Sunday, May 31, 2020

Water Cement ratio

Every time when a civil engineer goes to site the first few questions he ask is What is Water Cement Ratio? 

Things we are going to see in this article is water to cement ratio,  Roles of cement water ratio , What will be the Effects of too much water in concrete and How to calculate Water Cement Ratio?

Water Cement Ratio

The ratio of weight of water to the weight of  cement used in a concrete mix is known as water to cement ratio.

If a mix have low w/c (water cement) ratio, the mix tends to show high concrete strength and durability but the disadvantage of this is due to low water to cement ratio, the concrete shows low workability (ability of concrete to handle, transport and placing without any segregation)
To overcome the issues we use admixtures such as plasticizers and super- plasticizers to overcome the workability problem and make the concrete flow workable.
  • Usually we take w/c ratio 0.4 to 0.6 as per IS Code 10262 (2009) for nominal mix (M10, M15, M20,M25)
For concrete greater tha M25, we design-mix the concrete with proper admixtures as per requirements of strength, Durability and flow.

Role of water in concrete mix

When the water is mixed in a concrete mix, It results in chemical reaction between cement and water. This reaction is termed as "Heat of hydration".

The water added in mix uses 23-25% of the cement volume in the chemical reaction of hydration process and 15-18% turns up in a form of gel or slurry or water cement paste which fill the voids of the mix, making the concrete gain strength and durable.

Effects of adding too much water in concrete mix

Adding too much of water in concrete mix may lead to
  • Segregation of sand and aggregate from cement mix
  • As low as 10 percent more water gives you 17 percent less strength.
  • The excess water not consumed in chemical reaction of hydration will result in bleeding of concrete.

How to calculate water cement ratio?

Normally the water to cement ratio is provided by structural engineers with grading of concrete mix.
But in site conditions we can calculate water to cement ratio as

Consider we have to calculate for 0.50 w/c ratio.
Lets consider we need to take out water required for 1 bag of cement, ie. 50kg

So, we can calculate as, 
W/C = 0.5
W/50= 0.5
 W= 0.5x50
   W=25 liters.

So for 1 bag of cement at 0.5 w/c ratio, we require 25 liters of water for the mix. 

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