METHODS AND COST OF HEAVY CONCRETE WORK IN FORTIFICATIONS, LOCKS, DAMS, BREAKWATERS AND PIERS.
The construction problem in building concrete structures of ma.s.sive form and volume is chiefly a problem of plant arrangement and organization of plant operations. In most such work form construction is simple and of such character that it offers no delay to placing the concrete as rapidly as it can be produced. The same is true of the character of the structure, it is seldom necessary for one part of the work to wait on the setting and hardening of another part. As a rule, there is no reinforcement to fabricate and place and where there is it is of such simple character as not to influence the main task of mixing, handling, and placing concrete. Stated broadly, the contractor in such work generally has a certain large amount of concrete to manufacture, transport and deposit in a certain s.p.a.ce with nothing to limit the rapidity of these operations, except the limitations of plant capacity and management. Installation and operation of mixing and conveying plant, then are matters to be considered carefully in heavy concrete work.
In the following sections we have given one or more examples of nearly every kind of heavy concrete work excepting bridge foundations and retaining walls, which are considered in Chapters XII and XIII, and except rubble concrete work, which is considered in Chapter VI. In each case so far as the available records made it possible, we have given an account of the plant used and of its operation.
~FORTIFICATION WORK.~--Concrete for fortification work consists very largely of heavy platforms and walls for gun foundations and enclosures and of heavily roofed galleries and chambers for machinery and ammunition. The work is very ma.s.sive and in the majority of cases of simple form. A large number of data are to be found in the reports of the Chief of Engineers, U. S. A., on all cla.s.ses of fortification work, but the manner in which they are recorded makes close a.n.a.lysis of relative efficiencies of methods or of relative costs almost impossible.
The following data are given, therefore, as examples that may be considered fairly representative of the costs obtained in fortification work done under the direction of army engineers; these data are not susceptible of close a.n.a.lysis because wages, working force, outputs, etc., are nearly always lacking.
~Gun Emplacements, Staten Island, N. Y.~--The work comprised 5,609 cu.
yds. of concrete in two 12-in. gun emplacements, and 3,778 cu. yds. of concrete in two 6-in. gun emplacements. Concrete was mixed in a revolving cube mixer with the exception of 809 cu. yds. in the 6-in.
emplacements which were mixed by hand at a cost of 56 cts. more per cubic yard than machine mixing cost. The body of the concrete was a 1-3-5 Portland cement, beach sand and broken trap rock mixture. The floors and upper surface of the concrete had a pavement consisting of 6 ins. of 1-3-5 concrete surfaced with 2 ins. of 1-3 mortar. Wages are not given, but for the time and place should have been about $1.50 per 8-hour day for common labor. The cost of materials was:
Alpha Portland cement, per bbl. $1.98 Broken trap rock, per cu. yd. 0.81 12-in. emplacement, hauling sand per cu. yd. 0.175 6-in. emplacement, hauling sand per cu. yd. 0.20
The cost of the concrete in place was as follows:
12-in., per 6-in., per Body Concrete-- cu. yd. cu. yd.
Cement, at $1.98 per bbl. $2.546 $2.546 Broken stone, at 81 cts. per cu. yd. 1.041 1.041 Sand, at 17 and 20 cts. per cu. yd. 0.225 0.257 Receiving and storing materials at 11.6 cts. per cu. yd. and 8.4 cts. per bbl. 0.149 0.180 Mixing, placing and ramming 0.879 1.110 Forms, lumber and labor 0.477 0.950 Superintendence and miscellaneous 0.190 0.150 ------ ------ Total $5.507 $6.234 Concrete Pavement-- Materials $2.97 $3.06 Labor 4.63 4.72 ------ ------ Total $7.60 $7.78
[Ill.u.s.tration: Fig. 68.--Sketch Plans of Concrete Making Plant for Mortar Battery Platform.]
