What is the purpose of water cement ratio and then how to calculate?

In concrete mix design, the ratio of the amount of water to the amount of cement used (both by weight) is called the water to cement ratio (w/c). These two ingredients are responsible for binding everything together.

The water to cement ratio largely determines the strength and durability of the concrete when it is cured properly. The w/c ratio refers to the ratio of the weights of water and cement used in the concrete mix. A w/c ratio of 0.4 means that for every 100 lbs of cement used in the concrete, 40 lbs of water is added.

Typical w/c ratios are as follows:

• Normal for ordinary concrete (sidewalks and driveways): 0.6 to 0.7 
• Specified if a higher quality concrete is desired: 0.4

The practical range of the w/c ratio is from about 0.3 to over 0.8.

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

   

  The water to cement ratio is calculated by dividing the water in one cubic yard of the mix ( in pounds) by the cement in the mix (in pounds). So if one cubic yard of the mix has 235 pounds of water and 470 pounds of cement- the mix is a .50 water to cement ratio.

  If the mix lists the water in gallons, multiply the gallons by 8.33 to find how many pounds there are in the mix.