Block laying require mortar which is produced by mixing cement, fine aggregates (sand) and water. Mortar is a type of light weight concrete and as a result, can be batched by weight or by volume. In this write up, we will look at factors to note when estimating the amount of mortar needed for block work.
1. Type of block to be laid:
Unlike block estimation which is calculated in area, quantity of mortar is measured in volume, therefore the thickness and nature of the block needs to be accounted for. A 5 inches block will require mortar than a 6 inches hollowed block. Solid blocks consume more mortar than hollow ones.
2. Blocks are laid in interlocking fashion:
It is also very imperative to note that most blocks are laid in interlocking fashion and not otherwise. This is to ensure that the wall is firm as a result of the nature of the interlocked bonds produced by the mortar. Length of all blocks laid are not equal.
3. Batching of mortar by volume
Mortar like concrete can be batched by weight or by volume. Weight batching is more accurate than volume batching. Batching by weight will require you to know the amount of fine aggregates, cement and the water cement ratio needed to produce the mortar. For this calculation we will be considering batching by volume.
4. Height of Mortar
Height of mortar is important because the volume of mortar depends on block thickness minus (minus hollow length where one exist) multiplied by the block length multiplied by the height of mortar (bonding). Let us illustrate this with an example:
Consider a block wall of 3600 mm x 2000 mm with mortar height of 25 mm. The wall is to be constructed using a 9 inches hollow block, calculate the number of blocks and volume of mortar needed for the work. Take hollow width of the block to be 5 inches.
Area of wall to be constructed = 3600 x 2000 = 7200000 square mm = 7.2 square meters
Area of one 9 inches block = 225 x 450 = 101250 square mm = 0.1013
Number of blocks = 7.2/0.1013 =71.11 blocks = 72 blocks approximately.
To calculate the volume of mortar, we need to consider that laying a block will involve placing mortar on two sides of the block. i.e the left hand side and the bottom side of the block. using a thickness of 1 inch or 25 mm:
Volume of mortar for the left hand side = 225 x 255 x 25 =1265625 cubic millimetres = 0.00127 cubic metres
Volume of mortar for the bottom side = 450 x 25 x (225 -125) = 1125000 cubic millimetres = 0.00113 cubic metres
Volume of mortar for one block = 0.00127+0.00133 = 0.0024 cubic metres
Total volume of mortar for 72 blocks = 72 x 0.0024 = 0.1724 cubic metres
4. Measurements are not exact but approximate
What ever result you get can not be exact but approximate because of certain factors such as the water cement ratio used to produce the mortar as well as the nature of mortar placement. It is also to account of wastage especially where the blocks are of hollowed types.
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