The cost of forms is usually given per cubic yard of concrete, but this form of statement does not discriminate between thin and thick masses, nor between structures having a regular or irregular contour. It would be of advantage to all concerned, if the cost of materials and of labor for forms were each stated in three ways, viz.: (1) per cubic yard of concrete; (2) per square yard of finished sur face; and (3) per thousand feet of lumber used. Unfortunately, the records of the cost of work are seldom kept in this form, and the published data seldom give any detailed information as to the cost of the forms.
There is greater diversity in the cost of the forms than in any other element of the cost of concrete. In eighteen cases of culverts, bridge abutments, retaining walls, arches, canal locks, and reservoirs, the cost of lumber ranged from $16 to $20 per thousand feet. The amount of lumber varied from 6 to 70 feet per yard of concrete, usu ally from 12 to 25; and the cost of lumber ran from 9 to 88 cents per cubic yard of concrete, in most cases from 25 to 55. The cost of labor varied from $7 to $10 per thousand feet of lumber; and from 28 cents to $1.10 per cubic yard of concrete, without any uniformity about any intermediate values. In the eighteen cases the total cost of forms ranged from 32 cents to $1.95 per cubic yard, six being below 75 cents, six between 75 cents and $1.00, and six between $1.00 and $1.95.
In making estimates for bridge and culvert construction on a prominent western railway system, the cost of forms is taken at 35 to 85 cents per cubic yard, depending upon the cost of lumber and the contour of the structure.
The cost of labor in mixing concrete by hand and putting the same into place, exclusive of the cost of forms and of finishing the surface of the concrete, may be divided as follows: (1) loading the cement, sand, and stone into the wheel barrows, buckets, or cars employed to transport the materials from the stock piles to the mixing board; (2) transporting and dumping the materials; (3) mixing the materials; (4) loading the concrete; (5) transporting the concrete to place; (6) dumping; (7) spreading and ramming; (8) superintendence; (9) finishing the surface; and (10) general expense, including building cement house, sand bin, runways, etc., and interest, depreciation, etc. Gillette's Handbook of Cost Data, pages 269-80, analyzes each of the first seven items above, and presents the above summary, "under the assumption that the concrete is to be put into a deep foundation requiring wheeling a distance of 30 ft., that the stock piles are on plank 60 ft. distant from the mixing board, that the specifications call for 6 turns of gravel concrete thoroughly rammed in 6-in. layers, and that a gang of sixteen men at $1.50 a day each is to work under a foreman receiving $2.70 a day." "To estimate the daily output of this gang of laborers, divide the daily wages of the 16 men, expressed in cents, by the labor cost of the concrete in cents, and the quotient will be the cubic yards of output of the gang." "In street-paving work where no man is needed to help dump the wheelbarrows, and where it is usually possible to shovel concrete direct from the mixing board into place, and where half as much ramming as above assumed is usually satisfactory, we see that items 4 to 7, instead of amounting to 37 cts., are only one half of the
last item, viz., 7-1 cts. This makes the total cost of labor only 60 cts., instead of 90 cts. If we divide 2,400 cents (the total day's wages of 16 men) by 60 cents (the labor cost per cu. yd.), we have 40, which is the cubic yards of output of the 16 men. This greater output of the 16 men reduces the cost of superintendence to 7 cts. per cu. yd." The above examples are fairly representative of well organized work, but if the superintendence is inefficient the cost of labor may easily be 25 per cent more; and if the job is large, or if the men have long been in the gang, the above cost may be reduced a little.
In making estimates on a prominent western railroad system the cost of labor in mixing and placing concrete in bridge and culvert construction is taken at 90 per cent of the price paid per day for common labor. This includes the cost of unloading all concrete material and tools, building concrete-mixing platforms and run ways, mixing by hand and placing; but does not include the cost of excavating, the cost of forms or of train servi ?e.
When concrete is mixed by machinery, the ingredients and the concrete are sometimes handled by hand and sometimes the materials are fed into the mixer by machinery and the concrete is transported in buckets or cars moved by power. In the first case, the only economy of machine mixing over hand mixing is in the reduction of the cost of the mixing; but in the second case, nearly every item is materially reduced. The cost of machine mixing will depend upon the size of the mixer; but under ordinary circumstances the cost of mixing with hand feeding and bearing away will be about 10 cents per cubic yard; while with gravity feed from bins and power transportation the cost of mixing, exclusive of interest, depreciation, and cost of setting up and taking down the machine, may be as low as 3 cents per cubic yard, and a cost of 2 cents has been claimed.* With the price of labor stated in § 418, machine mixing with manual attendance will save about 20 cents per cubic yard; but ordinarily the men who do the mixing will receive more than the minimum wages, and hence the difference between hand and machine mixing will usually be greater than 20 cents per cubic yard; and if the price of labor is more than in § 418, the saving will be still greater. In Chicago the cost of machine mixing, under four different foremen on five or six jobs each of 200 to 500 cubic yards each, ranged from 28.5 to 38.5 cents per cu. yd., the average being 33.9; and for hand mixing under four foremen on several jobs, the range was from 49.0 to 58.3 cents per cu. yd., the average being 53.0.t Examples of Cost. A few examples showing the cost of concrete in massive construction immediately follows; and for additional data, consult the index under the title of the particular structure, as Abutment, Arch, Building, Culvert, Pier, Retaining Wall, etc., or the heading: Concrete, Cost of.