The very uniformity in size that makes cut stone expensive makes concrete blocks cheap, i.e., standardizes the forms and reduces their number. Absolute uniformity in size is not necessary to secure a satisfactory effect in pitch-faced stone work; it seems, however, to be more appropriate than random lengths in a face of smooth concrete blocks. In any event, uniformity in size facilitates the process of laying, especially when closing a gap by laying from both ends. When such a closure is made with stones of random length, it is almost invariably necessary to cut part of the last stone to make it fit. The cost of concrete blocks is naturally not affected by the thickness of the proposed joint. The cost per cu. yd. of the blocks depends upon the following factors:
The cost of producing at the block yard, the sand and crushed stone for 1 cu. yd. of concrete, i.e., the cost of say 0.9 cu. yd. of crushed stone (depending on the nature of the quarry, the quarrying and crushing plant and the methods). Assume for illustration that this cost is 75 cents, and that the cost of say 0.5 cu. yd. of sand, either pit or crushed from stone, is 25 cents.
The cost of the cement, say for a 1-2 1/2-4 1/2 mixture, 1.4 barrels at $1.50 per barrel, equals $2.10.
The cost of forms, yard for manufacturing and storing and whatever labor-saving equipment may be desirable.
Labor and power.
The third item depends very largely upon the number of cu. yd. of blocks it is proposed to manufacture, and the fourth item depends upon the equipment.
At the Kensico dam with about 60,000 cu. yd. of blocks to manufacture, the yard is 1100 ft. long by 150 ft. wide with several railroad tracks the entire length. Concrete materials are delivered to three mixers mounted upon a traveler which spans three rows of forms at a height just sufficient to clear them. One or two traveling cranes stack the blocks when they are hard enough to be handled, and the cranes also load onto cars the blocks to be sent to the dam. The entire labor cost in the yard of mixing and placing concrete, caring for, handling and storing the blocks has been stated as almost exactly $1 per cu. yd. The total cost per cu. yd. may then be about as follows:
The above assumptions as to cost of concrete blocks, though subject to change on account of local conditions, are probably fair enough to indicate that in most cases the total cost of concrete blocks would be less than the cost of cutting stone, to say nothing of the cost of quarrying the stone and the effect upon quarry methods, output and cost.
The thinner the joint desired the greater will be the margin of cost in favor of concrete blocks.