Concrete Construction Part 21

Piles delivered 12 cts. per ft.

Piles driven $5 each Cofferdam $37 per M.

Wet excavation $1.00 per cu. yd.

Concrete $8.00 per cu. yd.

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:

Per cent. Prorated Days. of time. plant cost.

Cofferdam 7 39 $74 Excavation 2 11 21 Foundation piles 4 22 42 Concrete 5 28 53 -- --- ---- Totals 18 100 $190

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:

Per cu. yd.

Cement, sand and stone $3.20 Foreman (at $5) 0.25 Labor (at $2) 1.00 Engineman (at $3) 0.15 Coal (at $3) 0.06 Carpenters (at $3) 0.24 Forms (at $23.50, used once) 0.80 Wire, nails, etc 0.02 Prorated plant cost 0.53 ----- Total $6.25

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.

[Transcriber's note: Table split to be less than 80 column width]

Abutment No. 1. Pier No. 2.

Wages Cost Cost per No. Total per No. Total per Hour. hrs. Cost. cu. yd. hrs. Cost. cu. yd.

Superintendent 70 24 $16.80 $0.03 47 $32.90 $0.09 Foreman 35 160 56.00 0.09 128 44.80 0.13 Laborers 15-20 2555 383.25 0.65 2038 313.60 0.92 Engineman 30 365 109.50 0.19 196 58.50 0.19 Timekeeper 40 86 34.40 0.06 46 18.40 0.06

Pier No. 3. Pier No. 4.

Wages Cost Cost per No. Total per No. Total per Hour. hrs. Cost. cu. yd. hrs Cost. cu. yd.

Superintendent 70 72 $50.40 $0.05 16 $11.20 $0.03 Foreman 35 324 113.40 0.12 54 18.90 0.06 Laborers 15-20 3513 526.95 0.56 940 141.00 0.44 Engineman 30 244 73.20 0.08 60 18.00 0.06 Timekeeper 40 81 32.40 0.04 10 4.00 0.01

Summary.

Wages Total Total Av.

Hrs. cost. per cu. yd.

Superintendent 70 159 $111.30 $0.05 Foreman 35 666 233.10 0.11 Laborers 15-20 9046 1364.90 0.62 Engineman 30 865 259.50 0.12 Timekeeper 40 223 89.20 0.04

~PNEUMATIC CAISSONS, WILLIAMSBURG BRIDGE.~--Mr. Francis L. Pruyn, a.s.soc.

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 cu. yd. bottom dump buckets were used in placing the concrete, on the North caisson the size of these was increased to 1 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:

Foreman $5.00 a.s.sistant foreman 2.50 Hoisters 2.50 Fireman 1.60 Laborer 1.50

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 2 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. s.p.a.ce was left below all bearing surfaces into which damp mortar was tightly rammed. Concreting the South caisson took 10 working days of 24 hours, the gangs working night and day in twelve 2-hour s.h.i.+fts; 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 14 days. The sand hogs worked in s.h.i.+fts 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 1 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 11 days including Sundays.

The average number of men in the sand hog gangs was 18, with one foreman, who received $5 for 1 hours work.

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

South Caisson (3,827 cu. yds.).

Materials. Quant.i.ty. Rate. Amount.

Cement 4,480 bbls. $1.57 $7,034.00 Sand 1,288 cu. yds. .60 773.00 Broken stone 3,421 cu. yds. 1.50 5,132.00 Water 36.00 ------- ------ --------- Total 3,827 cu. yds. $3.39 $12,975.00

Labor.

Mixing and placing 3,827 cu. yds. $0.90 $3,432.00 Plant charges 2,280.00 Plant labor 742.00 ------- ------ --------- Total plant 3,827 cu. yds. $0.79 $3,022.00 ------- ------ --------- Total cost 3,827 cu. yds. $5.08 $19,429.00 General expenses, 10% 3,827 cu. yds. .51 1,943.00 ------- ------ --------- Grand total 3,827 cu. yds. $5.59 $21,372.00

COST OF CONCRETING SHAFTS.

South Caisson.

Materials. Quant.i.ty. Rate. Amount.

Concrete 612 bbls. $1.57 $962.00 Sand 193 cu. yds. .40 77.00 Stone 493 cu. yds. 1.10 542.00 ------- ------ --------- Total 541 cu. yds. $2.92 $1,581.00 Labor.

Handling, mixing and placing 541 cu. yds. $0.96 $519.00 Plant charges, etc. 541 cu. yds. 1.06 576.00 ------- ------ --------- Total 541 cu. yds. $4.94 $2,676.00 General expenses, 10% 541 cu. yds. .49 267.00 ------- ------ --------- Grand total 541 cu. yds. $5.43 $2,943.00

COST OF CONCRETE IN WORKING CHAMBERS.

South Caisson. (1,435 cu. yds.) Materials. Quant.i.ty. Rate. Amount.

Cement for concrete 1,666 bbls. $1.57 $2,615.00 Cement for mortar 459 bbls. 1.57 720.00 Sand for both 670 cu. yds. .40 268.00 Broken stone 1,181 cu. yds. 1.10 1,299.00 ------- ------ --------- Total materials 1,435 cu. yds. $3.42 $4,902.00 Labor.

Top labor, mixing and placing 1,435 cu. yds. $1.09 $1,575.00 Pneumatic labor 1,435 cu. yds. 4.93 7,117.00 Compressor house labor 1,435 cu. yds. .19 275.00 ------- ------ --------- Total labor 1,435 cu. yds. $6.21 $8,967.00

Plant.

Coal at $2.40 per ton 1,435 cu. yds. .10 140.00 Concrete plant 1,435 cu. yds. .79 1,145.00 Pneumatic plant 1.435 cu. yds. 1.05 1,522.00 -------- ------ --------- Total plant 1,435 cu. yds. $1.94 $2,807.00 Totals 1,435 cu. yds. $11.57 $16,676.00 General expenses, 10% 1,435 cu. yds. 1.16 1,667.00 -------- ------- ---------- Grand total 1,435 cu. yds. $12.73 $18,343.00

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

COST OF CONCRETING CAISSON ABOVE ROOF (5,692 cu. yds.)

Materials. Quant.i.ty. Rate. Amount.

Cement 6,707 bbls. $1.57 $10,531.00 Sand 2,133 cu. yds. .40 845.00 Broken stone 4,938 cu. yds. 1.10 5,432.00 Water 51.00 ------- ------ --------- Total 5,692 cu. yds. $2.96 $16,859.00 Labor.