdam construction and excavation as well as of mixing and placing the concrete.

Regarding the item of plant rental, it should be said that the plant consisted of a pile driver, a derrick, a hoisting engine, and sundry timbers for platforms. There was no concrete mixer. Hence an allowance $5 per day for use of plant is sufficient.

It will be noted that no salvage has been allowed on the lumber for forms. As a matter of fact, all this lumber was recovered, and was used again in similar work.

Referring to the cost of cofferdam work, we see that, in order to excavate the 58 cu. yds. inside the cofferdam, it was necessary to spend $284, or nearly $5 per cu. yd. before the actual excavation was begun. The work of excavating cost only 57 cts. per cu. yd., but this does not include the cost of erecting the derrick which was used in raising the loaded buckets of earth, as well as in subsequently placing the concrete. The sheet piles were not pulled, in this instance, but a contractor who understands the art of pile pulling would certainly not leave the piles in the ground. A hand pump served to keep the cofferdam dry enough for excavating; but in more open material a power pump is usually required.

The above costs are the actual costs, and do not include the contractor's profits. His bid on the work was as follows:

In order to ascertain whether or not these prices yielded a fair profit, it is necessary to distribute the cost of the plant transportation and rental over the various items. We have allowed $120 for plant transportation and rental, and $70 for setting up and taking down the plant, or $190 in all. The working time of the plant was as follows:

As above given, the labor on the 7,900 ft. B. M. in the cofferdam cost $126, or $16 per M.; but this additional $74 of prorated plant costs, adds another $9 per M., bringing the total labor and plant to $25 per M., to which must be added the $20 per M. paid for the timber in the cofferdam, making a grand total of $45 per M. This shows that the contractor's bid of $37 per M. was much too low.

The labor on the excavation cost 57 cts. per cu. yd., to which must be added the prorated plant cost of $21 distributed over the 58 cu. yds., or 36 cts. per cu. yd., making a total of 93 cts. per cu. yd. This shows that the bid of $1 per cu. yd. was hardly high enough.

The labor on the 24 foundation piles cost $80, or $3.33 each. The prorated plant cost is $42, or $1.75 per pile, which, added to $3.33, makes a total of $5.08. This shows that the bid of $5 Per pile for driving was too low. However there was a profit of 2 cts. per ft., or 80 cts. per pile, on the cost of piles delivered.

The concrete amounted to 100 cu. yds. Hence the prorated plant cost of $53 is equivalent to 53 cts. per cu. yd. Hence the total cost of the concrete was:

Since the contract price for concrete was $8 per cu. yd., there was a good profit in this item.

BACKING FOR BRIDGE PIERS AND ABUTMENTS.—Six piers and two abutments of the City Island bridge were constructed in 1906 at New York city, of masonry backed with 1-2-4 concrete below and 1-3-5 concrete above high water. The piers and abutments were all sunk to rock or hard material by means of timber cofferdams. Table XVI gives the labor cost of mixing and placing the concrete backing for one abutment and three piers, after the materials were delivered on the scows. The concrete was mixed by a rectangular horizontal machine mixer and deposited by 2-cu. yd. bottom dump buckets handled by derrick scows and stiff leg derricks. The high cost of concreting on Pier 2 was due to the fact that the concrete was improperly deposited and had to be removed and the higher cost in Abutment 1 was probably due to the fact that the abutment was so long and narrow that it was difficult to handle the bucket.

Table XVI.—Cost of Concrete Backing for Masonry Piers.

