Nowadays, most of the modern construction companies use computer-aided software for their estimating and costing solution. But we often estimate materials on a project manually for purchasing purpose.
When we need to cast concrete members of a building, we either use ready-mix concrete or prepare the concrete manually on site based on the mix design.
Mix design is commonly referred to the mix proportion of concrete ingredients.
The question is, how do we calculate concrete ingredients for different-ratio concrete?
How to Estimate Materials for Different Ratio Concrete?
The most common ratio of concrete ingredients used in a building construction project is 1:1½:3 and 1:2:4.
When we mix cement, sand and stone chips at 1:1½:3 ratio, the concrete strength of 28 days cylinder test result comes around 3500 psi. If we mix cement, sand and brick chips at 1:2:4 ratio the 28 days cylinder test result will come around 3000 psi. However, the concrete strength mostly depends on the quality of materials used to prepare concrete.
when we prepare concrete on site we need to measure the concrete ingredients separately based on the proportion specified in the mix design.
I will show you how to estimate concrete ingredients for 1:1½:3 proportion. After learning this process you will be able to estimate materials for any concrete proportion.
So, the dry volume of concrete is,
= 100 x 1.54 = 154 cubic feet.
[1.54 is a factor used to convert the wet volume of concrete to the dry volume of concrete. I’ve talked about this before on the post “what is the dry volume of concrete“]
Sum of ratio 1:1½:3= 5.5.
So, the required Cement is,
= (154÷5.5) x 1
=28 cubic feet.
The required quantity of Sand is,
= (154÷5.5) x 1.5
= 42 cubic feet.
The required Stone chips are,
= (154÷5.5) x 3
= 84 cubic feet.
As we know, Cement is available as 50-kilo bag in the market and the volume of 50-kilo cement bag is 1¼ cubic feet. So the required cement is 28 divided by 1¼ equal to 22.4 bags.
Cement- 22.4 bags,
Sand- 42 cubic feet,
Stone chips- 84 cubic feet.
In this estimation, I use cubic feet as the concrete measuring unit. If you want to use the cubic meter, the same method can be applied. But that will be time-consuming.
The easiest way to estimate concrete materials for the different unit is to apply the above result as a percentage. That means, Cement content for 100 unit of 1:1½:3 ratio concrete is 28% (unit will be same as the concrete unit), Sand is 42% and Stone chips is 84%.
Now let’s calculate the required water content for preparing concrete. Suppose, the water-cement ratio for concrete is specified 0.45. That means, water/cement = 0.45, or W/C = 0.45.
For one bag of cement, the required water is,
=0.45 x 1.25 (as we know, 1 bag cement equal to 1.25 cubic feet),
=0.5625 cubic feet.
We know one cubic feet water is equal to 28.31685 liters,
So the required water is,
= 0.5625 x 28.31685
say, 16 liters.
So, preparing concrete with one bag of cement, the required water content is 16 liters for 0.45 w/c.
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