Water - Cement Ratio

According to Wikipedia, water - cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix.

Duff A. Abrams is the person who developed and published the notion of water - cement ratio in 1918.

But to understand the concept of water - cement ratio, it is necessary to understand how water reacts with cement.

Cement basically consists of 4 major compounds called Bogue's compounds. There are many other compounds present in cement in lesser concentrations, but to makes things easy we will stick to the major compounds only, namely:

• Tricalcium Silicate (C3S) 54.10%

• Dicalcium Silicate (C2S) 16.60%

• Tricalcium Aluminate (C3A) 10.80%

• Tetra Calcium Aluminoferrite (C4AF) 9.10%

The percentages mentioned is the respective compounds composition in modern cements.

As you can see tricalcium silicate and dicalcium silicate constitutes about 70% of the cement and are also greatly responsible for strength.

Water requirement for hydration

C3S and C2S together requires about 23% of water by the weight of total cement. and 15% of water by weight of cement is required for filling up gel pours.

So, in total 38% of water by weight of cement is necessary for the complete hydration of cement.

• Anything less than 38% means not all of the cement is hydrated,

• Anything more that 38% means water getting into capillary cavities of formation of capillary cavities.

But it is important for my readers to understand that plasticizers and superplasticizers are used to increase the workability, and though complete hydration is not achieved, modern concrete mix designs with much lower water - cement ratio are used to achieve much higher strengths. This a completely different topic and needs to be discussed separately.

What happens in a site where cement is locally batched and mixed?

Workability is a very important factor of cement. With lower water - cement ratios, workability is greatly reduced. Thus, it is common tendency of most masons and workers to add water randomly to increase workability. It is a major cause of capillary cavity generation in concrete which will ultimately make the concrete weak. Also my blog on concrete pour during rainy season will give you a better idea of defects of excess water in cement.

It is the responsibility of the site engineer to make his masons understand the importance of water - cement ratio and defects of excess water in concrete.

Also, it was mentioned earlier, that cement only requires 38% of water by its weight for complete hydration. But it is common practice to use 45% to 60% of water by weight. This is done to achieve proper mix. This increased water content is also taken with the view that surface of aggregates needs to be wet or the excess water is usually required by the aggregates present in the mix.

Water requirements for different nominal mix

Duff A. Abrams is the person who developed and published the notion of water - cement ratio in 1918.

Reference:-

Concrete Technology - Theory and Practice, by MS Shetty. (Published by S Chand Publications)