~Mortar Battery Platform, Tampa Bay, Fla.~--The platform contained 8,994 cu. yds. of concrete composed of a mixture of Portland cement, sand, sh.e.l.ls and broken stone. The broken stone and cement were brought in by vessel and the sand and sh.e.l.ls were obtained from the beach near by. The plant for the work was arranged as shown by the sketch, Fig. 68. Sand, stone and sh.e.l.ls were stored in separate compartments in the storage bins. Box cars, divided into compartments of such size that when each was filled with its proper material, the car would contain the proper proportions for one batch of concrete, were pushed by hand under the several compartments of the bin in succession until charged; then they were hooked to a cable and hauled to the platform over the mixer and dumped. The charge was then turned over with shovels and shoveled into the hopper of a continuous mixer, located beneath. Two cars were used for charging the mixer, running on separate tracks as shown. The mixer discharged into buckets set on flat cars, which were hauled by mules under the cableway, which then lifted and dumped the bucket and returned it empty to the car. By using three bucket cars, one was always ready to receive the mixer discharge as soon as the preceding one had been filled, so that the mixer operated continuously. The cableway had a working span of 270 ft., the cable being carried by traveling towers 69 ft. high; the cableway was very easily operated back and forth along the work. The cableway complete, with 497 ft. of six-rail track for each tower, cost $4,700. The cost of materials and labor for the 8,994 cu.
yds. of concrete was as follows:
Per cu. yd.
1 bbl. cement at $2.46 $2.46 0.89 cu. yd. stone, at $2.95 2.622 0.315 cu. yd. sh.e.l.ls, at $0.45 0.142 0.51 cu. yd. sand, at $0.12 0.062 Mixing and placing 0.693 ------- Total $5.979
The above batch tamped in place to 30 cu. ft., or 1-1/9 cu. yds., which gives the cost as follows:
Per cu. yd.
Cost of concrete tamped in place $5.381 Cost of form work 0.370 ------- Total cost $5.751
In the preceding prices of cement and stone, 59 cts. and 29 cts. per cubic yard, respectively, are included for storage. The costs of sand and sh.e.l.ls are costs of screening and storing. Rough lumber for forms cost $10.25, and dressed lumber $12.75 per M. ft. B. M.
~Emplacement for Battery, Tampa Bay, Fla.~--The emplacement contained 6,654 cu. yds. of Portland cement, sand, sh.e.l.ls and broken stone concrete. The plant arrangement is shown by Fig. 69. The sand and sh.e.l.ls were got near the site, using an inclined cableway running from a 40-ft.
mast near the mixer to a deadman at the sh.e.l.l bank. All the sand for the fill around the emplacement was obtained in the same way. The other materials were brought by vessel to a wharf, loaded by derrick onto cars operated by an endless cable, and taken to the work. The storage bins and mixing plant were operated much like those for the mortar battery work, previously described. A cube mixer was used, and the concrete was handled from it to the work by a crane derrick covering a circle of 100 ft. in diameter. The cost of materials and concrete was as follows:
Cement, plus 7 cts. for storage per bbl. $ 2.532 Stone, plus 38 cts. for storage per cu. yd. 3.047 Sh.e.l.ls, excavating and storage. 0.481 Sand, excavating and storage. 0.250 Lumber, rough per M. ft. B. M. 10.25 Lumber, dressed per M. ft. B. M. 12.75
[Ill.u.s.tration: Fig. 69.--Sketch Plans of Concrete Making Plant for Battery Emplacement.]
A batch made up as follows, tamped in place to a volume of 30 cu. ft. or 1-1/9 cu. yds.:
1 bbl. cement, at $2.532. $ 2.532 0.315 cu. yd. sh.e.l.ls, at $0.481. 0.151 0.51 cu. yd. sand, at $0.25. 0.130 0.89 cu. yd. stone, at $3.047. 2.710 Mixing and placing. 0.761 -------- Total for 30 cu. ft. $ 6.284
This gives a cost per cubic yard of concrete in place as follows:
Concrete in place, per cu. yd. $ 5.655 Forms, per cu. yd. of concrete. 0.220 -------- Total cost of concrete per cu. yd. $ 5.875
~United States Fortification Work.~--The following methods and cost of mixing and placing concrete by hand and by cubical mixers is given by Mr. L. R. Grabill for U. S. Government fortification work done in 1899.