PNEUMATIC CAISSONS, WILLIAMSBURG BRIDGE.—Mr. Francis L. Pruyn, Assoc. M. Am. Soc. C. E., gives the following costs of concreting the pneumatic caissons for the Brooklyn tower of the Williamsburg bridge at New York city. The work comprised the mixing and placing of some 13,637 cu. yds. of concrete in two caissons. Table XVII shows the itemized costs for one caisson and Table XVIII shows them for the other caisson. The methods of work were as follows:

After each caisson was built it was towed to its proper site, where it was held in place by temporary pile dock built completely around it. On these docks the concrete was placed; a 2 cu. yd. cubical mixer of the usual pattern being used for mixing. The concrete materials, consisting of sand, stone and cement was handled direct from barges alongside, into the mixer. The concrete was placed by a derrick located in the center of the caisson, which was a bad feature as the caisson was usually out of level and considerable difficulty was experienced in swinging the derrick. On the South caisson ¾ cu. yd. bottom dump buckets were used in placing the concrete, on the North caisson the size of these was increased to 1½ cu. yd. which reduced the cost of placing 15 cts. per cu. yd. There were placed in the South caisson 3,827 cu. yds. in 32 days of actual working time—120 cu. yds. per day of 10 hrs. The gross time was 2 months. On the North caisson 5,693 cu. yds. were placed in 46 days worked—124 cu. yds. per day. The gross time was 4 months.

The rates of labor were as follows per 10-hour day:

Proportions concrete were 1: 2.5: 6.

The low price of sand in the North caisson was brought about by the finding of good building sand in the excavation for the anchorage, which work was done by the same contractor.

When the caissons had been sealed the iron material shafts were removed. This left holes 5 ft.×6 ft. extending from the roof of the caisson up to Mean H.W. which were filled with concrete. These shaft holes were 80 ft. deep on the South caisson and 100 ft. deep on the North caisson. They were partially filled with water and the concrete had to be placed with considerable care. Wooden chutes were used on the South caisson; they rested on the caisson roof, were filled with concrete and then raised allowing concrete to flow out at the bottom. The shaft holes were too deep on the North caisson for chutes and 20 cu. ft. bottom dump buckets were used. They had to be lowered to bottom of shaft each trip before dumping, a slow operation, which greatly added to the cost. Proportion for concrete 1-2.5-6.

The proportion for concrete in working chamber was the same as for all other concrete. The specifications called for 6 in. of mortar, of 1 part of cement to 2½ parts of sand, between the concrete and all bearing areas; that is, under the cutting edge and directly under the roof of the working chamber. The concrete was mixed in the cubical mixer and dumped on the bottom door of the material lock, the top door of the lock was then closed, the bottom door opened and the concrete fell through the shaft to the working chamber. It was then shoveled by the sand hogs into place. A 6-in. space was left below all bearing surfaces into which damp mortar was tightly rammed. Concreting the South caisson took 10¼ working days of 24 hours, the gangs working night and day in twelve 2-hour shifts; 1,566 cu. yds. of concrete and mortar were placed, or at the rate of 140 cu. yds. per 24 hours. The gross time including Sundays was 14½ days. The sand hogs worked in shifts of 2 hours each and received $3.50 for the two hours work. The twelve foremen received 1 dollar more: the average gang consisted of 12 sand hogs.

On the North caisson the organization was much better, owing to the experience gained on the first caisson; and in spite of the fact that the sand hogs, on account of the increased depth, received $4.00 for 1½ hours' work, or an increase of $22.00 per man per 24 hrs. over that on the South caisson, the work was done for less money. There were placed 1,566 cu. yds. of concrete in 7 working days of 24 hrs., or at the rate of 224 cu. yds. per day. The gross time was 11½ days including Sundays. The average number of men in the sand hog gangs was 18, with one foreman, who received $5 for 1½ hours work.

TABLE XVII.—ITEMIZED COST OF CONCRETING SOUTH CAISSON FOR BROOKLYN TOWER OF THE WILLIAMSBURG BRIDGE: COST OF CONCRETING CAISSONS ABOVE ROOF.

TABLE XVIII.—ITEMIZED COST OF CONCRETING NORTH CAISSON FOR BROOKLYN TOWER OF THE WILLIAMSBURG BRIDGE:

COST OF FILLING PIER CYLINDERS.—The following costs were obtained in mixing and placing concrete in steel cylinder piers. The sand and gravel were wheeled 100 ft. to the mixing board at the foot of the cylinder, mixed and shoveled into wooden skips, hoisted 20 ft. by horsepower and dumped into the cylinder. The foreman worked on the mixing board and the men worked with great energy. The costs were as follows: