| Item. | Total. | Per cu. yd. |
| 1,910 days labor mixing | $3,175 | 45 cts. |
| 1,740 days labor placing | 2,660 | 38 cts. |
| ——— | ———— | |
| Total | $5,835 | 83 cts. |
Haulage.—The costs given comprise in mixing, the cost of delivering the materials to the mixer, and, in placing, the cost of hauling the concrete away. A Robins belt conveyor was used to deliver materials to the gravity mixer and this accounts, in a large measure, for the lower cost of mixing by gravity. The mixed concrete was hauled from both mixers in dump cars pushed by men.
Form Work.—The labor cost of forms for 19,300 cu. yds. of concrete placed in 1903 was $16,800, or 87 cts. per cu. yd. of concrete. The total labor days consumed on form work was 6,340 at $2.70 per day. The total cost of concrete in place for mixing, placing and form work was $1.46 per cu. yd., not including lumber in forms, fuel, interest and depreciation.
CHAPTER XVII.
METHODS AND COST OF CONSTRUCTING ARCH AND GIRDER BRIDGES.
The construction problems in arch and girder bridges of moderate spans are simple, and with the exception of center construction and arrangement of plant for making and placing concrete, are best explained by citing specific examples of bridge work. This is the arrangement followed in this chapter.
CENTERS.—The construction of centers is no less important a task for concrete arches than for stone arches. This means that success in the construction of concrete arches depends quite as much upon the sufficiency of the center construction as it does upon any other portion of the work. The center must, in a word, remain as nearly as possible invariable in level and form from the time it is made ready for the concrete until the time it is removed from underneath the arch, and, when the time for removal comes, the construction must be such that that operation can be performed with ease and without shock or jar to the masonry. The problem of center construction is thus the two-fold one of building a structure which is immovable until movement is desired and then moves at will. Incidentally these requisites must be obtained with the least combined expenditure for materials, framing, erection and removal, and with the greatest salvage of useful material when the work is over. The factors to be taken count of are it, will be seen, numerous and may exist in innumerable combinations.
