The above examples give about average con ditions, and show the cost of labor on hand-mixed concrete in heavy work where mixing and laying can go on continuously. If labor is cheap (and efficient), the unit-cost may be less, and vice versa. If materials can be deposited for easy handling, as when they are laid close to the mix ing board and need only to be measured, the unit cost will be reduced accordingly; whereas long hauls or high lifts, either before or after mix ing, will add to the cost very materially. If the gang cannot be continuously employed, costs may be two or three times as much as the above. Concrete deposited in narrow forms will also cost more per cubic yard than in massive work.
With mechanical mixers, the cost of mixing concrete will be less than by hand-mixing, though the extra cost of skilled workers to run the engine and mixer helps to balance the costs. Batch mixers should turn out about 20 batches per hour.
Current prices for which similar work is be ing done in localities situated about the same distance from the source of supply, afford a sound basis upon which to gauge the cost of work. It is best for the engineer not engaged in the estimating of cost to the contractor or manu facturer, to use as a base the unit-cost of work in place, rather than to analyze the elements that go to make up the cost, such as material, labor, freight, hauling, profit, etc. The contractor's profit is an elastic factor, depending upon the size of the work, the risk, and many other con siderations. The cost of manufacture is variable. Some shops can make heavy work cheaper than others, while others can handle light work more economically.
It is not the purpose here to analyze the cost in shops and mills, so much as to give more general data for determining the probable cost of ordinary building and bridge work, as well as to point out some of the special cases where costs are apt to be more or less than the average. Average costs will prevail near the railroads and within radii of 50 or 100 miles of the commercial centers. Freight rates average about cent to cents per ton-mile. Long pieces requiring several cars and not weighing enough to load them to their normal capacity will cost more per ton than materials that can be shipped in full carloads. Partial carloads are charged at a minimum carload rate—say one-half of the ca pacity of the car. Where more than one car is required, one car is charged at this minimum rate, and each other car at one-half of this amount, if the actual weight of the material shipped is not over that total.
Hauling under ordinary conditions costs about 50 cents per ton for structural material.
The actual cost of hauling crushed stone miles in some macadam paving, was found to be 26.6 cents per ton when drawn from the crusher bins, and 31. cents per ton when drawn from the piles. The contract price was cents per ton.
The hire of a dumping wagon and team and driver is about $4.00 per day; that of a horse and cart and driver is about $3.00 per day.
The actual cost, with stone free at the quarry, of laying macadam pavement (5-inch layer, large-sized stone, rolled; to 3-inch layer of medium-sized stone, sprinkled and rolled; about inch of fine screenings, sprinkled and rolled) was 42 cents per square yard. The average weight of stone was 3 tons per square yard. ("Engineering News," Oct. 8, 1903).
The actual cost of quarrying and crushing stone in the above-mentioned work was 42 cents per ton, in which coal delivered cost $4.00 per ton; a driller, $1.75 per day; helper, $1.50 per day; engine man, $2.00 per day.
The cost of mixing materials and laying the same in making the Buffalo breakwater was as follows: Laying materials 17.4 cents per cubic yard Mixing materials 12.9 Li if Li Placing mixed materials 14.6 " Li it Total 44.9 cents per cubic yard Sometimes the gravel and sand for. a piece of work can be found at or near the site, thus greatly reducing the cost of concrete made of the same.
Bricks may be hauled direct from the works, without the expense of loading and unloading on cars.
Extra hazardous work should have some thing added to the estimated cost to allow for the risk taken by the contractor. Work that must be finished in a short time should have the esti, mate increased, especially if a penalty attaches for failure to complete by a specified time. If the season is a poor one for the class of work, still more expense is liable to be incurred. Erecting bridges over streams in flood-time may be at tended by serious difficulties and expensive de lays.
Large contracts, as a rule, cost less per unit than small ones. The placing and removing of the contractor's plant on a job often requires considerable time. If the magnitude of work does not justify bringing labor-saving machinery to the site, the extra labor will make the smaller job more expensive. Large orders of materials may be placed at lower rates than small ones.