_Hand Mixing and Placing._--The work was done by contract, using a 1 cement, 2 sand, 2 pebbles and 3 stone mixture turned four times. A board large enough for three batches at a time was used; one batch was being placed, one being mixed and one being removed at the same time so that the mixers moved without interval from one to the other. Two gangs were worked, each mixing 64 batches of 0.75 cu. yd., or 48 cu. yds. of concrete per day at the following cost:
Per Per Hand Mixing 9,000 Cu. Yds.-- day. cu. yd.
6 men wheeling materials $ 7.50 $0.16 8 men mixing 10.00 0.21 8 men wheeling away 10.00 0.21 6 men placing and ramming 7.50 0.16 1 pump man 1.25 0.02 1 waterboy 1.00 0.02 1 foreman 2.00 0.04 ------- ------- Totals $39.25 $0.82
The entire cost of plant for this work was about $500.
_Machine Mixing and Placing._--The concrete was mixed in a 4-ft. cubical mixer operated by a 12 hp. engine which also hauled the material cars up the incline to the mixer. These cars pa.s.sed by double track under the material bins where the compartments of the car body were filled through trap doors; they then pa.s.sed the cement house where the cement was placed on the load, then up the incline to the mixer and dumped, and then empty down an opposite incline. Seven turns of the mixer mixed the charge which was discharged into iron tubs on cars hauled by horses to two derricks whose booms covered the work. One gang by day labor mixed and placed 168 batches of 0.7 cu. yd., or 117.6 cu. yds. per day at the following cost:
Per Per Machine Mixing 4,000 Cu. Yds.-- day. cu. yd.
32 men at $1.25 $40.00 $0.34 1 pumpman 1.25 0.01 1 teamster and horse 2.00 0.02 2 waterboys at $1 2.00 0.02 1 engineman 1.70 0.02 1 derrickman 1.50 0.01 1 fireman 1.50 0.01 1 foreman 2.88 0.03 Fuel (cement barrels largely) 1.25 0.01 ------- ------- Totals $54.08 $0.47
The cost of the plant was about $5,000.
[Ill.u.s.tration: Fig. 70.--Concrete Making Plant for Constructing Lock Walls, Cascades Ca.n.a.l.]
~LOCK WALLS, CASCADES Ca.n.a.l.~--Four-fifths or 70,000 cu. yds. of lock masonry was concrete, the bulk of which was mixed and deposited by the plant shown by Fig. 70. The concrete was Portland cement, sand, gravel and broken stone. Cement was brought in in barrels by railway, stored and tested; from the store house the barrels were loaded onto cars and taken 250 ft. to a platform onto which the barrels were emptied and from which the cement was shoveled into the cement hopper and chuted to cars which took it to the charging hopper of the mixer. The stone was crushed from spalls and waste ends from the stone cutting yards, where stone for wall lining and coping and other special parts was prepared. These spalls and ends were brought in cars and dumped into the hopper of a No. 5 Gates crusher, with a capacity of 30 tons per hour. From the crusher the stone pa.s.sed to a 2-in. screen, the pieces pa.s.sing going to a bin below and the rejections going to a smaller Blake crusher and thence to the bin. The dust and small particles were not screened out.
The sand and gravel were obtained by screening and washing pit gravel.
The gravel was excavated and brought in cars to the washer. This consisted of a steel cylinder 2 ft. 6 ins. in diameter and about 18 ft.
long, having an inclination of 1 in. per foot. An axial gudgeon supported the cylinder at the lower end and it rested on rollers at the other end and at an intermediate point. The gravel was fed by hopper and chute into the upper end and into this same end a 3-in. perforated pipe projected and extended to about mid-length of the cylinder. The cylinder sh.e.l.l was solid and provided with internal fins for about half its length from the feed end. For the remainder of its length nearly to the end, the sh.e.l.l was perforated with 2-in. holes. For a length of 4 ft.
beyond mid-point it was encircled by a concentric screen of 1/8-in.
holes, and this screen for 3 ft. of its length was encircled by another screen of 30 meshes to the inch. The pit mixture fed into the cylinder was gradually pa.s.sed along by the combined inclination and rotation, being washed and screened in the process. The sand fell into one bin and the gravel into another, and the waste water was carried away by a flume. The large stones pa.s.sed out through openings at the lower end of the sh.e.l.l and were chuted into cars. The cars came to the mixer as clearly shown by Fig. 70.
