Mix design is commonly referred by mix proportion in a selective zone where same materials are used in concrete. Suppose, in Bangladesh, most of the structural designers refer concrete mix design as ratio in drawing. Because materials of same properties are used in all over the Bangladesh.
The question is,
How do we estimate materials for different ratio concrete?
The most common ratio referred here in Bangladesh for column concrete 1:1.5:3 and for slab 1:2:4. When we mix cement, sand and stone chips at 1:1.5:3 ratio, the concrete strength of 28 days cube test’s result comes around 3500 psi. If we mix cement, sand and brick chips at 1:2:4 ratio the 28 days cube test result will come around 3000 psi, which is referred for slab concrete.
I will estimate materials for 1:1.5:3 ratio concrete. After learning this process you will be able to estimate materials for any concrete ratio.
Dry volume of concrete= 100 x 1.54 = 154 cft.
Sum of ratio 1:1.5:3, 1 + 1.5 + 3 = 5.5.
So, Cement content in concrete = (154/5.5) x 1=28 cft.
Sand content= (154 / 5.5) x 1.5 = 42 cft.
Stone chips = (154/5.5) x 3 = 84 cft.
As we know, Cement is available as 50 kg bag in the market. The volume of 50 kg cement bag is 1.25 cft. So the required cement is 28 divided by 1.25 equal to 22.4 bag.
Cement : 22.4 bag,
Sand : 42 cft,
Stone chips: 84 cft.
In this estimation, we use cubic feet as our concrete unit. If you want to use cubic meter, same method can be applied. But that will be time-consuming. The easiest way to estimate concrete materials for different unit is, apply the above result as percentage. That means, Cement content for 100 unit of 1:1.5:3 ratio concrete is 28% (unit will be as concrete unit), Sand is 42% and Stone chips is 84%.
Now lets calculate the water content of concrete. Suppose, water-cement ratio for concrete is specified 0.45. That means, water/cement = 0.45, or W/C = 0.45.
for 1 bag cement, water is, = 0.45 x 1.25 (as we know, 1 bag cement equal to 1.25 cft),
Water = 0.5625 cft.
We know 1 cubic feet water is equal to 28.31685 litre,
So we can write, water = 0.5625 x 28.31685 = 15.92 litre, say, 16 litre.
So One bag cement needs 16 liter of water for 0.45 W/C ratio.
Here, one thing should be cleared that someone assume the dry volume of concrete is equal to one and half times of wet volume. But it is better to use 1.54 for calculating dry volume.