What is the Water Cement Ratio of Concrete?
The Water Cement Ratio of concrete is defined as the ratio of the weight of water to the weight of cement used in the concrete mix.
Generally, the water to cement ratio for the nominal mix ( M10, M15, M20, and M25) falls under 0.4 to 0.6 as per IS Code-10262 (2009).
Lower water to cement ratio results in higher strength and durability but makes the concrete mix hard to work with and form. But this problem can be resolved by the use of plasticizers and superplasticizers.
Adding too much water to the concrete mix will result in the segregation of sand and aggregate from the cement paste.
Also, the water that is not consumed during the hydration reaction will start leaving the concrete with time, resulting in microscopic pores in the concrete and reducing the final strength.
Concrete having too much water in the mix will experience more shrinkage as excess water leaves, which will result in internal cracks and visible fractures on the concrete surface which again will reduce the final strength of concrete to a large extent.
Source of undesirable water in the concrete
Undesirable water can be added in a concrete batch either by mistake or deliberately during the manufacturing process, transportation, and placement of concrete. The extra source of water in the concrete is as follows:
1. The extensive way of water addition in the concrete is from aggregates. The aggregate moisture causes significant variation in the mixing water content. If the aggregates are stored in the outdoor environment then rain and sun exposure can cause water variation. Therefore, it is important to monitor aggregate water content constantly to ensure the accurate water-cement ratio because aggregates contain the largest volume in the concrete mix design.
2. In the construction site, concrete is accepted based on the slump level or visual consistency. After loading the material in the truck, the operator does the visual inspection and slump test. After that, the truck is allowed to go to the desired place the distance might change the workability of the concrete because of the hydration process heat. Therefore, the truck driver adds water in the truck to ease their work but it should be done under the supervision of an expert or the best way is to add a superplasticizer to get the desired slump on the construction site.
3. The addition of water on the construction site is also the cause of variation in the w/c-ratio. Water is added to the concrete to get the desired workability.
4. Another source of unwanted water in the concrete is the repeated use of the same concrete truck. After pouring the concrete on the construction site, the truck owner washes his truck to remove the old concrete. This process allows the large volume of water to stay at the bottom of the drum. Therefore, when the concrete truck returns to the plant to be loaded again, the volume of water remains at the bottom and causes the variation in the w/c-ratio.
There are many ways of addition of extra water to the concrete mix, which causes a change in the designed specification of concrete. Therefore, the test to measure the water-cement ratio of fresh and hardened concrete is important to avoid the error.