Foundation for Sea-Wall. The following is the analysis of the composition and cost of the concrete employed for the founda tions of the sea-wall at Lovell's Island, Boston Harbor: $ The proportions for this concrete were 1 cement, 3 sand, and 4 gravel. It was unusually cheap, owing partly to the use of pebbles instead of broken stone. If the broken stone had been used, it would have cost probably 4 to 6 times as much as the gravel. The amount of labor required was also unusually small, this item often being 2 to 2i times as much as in this case.
Foundation for Blast Furnace. The following is the analysis* of the cost of nearly 10,000 yards of concrete as laid for the founda tions of a blast furnace plant near Troy, N. Y. The concrete con sisted of 1 volume of packed cement to 7 of sand, gravel, and broken stone. The concrete was carried from 15 feet below the surface to 13 feet above the surface. No forms were used. The labor was performed by men who had worked about the blast furnace and who expected that kind of work again as soon as the repairs were com pleted; and the price per day paid for the labor in mixing and placing the concrete was unusually small, but the time per cubic yard was proportionally larger than usual—compare this example with that in § 421 and § 423.
Retaining Wall. The following is the cost of constructing the concrete retaining wall on the Chicago Sanitary Canal. f The average height of the wall was 10 ft. in Sec. 14, and 22 ft. in Sec. 15. The thickness on top was 6 ft., and at the bottom it was equal to half the height. The stone was taken from the adjacent canal excavation. The body of the wall was made with natural cement, but the coping and facing, each 3 inches thick, were made with Portland cement. The proportions were 1 volume of cement, 1 volumes of sand, and 4 volumes of unscreened limestone. The cost of plant employed in Sec. 14 was $9,600, and in Sec. 15 was $25,420. The contract price for the concrete in Sec. 14 was $2.74, and in Sec. 15 $3.40 per cu. yd.
Arch Culverts. Table 42 shows the cost of concrete in five arch culverts built under a railroad trestle by negro labor.* The proportions of the concrete were 1 :3:6. The mixing and placing were done by hand, except that in the last 12-foot culvert the mixing was done with a machine.
Cost of Mixing and Placing. Table 43 gives the details of the cost per cubic yard of the labor required in mixing and laying concrete for the Buffalo, N. Y., breakwater.* The total amount of concrete laid was 14,587 cu. yd. The conditions under which the work was done varied considerably from year to year, which accounts for the difference in the cost. The work summa rized in Table 43 was done by day's work, and is unusually high; but in 1902 under a contractor most of the work of transporting and mixing was done by machinery, when the cost of mixing and placing was reduced to 45 cents per cubic yard exclusive of fuel, forms, and plant rental.t The latter is about the usual cost when most of thg work is done by machinery.
Labor to Mix and Lay. Table 44, page 216, gives the details of the labor required in mixing and laying concrete in the construc tion of the Boyd's Corner dam in New York and a reservoir in St.
Louis, Mo. $ Economic Concrete. Sometimes there is a question as to the relative economy of concrete made with natural cement and with portland, although owing to the great decrease in the pr.'ce of portland cement in the past few years, portland-cement concrete is usually the more economical. However, if such an investigation is to made, proceed as follows: The crushing strength of both natural cement and portland-cement concrete is given in Table 29, page 195, with both broken stone and gravel. A study of these results shows that the relative strength of natural and portland concrete is different at different ages. For example, taking averages for 10 days, the portland concrete was 6 times as strong as the natural concrete; while at a year the portland concrete was only 3 times as strong as the natural concrete. At 45 days and also at 6 months, the portland concrete was 4 times stronger than the natural concrete; and at 3 months 5 times as strong. Taking averages for like dates and com positions, the portland-cement concrete was 3.7 times as strong as natural-cement concrete. Table 28, page 158, may be employed to find the ingredients per cubic yard for natural cement as well as for portland; and hence the cost of materials for each may be easily computed. If the cost of a cubic yard of portland-cement concrete is more than 3.7 times that of a cubic yard of natural-cement concrete, then the latter is on the average the more economical; but if the portland-cement concrete costs less than 3.7 times that of the nat ural-cement concrete, then the former is on the average the more economical.
However, uniformity of product is more important than average strength, and for this reason alone portland cement is usually pre ferred to natural. Of course the relative cost will vary with the condition of the cement market and with the locality.
Sometimes a similar question arises as to the relative economy of gravel or broken stone for concrete. The relative strengths of gravel and broken-stone concretes are stated in § 284. The relative economy of concrete made with broken stone and gravel will vary with the cost of each; but as a rule, when gravel costs less than 80 per cent of that of broken stone, gravel is more economical. However, some engineers prefer broken stone to gravel, because of the danger that the latter may be unduly dirty— see § 286.
The following example, from actual practice, illustrates the possibilities in the way of combinations between portland and natural cements, and gravel and broken stone. The specifications called for a concrete composed of 1 volume of natural cement, 2 volumes of sand, and 4 volumes of broken stone. The contractor found that at current prices a concrete composed of 1 volume of portland cement and 9 volumes of gravel would cost about the same as the concrete specified. A test of the strength of the two con cretes showed that at a week the portland-gravel concrete was 1.52 times as strong as the natural-cement and broken-stone concrete; and at a month 1.59 times as strong. Therefore the portland-gravel concrete was the more economical, and was used.