If you are planning a large construction project involving concrete blocks, such as building a retaining wall, then you need to plan everything out in detail. You need to know how much mortar to mix up to do the job without having to interrupt everything to make more. You need to know how much cement, lime and sand to purchase beforehand to make that mortar. To learn how to make these plans, consider the example of mortaring 600 concrete blocks into place.
Types of Concrete Blocks
Concrete blocks come in many sizes. One common size is the 8-inch cube. There are two types of these; the half block has six solid sides, while the half sash has four solid sides and a hole running from one of the remaining two sides to the other.
Types of Joints
There are two types of joints between blocks. One is the joint between rows. Each row sits on top of mortar that has been spread on the top of the row beneath it. The other type of joint is between adjacent blocks in the same row. Each row contains as many adjacent joints as there are blocks, minus one.
Mortar between Rows
To calculate how much mortar is needed between rows, multiply the width of the tops of the blocks by their lengths. In the example, this is 8 inches multiplied by 8 inches, which equals 64 square inches. However, mortar is usually calculated in cubic feet, so you need the area in cubic feet. Because 8 inches is two-thirds of a foot, the area is 2/3 multiplied by 2/3, or 0.44 square feet. As there are 600 blocks, the total surface area of the row is 266.67 square feet. To lay the next row, you need 1 inch of mortar on top of this row. This is the same as 1/12 foot, so the total volume of mortar you need is 22.22 cubic feet. However, this is only the mortar that you need at the start of the process. Once laid down, they are supposed to be tapped down until there is only 3/8 inches of mortar in the joint. The excess is then scraped away. This leaves you with a need for 8.33 of mortar between the rows.
If you are using half sashes, then this figure will be too high. Half sash blocks are stacked with the holes on top and bottom. Because adjacent joints are offset by a half block every other row, only the strips on the sides will rest on each other. To calculate the surface area for half sashes, subtract the width of the hole from 2/3 of a foot, multiply the result by 2/3 and multiply that by 600.
The joints between each block also need 3/8 inches of mortar. Each block has a surface area of 0.44 square feet on the sides. This leaves 0.014 cubic feet of mortar between each block. A row of 600 blocks requires 8.24 cubic feet of mortar between the blocks.
Total Materials Needed
The total mortar needed between all the joints for the example is 8.33 cubic feet plus 8.24 cubic feet, for a total of 16.57 cubic feet of mortar. If you are using a standard general purpose mortar, you are going to need 2 cubic feet of cement, 2 cubic feet of lime and 12 cubic feet of sand. For a stronger type S mortar, you need 2.76 cubic feet of cement, 1.4 cubic feet of lime and 12.43 cubic feet of sand. The strongest mortar, type M, needs 3.12 cubic feet of cement, 1.04 cubic feet of lime and 12.43 cubic feet of sand.