The concrete plant used at Lock No. 31, Coosa River Improvement,2 was erected in a three-story shed. The top story served as a cement storage room and two hoppers were arranged in the floor to receive the cement for the mixers below. Level with the floor of the second story were two other hoppers immediately below the cement hoppers, to receive the sand and broken stone, while in the first story or basement the mixers were suspended' at a height sufficient to allow concrete cars to pass under them. The following description is from the report of the designer, Mr. Charles Firth, U. S. Asst. Engineer:1
1 Engineering News, Aug. 27, 1903.
2 Major F. A. Mahan, Corps of Engineers, U. S. A., in charge.
"The cars used in handling the sand and broken stone are of the side dump pattern and are brought into the charging room on either side of the hoppers. The cement is drawn from the cement room overhead in proper quantities, through vertical chutes arranged somewhat on the principle of the old-fashioned powder flask.
"The water is added to the materials as they enter the mixers, and the quantity, which will probably be variable with the temperature, is controlled by valves on the mixing floor, the operators being governed by indicators, which show the quantity used. The mixers are cubical boxes four feet on each side, inside measurement, made of steel plate five-sixteenths of an inch thick, with 2 1/2 by 2 1/2-inch angle irons. Each mixer is provided with a door in one corner, twenty-two inches square, fastened with a tempered steel spring catch, and held open when required with a hinged screw bolt. The shaft which revolves the mixers is three inches square. It is securely fastened to them by trunnion castings at diagonally opposite corners. The whole is driven by a 10 by 16 inch horizontal engine, and thrown in and out of gear by ordinary friction gearing with friction and brake levers.
"After a sufficient number of revolutions in the mixers, the concrete is dumped into the concrete cars below, which are of the center dump pattern".
The method given of measuring the cement is not recommended, as the charge of cement, if not a full barrel, should always be weighed. The three-story arrangement by which the materials were handled almost entirely by gravity was made possible by the high bank at the side of the lock pit.
1 Annual Report, Chief of Engineers, U.S.A., 1894, p. 1292.
The total Cost of the plant, exclusive of the boilers, is stated to have been about $8,000, and the average output about two hundred cubic yards of concrete per day of eight hours. The Cost of mixing, depositing and ramming 8,710 cubic yards of concrete in the construction of lock walls was at the rate of $0,884 per cubic yard.
In the construction of the defenses at Portland, Maine,1 a five-foot cubical mixer was used. Sand and stone were delivered, by bucket conveyors, in bins directly over the mixer. "Immediately under these bins were two measuring hoppers for stone and sand, respectively, and an additional hopper for cement. From these measuring hoppers the charge was dumped into the mixer and thence, when mixed, into a car immediately under it. This car delivered the mixed batch by means of a hoisting engine and an inclined track to the site of the battery under a fifty-five foot derrick, which placed it in the work at the point required. Two barrels of cement, sixteen cubic feet of sand, and thirty-two cubic feet of stone constituted a batch. * * * The usual number of men engaged in the operation of mixing and placing was as follows: — Two master laborers, three steam engineers, two stokers and twenty-five laborers." It is said that 200 barrels of cement, or 100 batches, could be mixed and placed in a day of eight hours. This would make the labor Cost of this portion of the work 50 or 60 cents per cubic yard. The Cost stated, however, varies greatly according to the amount of detail in construction, and the lowest Cost given for " labor of mixing and placing" is $1.15 per cubic yard.
A cubical mixer used in the construction of the defenses at San Francisco 2 mixed 250 cubic yards per day with seven men, engineer, fireman, and five men to feed and dump mixer, at a labor Cost of $14.67 per day, or about six cents per cubic yard, exclusive of Cost of transportation and ramming. The materials and concrete were handled on cars run almost entirely by gravity.
In the construction of the Buffalo Breakwater,1 the mixing plant, consisting of a cubical mixer with necessary engines and boilers and two derricks,was mounted on a dismantled lake schooner which could be placed beside the section of the breakwater under construction. The broken stone was delivered in a canal boat which could be tied up alongside the schooner, and outside of the canal boat lay the material scow. The latter was made from an old dump scow, the decked pockets serving as bins for cement, sand and gravel.
1 Report of Charles P. Williams to Maj. Solomon W. Roessler, Corps of Engrs., U. S. A., in charge. Report Chief of Engineers, 1900, p. 745.
2 Maj. Charles E. L. B. Davis, Corps of Engineers, U. S. A., Report Chief of Engrs., 1900, p. 980.
Into a steel bucket on the scow were loaded, by wheelbarrows, the following materials:
5.4 cu. ft. (1 1/2 bbls.) cement. 10.8 cu. ft. sand.
5.4 cu. ft. gravel. 21.6 cu. ft. total.
Into a similar bucket on the canal boat 21.6 cubic feet of broken stone were shoveled. As these buckets were filled, they were hoisted by one of the derricks and dumped into the cubical mixer. The latter discharged the mixed concrete into a skip and a derrick deposited the concrete in place. The Cost of labor per cubic yard of concrete is as follows:
Cost per Hour.
Cu. Yds. per Hour.
Cost of Labor per Cu. Yd.
Loading material into buckets from scows
mixing, including engine men and derrick
Placing, including foreman.....
The above does not include Cost of fuel, nor of transporting materials from the storehouses or yards to the site of the work.