The Cost of mixing concrete in large quantities is seldom less than 30 cents per cubic yard if allowance is made for plant. This is for mixing alone, not including placing. The Cost for hand mixing is seldom less than 50 cents per cubic yard. In mixing concrete in small quantities, as for cement block making, these figures should be doubled. For a large output there is thus a chance of saving about 40 cents a cubic yard, or from one to two cents a block, by installing a small mixer to run by motor or gasoline engine. The more important item of Cost, however, is forming the block and conveying to the curing shed. By suitable cars and tracks and organization of force there is an opportunity of making a considerable saving in this part of the work.

As a general average it may be stated that in making blocks by the hand-tamped process 3 men can make, in one day, 60 blocks weighing 200 pounds each, or 100 blocks weighing 100 pounds each. With wages at $2.00 per day this represents a Cost of 10 cents each for the large blocks and 6 cents each for the small blocks. If the blocks have 30 per cent core space, or 70 per cent solid, we would have the following approximate costs of labor in making: —

8X 8x24or9X 8X24........... 6 cents each.

8 X 10 X 24 or 9 X 10 X 24........... 7 " "

8 X 8 X 32 or 8 X 10 X 32........... 7 " "

8 X 12 X 24 or 9 X 12 X 24.......... 8 " "

9 X 8 X 32.................. 8 " "

8 X 12 X 32 or 9 X 10 X 32........... 9 " "

9 X 12 X 32.................. 10 " "

We have already found the Cost of materials in an 8 x 10 x 32 inch block, assuming certain prices for cement, sand, and gravel, to be 15.7 cents when made of mortar composed of 4 parts sand to one cement by weight, and 12.4 cents when made of concrete containing enough 1 to 3 mortar to fill the voids in the gravel. The Cost of labor to make such a block is shown to be about 7 cents, giving a total cost of 22.7 cents or 19.4 cents according to the material used. In addition to the above is the Cost of delivery to the purchaser, a reasonable allowance for breakage of from two to five per cent, and incidental expenses.

799. Laying Concrete Blocks

In order that the laying of the blocks in the wall, once started, shall continue uninterrupted, all necessary specials must be on hand promptly, and the chipping of regular blocks to fit special places should be avoided if a strong wall of good appearance is to result. To this end each course of blocks must be carefully planned and a drawing made showing the number and location of all specials.

The mortar for laying should be of Portland cement, but as such mortar is brash unless very rich, it is well to add some wellslaked lime paste. To one part Portland cement and three parts sand add one half part of lime paste that has been thoroughly slaked and strained to remove lumps; or to a 1 to 4 mortar one part lime paste may be added. It is better not to lay blocks in freezing weather, but if it becomes necessary to do so, the blocks should be warmed and the lime should be omitted from the mortar, which must be richer in cement, not more than 2 1/2 parts sand being used to one part cement by weight. In dry and warm weather the block should be moistened just before laying. In spreading the mortar keep the bed back from the face of the joint about one inch to leave room for pointing. Be careful to have a full mortar bed, as the joint is at best the weak point in the wall.

Cost Of Laying

The Cost of mortar alone will vary from one to three cents per block. One mason with two helpers to carry and hoist blocks and mix mortar, will lay from 8 to 12 blocks per hour, making the Cost of labor for laying from 8 to 12 cents per block.

800. Comparison Of Cost Of Concrete Block And Brick

We have now considered the elements of Cost in detail, and may compare the cost of concrete block and brick. A ten-inch concrete block wall is usually considered as strong as a thirteen-inch brick wall, and we will use one hundred 8 x 10 x 32 inch blocks in the comparison, with prices of materials already assumed.

Concrete in 100 blocks, 70 per cent solid......... $12.40

making and curing................... 7.00

Incidental expenses, 4 per cent............. .80

Breakage, 4 per cent................. .80

Profit, at 9 cents per block............... 9.00

Delivery to building ................. 3.00

Mortar to lay.................... 2.00

Labor to lay..................... 10.00

Total Cost, wall of 100 blocks or 180 sq. ft. face . . $45.00

Cost per 100 sq. ft................. $25.00

To lay 100 sq. ft. of brick wall requires 2,100 brick.

2,100 brick at $9.00 per thousand..........$18.90

Mortar to lay 2,100 brick.............. 4.10

Labor to lay 2,100 brick...............11.00

Cost of 100 square feet of brick wall ..... .. $34.00

The Cost of a brick wall is thus about 35 per cent in excess of the Cost of a wall of concrete blocks of equal strength. If pressed brick are used for the face at $20.00 per thousand, the Cost will be increased by $10.00, allowing for additional cost of laying.

A comparison of 8-inch walls of concrete block and brick will be somewhat less favorable unless the extra strength of the concrete blocks is considered. Using blocks 8 X 8 X 24 we have:—

Concrete in 100 blocks, 70 per cent solid......... $7.42

making and curing.................. 6.00

Incidental expenses 4 per cent and breakage 4 per cent . . 1.08

Profit, at 4 cents per block............... 4.00

Delivery to building ..........,...... 1.75

Mortar to lay.................... 1.35

Labor to lay, at 8 cents per block............ 8.00

Cost of 100 blocks in place, or 133 sq. ft. of wall . . .$29.50

Cost per 100 sq. ft. of 8-inch wall.......... $22.20

To lay 100 sq. ft. of 8-inch wall requires 1,400 brick.