Engineers, site managers, and quantity surveyors are always faced with the challenge of specifying as accurately as possible, the quantity of materials needed to execute a specific item of work. In this article, we are going to explain how you can estimate the quantity of mortar (cement and sand) needed to lay blocks per square metre of wall.
It is well known that we need about 10 blocks to build one square metre of wall.
Area of wall = 1 m2
Planar area of one standard block (Nigeria) = 450mm x 225mm = 0.101 m2
Therefore, the number of blocks required (disregarding mortar) = 1/0.101 = 9.99 (say 10 blocks)
Now, how do we estimate the quantity of mortar needed to lay the blocks? First of all, let us look at the dimensions of a typical 9 inches block with holes (the most popular block for building in Nigeria).
From the image above, we can say that the cross-sectional area of the block that receive horizontal mortar is;
Ab = (0.45 x 0.225) – 2(0.15 x 0.125) = 0.0637 m2
Now, let us assume that the mortar will be 25 mm (1 inch) thick
The typical arrangement of blocks in a one square metre wall is shown below;
Therefore, we can estimate the volume of mortar required to build one square metre (1 m2) of wall as follows;
Vertical mortar = 8 x (0.025 x 0.225 x 0.275) = 0.0123 m3
Horizontal mortar = 10 x (0.0637 x 0.025) = 0.0159 m3
Total = 0.0282 m3
Therefore the volume of mortar required to lay one square metre (1 m2) of 9 inches block (with hole) can be taken as 0.03 m3 for all practical purposes.
To go further, let us assume that we have 150 m2 of wall, and we want to estimate the quantity of cement and sand needed to lay the blocks.
The volume of mortar required = 0.03 x 150 = 4.5 m3
Typical mix ratio for mortar (laying of blocks) is 1:6
Quantity of cement required = 1/7 x 1440 kg/m3 = 205.71 kg
Making allowance for shrinkage between fresh and wet concrete = 1.54 x 205.71 = 316.79 kg
Quantity of cement required in bags = 316.79/50 = 6.33 bags
For 4.5m3 of mortar;
Provide = 4.5 x 6.33 = 29 bags of cement + allowance for waste as appropriate.
Quantity of sand required = 6/7 x 1600 kg/m3 = 1371.428 kg
Making allowance for shrinkage between fresh and wet concrete = 1.54 x 1371.428 = 2112 kg
For 4.5m3 of mortar;
Provide = 4.5 x 2112 = 9504 kg of sharp sand + allowance for waste as appropriate.
Therefore you need about 10 tonnes of sharp sand and 29 bags of cement for laying 150m2 of 9 inches block wall.
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