COST OF CONCRETE. The cost of the materials varies with the locality and the conditions of the markets; and hence it is unwise to cite examples or to attempt any generalizations. When the prices are known estimates may be easily prepared by the use of Tables 36 and 37 or 38. pages 372, 373, 374.
For detailed data on the cost of mixing and laying concrete in miscellaneous engineering construction, see the author's Treatise on Masonry Construction, pages 112v-112z. The following relate to the cost of mixing and laying concrete for pavement foundations In a small western city the average cost to the contractor of mixing and laying a thickness of 6 inches of concrete during two years was about 7 cents per square yard, for 1 part cement, 2 parts sand, and 4 parts broken stone, turned six times exclusive of casting into place. With gravel instead of broken stone. the cost was about 6 cents per square yard; and with four turnings instead of six, the cost was about half a cent less than the prices above. All the mixing was done with shovels. The wages of common laborers was $1.50 for 10 hours.
In a large western city the average cost to various contractors of mixing and laying a thickness of 6 inches of concrete was 5.1 cents per square yard. The mixing was done with hoes, the specifi cations requiring that the concrete should be mixed until each particle of the stone was completely covered with mortar. The wages of common laborers was $1.50 for 10 hours.
The following example * gives the detailed cost and distribution of the labor of laying a 6-inch concrete pavement foundation, in hours per square yard.
The sand and stone were dumped in the street upon boards, and were hauled in wheel-barrows about 40 feet to the mixing boards_ The mortar was turned three, and the stone three or four times. Two gangs under separate foremen worked side by side in the same street.
The same correspondent gives another example which required 0.57 hours per cu. yd., in which case the mortar was turned only once and the stone twice, water being used in abundance.
The cost of labor in mixing and laying concrete is often 8 or 9 cents a square yard. For the most economical work the sand and stone should be deposited in ridges on the subgrade near the middle of the street; and if they are piled on the parking, the cost will be considerably greater than above.
Portable concrete-mixing machines have been tried for pavement fOundations, but have not been very successful. Appar ently the hand labor required with the machine is about equivalent to that required to mix the concrete by hand directly; in other words, the cost of shoveling the ingredients from the ground up and into the machine is about equivalent to the cost of mixing by hand. Of course, with a machine, interest and depreciation will amount to considerable particularly as the machine will probably be in use only a comparatively few days during the year. However, or three concrete-mixing machines have recently been introduced which give promise of success for pavement work.