Saturday, 11 August 2018

How To Estimate The Amount Of Cement Mortar Required To Lay Blocks

materials for mortar Required To Lay Blocks

As a builder, experienced site engineer, i can tell you on this page that one of most difficult moment of a building contractor or a builder is that moment when the client asks "How many Blocks will i need to build my 2 bedroom flat?" or How many bags of cement will i need to mold blocks or How many bags of cement will i need to lay the block. A this point, most contractors become stunned, to save the day, they quickly say "I'll get back to you". With this post today, you'll be learning How to estimate the amount of mortar Required To Lay Blocks. 

This means that by the time you're done reading this post, you will be able to answer questions like: How many bags of cement do i need to lay the blocks? How many trips of sand do i need for the block work

Just sit back and learn with me.

How Many Blocks are there in One square metre?

When i wrote this article on how many block are there in 1 square metre, i explained that there are about 10 number of blocks in every 1 square metre.

Want More proof?

Ok, walk with me dear.

What is the area of one 9 inch block in Nigeria?

In Nigeria, the area of a 9 inch block = 450mm x 225mm. The product of this gives us 0.101m2 isnt it?

Now to get the number of blocks to cover 1 square metre, we simply divide 1m2 by 0.101.

This gives us 9.99 isnt it? 
Can we just assume its 10 blocks since we cannot have 9.9 blocks?

Now lets see we can estimate the quantity of mortar required to law the blocks.

Before we proceed, lets quickly take a good look at the dimensions of a 9 inch block with voids (9 is very popular, known because it is usually used for building in Nigeria)

longitudinal section of a 9 inch block

Having seen the above image comprising of the various sections of a 9 inch hollow block, lets calculate the area of the block that will receive mortar.

How to Calculate the amount of mortar required by a 9 inch block

Ab = Area of the whole block - area of Voids

Ab = (0.45 x 0.225) - 2(0.15 x 0.125) = 0.0637 m2 

Whats the required thickness of martar?

The required thickness of mortar is usually 25mm

From the above, we can say that for every 9 inch block, there is need for 0.0637 x 25 and this gives us a 1.6m3 of mortar.

We will get back to this later on.

Now, let us assume that the mortar will be 25 mm thick

The typical arrangement of blocks in a one square metre wall is given below;

Arrangement of blocks in one square metre

Having seen the arrangement of blocks in a square metre, the number of blocks, lets now calculate the volume of mortar for 1 square metre of block

From the vertical section of a 1 square metre wall, the volume of mortar will be:

Vertical Mortar Requirements = 8 x (0.025 x 0.225 x 0.275) = 0.0123 m3
Horizontal mortar Requirements = 10 x (0.0637 x 0.025) = 0.0159 m3

Now, Adding Vertical and Horizontal Mortar requirements, we get a total of  = 0.0282 m3

To go further, let us assume that we have 300 m2 of wall, and we want to estimate the quantity of cement and sand for mortar needed to lay the blocks.

What Volume of Mortar is required to Lay Blocks on a 300m2 wall?

The volume of mortar required = 0.03 x 300 = 9 m3

What is the Mix Ratio For Mortar?
Typical mix ratio for mortar (laying of blocks) is 1:6

Materials For Cement Mortar

For Cement
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 9m3 of Mortar;

You Must Provide = 9 x 6.33 = 57 bags of cement + allowance for waste as appropriate.

For Sharp Sand

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 9m3 Of mortar;

You Must Provide = 9 x 2112 = 19004 kg of sharp sand + allowance for waste as appropriate.

So you can order 20 tonnes of sharp sand and 57 bags of cement for this job.

Now i am sure you understand How to estimate the amount of mortar Required To Lay Blocks (Cement and Sand) covering one square metre as well as make estimates for cement mortar.

Note: This post was made using a 9 inch block as a case study, the volume of mortar will differ if you're using a 6 inch hollow block. 

Also note that this post was made with the Nigerian contractors in mind.

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