The stone and gravel cars were side dump and the cement car was bottom dump. The mixers were of the cube type 4 ft. on each edge and operated by a 712-in. double cylinder engine at nine revolutions per minute. The usual charge was 32 cu. ft. of the several ingredients, and it was found that 15 revolutions requiring about 1 minutes were sufficient for mixing. The average work of one mixer was 17 batches or about 13 cu.
yds. per hour, but this could be speeded up to 20 batches per hour when the materials were freely supplied and the output freely removed. Two cars took the concrete from the mixer to the hopper, from which it was fed to the work by chute. The hopper was mounted on a truck and the chute was a wrought iron cylinder trussed on four sides and having a 45 elbow at the lower end to prevent scattering. The chute fed into a car running along the wall and distributing the material. It was found impracticable to move the chute readily enough to permit of feeding the concrete directly into place. As the concreting progressed upward the trestle was extended and the chute shortened. It was found that wear would soon disable a steel chute so that the main trussed cylinder had a smaller, cheaply made cylinder placed inside as a lining to take the wear and be replaced when necessary.
The plant described worked very successfully. Records based on 9,614.4 cu. yds. of concrete laid, gave the following:
Cu. yds.
Concrete mixed by hand 1,777.0 Concrete mixed by machine 7,837.4 Total concrete laid 9,614.4 Concrete placed by derricks 2,372.0 Concrete placed by chute 7,242.4 Concrete 1-2-4 mixture 156.0 Concrete 1-3-6 mixture 1,564.0 Concrete 1-4-8 mixture 6,892.0
The average mixture was 1 cement, 3.7 sand, 4.8 gravel and 2.6 broken stone. The average product was 1.241 cu. yds. concrete per barrel of cement and 1.116 cu. yds. of concrete per cubic yard of stone and gravel. The average materials for 1 cu. yd. of concrete were: Cement 0.805 bbl., sand 0.456 cu. yd., gravel 0.579 cu. yd., and stone 0.317 cu. yd.
The cost of these 9,614.4 cu. yds. of concrete in place was:
Hand Mixed and Placed by Derrick-- Per cu. yd.
Labor mixing 1,777 cu. yds $1,072 Repairs, fuel, etc 0.016 ------- Total cost mixing $1,088 Labor placing 2,372 cu. yds. 0.6025 Fuel, tramways, etc. 0.1958 ------- Total cost placing $0.7983
Machine Mixed and Placed by Chute-- Labor mixing 7,837 cu. yds. $0.388 Repairs, fuel, etc 0.046 ------ Total cost mixing $0.434 Labor placing 7,242 cu. yds 0.414 Fuel, tramways, etc. 0.045 ------ Total cost placing $0.459 Materials and Supplies 9,614 cu. yds.-- Timbering $0.145 Cement 3.289 Sand and gravel 1.073 Broken stone 0.536 Cement testing, repairs, etc. 0.223 ------ Total $5,266 Plant and Superintendence, 9,614 Cu. Yds.-- Engineering, superintendence, repairs, etc. $1,508 20% cost of plant 0.165 ------ Total $1,673
The comparative cost of hand and machine mixing and handling was thus:
Item-- Hand. Machine.
Mixing per cu. yd. $1.088 $0.434 Placing per cu. yd. 0.798 0.459 Materials, etc., per cu. yd. 5.466 5.466 Plant, etc., per cu. yd. 1.673 1.673 ------ ------ Totals $9.025 $8.032
The average total costs of all the concrete placed were: