PARAM VISIONS

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What is the Water cement ratio in construction? - importance of w/c ratio & its calculation.

1. What is the water-cement ratio?

The ratio of the amount of water by weight to the amount of cement by weight taken in the concrete mix is defined as the water-cement ratio.

When water is added to the concrete, the hydration reaction starts between the water and the cement present in the concrete mix. The amount of water required to complete this hydration process is about 0.35kg. of water for every 1kg. of cement. But adding such a low amount of water makes the concrete stiff without workability. So from practical consideration, the w/c ratio ranges between 0.4 to 0.7  depending upon the design mix, ingredient properties, and atmospheric exposure.

2.  How to calculate the amount of water needed for one bag of cement in the concrete mix?

For the calculation purpose (theoretical), we consider the w/c ratio for one bag of cement as 0.5

So, the amount of water added

= w/c ratio × wt. of the cement

= 0.5 × 50 kg.

= 25kg of water

As the density of water is 1kg/liter

= 25 liters of water

So, 1 bag of cement needs 25 liters of water.

3.  What is the quantity of water and cement required for different mix ratios or grades of concrete?

Let us now observe, the amount of water and cement required for the given w/c ratio in a cubic meter of concrete.

Grade of concrete       mix ratio         w/c ratio        cement in kgs           water in liters

M5                       1:5:10               0.60                  137.5                         82.5

M7.5                     1:4:8                 0.55                  169.0                         93.0

M10                       1:3:6                 0.50                  220.0                        110.0

M15                      1:2:4                 0.50                  314.0                        157.0

M20                       1:1.5:3              0.45                  400.0                         180.0

M25                       1:1:2                 0.40                  550.0                         220.0

Note: The above-given values are approximate calculations, for your reference. The amount of water content varies according to the moisture content of the aggregates, admixtures added, workability needed, design mix according to the site conditions.

4.   What is the importance of the water-cement ratio in the concrete?

OR

5.  What happens if the water-cement ratio is not controlled in the concrete mix?

If we add more water to the concrete mix, it leads to the segregation of the concrete by bleeding. The concrete so produced will have lower strength, reduced life span, and weak bonding strength. So it becomes mandatory to maintain the specified w/c ratio to have concrete of good quality.

 Bleeding of concrete.

By adding plasticizers and superplasticizers in the concrete mix, we can increase the workability of concrete by maintaining the designed w/c ratio.

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