795. Cost Of Blocks

Concrete blocks are usually made to lay courses eight inches or nine inches high, and are for 8-inch, 10-inch, and 12-inch walls. The blocks themselves may be of any length, but 24 inches and 32 inches are the lengths most commonly used. The lengths and heights above mentioned include the thickness of one joint, that is, each block when laid in the wall measures 24 or 32 inches long from center to center of end joints, and eight inches or nine inches high from center to center of longitudinal joints. These dimensions have been selected for convenience in laying out the work, the lengths mentioned being exactly divisible into halves, quarters, and eighths.

Blocks of the one-piece system are usually made with cores occupying 20 to 40 per cent of the entire volume, that is, from 60 to 80 per cent of the entire volume of the block is solid. With the two-piece system the spaces sometimes reach 50 per cent of the entire volume.

Volume Of Mortar Or Concrete In One Block, With Varying Proportions Of Core Space

796. The weight of concrete varies considerably with the character of materials and method of making, the usual weight with rock aggregate being 140 to 150 pounds per cubic foot. At 150 pounds per cubic foot the weight of blocks 70 per cent solid and of the sizes given in Table 162 is as follows: —

Weight Of Blocks, 70 Per Cent Solid, In Pounds

The Cost of materials in a cubic yard of Portland cement mortar of various compositions is given in Table 62, page 197. From this table is deduced Table 163 giving the Cost of materials in one cubic foot of mortar containing 2, 3, 4, or 6 parts sand by weight to one cement, the cost of cement varying from $1.20 to $3.00 per barrel.

Table 162 gives the volume, in cubic feet, of solid mortar or concrete in one block of each of the usual sizes when the voids or core spaces occupy 50 to 20 per cent of the total volume of the block.

Table 162.

Table 163. Cost Of Cement And Sand In One Cubic Foot Of Mortar. (Sand At 75 Cents Per Cubic Yard)

Taking from Table 162 the volume of solid mortar or concrete in a given block, and from Table 163 the Cost of the ingredients in a cubic foot of mortar of the given proportions and at the given price of cement, the product of these two gives the cost of cement and sand in one block.

Example

What will be the Cost of the materials contained in a hollow block 8 x 10 x 32, in which the cores occupy 30 per cent of the space, if the mortar is composed of 4 parts sand by weight to one cement, the cement costing $2.00 a barrel and the sand $1.00 a cubic yard?

From Table 162 we find the volume of solid mortar in such a block is 1.038 cu. ft. From Table 163 the Cost of the materials in a cubic foot of mortar of proportions stated is 14.2 cents if sand costs 75 cents a cubic yard, and at $1.00 per cubic yard for sand the cost of mortar per cubic foot will be about nine-tenths of a cent more, or 15.1 cents. Then 15.1 x 1.038 = 15.7 cents, the Cost of materials in one block.

797. For blocks made of concrete instead of mortar, it is more convenient to use the cubic yard as the volume unit. Table 164, derived directly from 162, gives the solid volume, in cubic yards,

Volume In Cubic Yards, Of Solid Mortar Or Concrete In 100 Blocks

The cost of concrete is discussed in detail in Art. 44. In order, however, to give the approximate Cost of materials per block in the simplest way, Table 165 has been prepared covering the usual proportions. With broken stone from which the screenings have been removed, the left half of the table should be used. With broken stone properly graded from coarse to fine, or with gravel, the right half may be employed. This table is based on a Cost of 75 cents per cubic yard of sand and $1.00 per cubic yard of broken stone. When higher prices prevail, add about one-third the excess in cost of sand and the entire excess in Cost of stone or gravel for approximate results. If greater accuracy is desired, see Art. 44.

We may now determine the Cost of materials in a concrete block as follows: —

Example

Block 8 x 10 x 32, 30 per cent space. Proportions, 1 cement to 3 parts sand by weight, and sufficient mortar used to fill voids in gravel. Cost of cement $2.00 per bbl., sand $1.00 per cu. yd., and gravel $1.50 per cu. yd. What is the Cost of materials per block?

From Table 165 we find the Cost of materials in such concrete to be $2.63 per cu. yd. with sand at 75 cents and gravel $1.00 of 100 blocks of various sizes, and containing 50, 40, 30, or 20 per cent core space.

Table 164.

Per cu. yd. For the prices in the example we must add 25/3 + 50 = 58 cents, giving $2.63 + 58 - $3.21 per cu. yd. of rammed concrete. From Table 164 we find the contents of 100 blocks of the given size to be 3.85 cu. yds. Then 3.85 X $3.21 = $12.36 or 12.4 cents per block.

Table 165. Cost Of Materials In One Cubic Yard Of Concrete. (Mortar Sufficient To Fill Voids)

Cost of sand assumed 75 cents per cubic yard. Cost of stone or gravel assumed $1.00 per cubic yard.

798. The labor Cost of making blocks depends upon so many elements that average figures are lit.tle more than an indication of the probable actual Cost. The method of making — hand tamped, pressed or poured — is of course a most important element. The number of blocks turned out per man varies greatly with the style of machine and the facility of operation. The method of mixing the concrete, by hand or machine, and the perfection of the arrangements for ease in manipulating and handling the finished blocks, not to mention the efficiency of the laborers, all have an important effect on the labor Cost per block.