Important of Water Cement Ratio - Calculation

What is Water Cement Ratio

It the ratio of Water and cement ( By weight or by volume ) in a concrete mix. It is usually expressed in liters of water per bag of cement (50kg). 

After carrying out a number of experiments by Duff Abrahams, he establish a relationship between and cement. Which is known as water cement ration Law. This law is valid only when the concrete is of workable plasticity.

According to this law the strength of the concrete totally depends upon the amount of water used in the preparation of concrete mix and is quite independent of the proportion of cement and aggregates. The strength of the concrete is inversely proportional to the water cement ratio. In other words, the strength of concrete decreases as the water cement ration increases. 

It may be noted that when the water cement ratio become less than 0.45, the concrete is not workable and causes honeycombed structure containing a large number of voids.

If the water cement ratio is more, than the capillary voids will be more in the physical structure of hydrated cement. The strength and durability of concrete will also be less. 

Typical Water-Cement Ratios in Concrete Mixes 

Water-cement ratio is not same for all type of concrete mixes, different water-cement ratios are employed for different application of concrete.

• Normal for ordinary concrete ( Sidewalks and driveways ) : 0.6 to 0.7
• If a higher quality concrete is desired : 0.4

The practical range of water-cement ratio is from about 0.3 to 0.8

•  A ratio of 0.3 is very stiff (unless a plasticizer is used).
• A ratio of 0.8 makes very wet and fairly weak concrete.

Typical compressive strength when concrete is properly cured are

• 0.4 water-cement ratio - > 5600psi
• 0.8 water-cement ratio - > 2000psi

How to Calculate Quantity of water in Concrete

To calculate the quantity of water required in concrete, first we need to know the quantity of cement needed.

Lets calculate for 50kg of cement i.e. one bag of cement.

We take 0.42 as the ratio of W/C

Quantity of water = 0.42 X 50 = 21 liters 

So the required quantity of water per cement bag of 50kg is 21 liters.

Test on Concrete Water-Cement Ratio 

Now we have calculated the water cement ratio for 1 bag of cement from the above steps, now we have to test is practically.

For this we need to test the workability and consistency of mix through Slump Cone Test.

We have written a detailed guide on Concrete Slump Cone Test don't forget to read it.

Hope you learn in detail on Water-Cement Ratio for concrete mix and how it is important to know.

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