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The most important thing that determines the strength of the concrete mix is the water cement ratio.
The water cement ratio is the ultimate factor for the strength and durability of the concrete when it is cured properly. For instance, if the water cement ratio is 0.40, it means for every 50 Kg of cement(1 Bag) used in the concrete, 20 Liters of water is to be added.
Water cement ratio = Weight of water
Weight of cement
For example if the water-cement ratio is 0.50 for concrete and cement added is 50 kg (weight of 1 bag of cement).
Water required for concrete will be:
Water / cement = 0.50
Water / 50kg = 0.50
Water = 0.50 x 50 = 25 liters.
Similarly for W/C = 0.40
Water = 0.40 x 50
Water = 20 litres
As you see, the water is reduced as we decrease the water-cement ratio. It is a fact that when water is reduced in concrete, the compressive strength of the concrete increases. But there is some limit to the water cement ratio. The minimum water cement ratio is 0.30 - 0.35, beyond this the concrete becomes too stiff and impractical to handle.
Here’s how you can test the water cement ratio in concrete to ensure you have the best cement for house construction :
A simple and practical way of controlling the water content is by means of a slump test carried out by a contractor, explained below.
To conduct this test a Slump Cone of steel: 30cm high, 20cm diameter at the base, 10cm diameter on the top and provided with handles is used. Concrete is filled in the cone in layers of 7.5cm at a time, each layer being tamped 25 times with a metallic tamping rod 16mm in diameter and 60cm long. Just after the slump cone has thus been filled then it is lifted. The extent to which the concrete drops is called the slump. It is measured from the top of the cone to the top of the concrete after the cone has been removed.
The usual values of the slump of concrete used for various purposes are given below and depend upon the method of compaction possible in each case. Where there is no obstruction by way of reinforcement etc., in the movement of concrete or where the concrete can be rammed hard in the case the smaller value of slump is required.
Mass concrete and road work : 2.5 to 5 cm
Ordinary beams and slabs : 5 to 10 cm
Columns, thin vertical sections
and retaining walls etc. : 7.5 to 12.5 cm
Also Read : Concrete and it's types.
1. How does the water cement ratio affect strength ?
The lower the water to cement ratio, the fewer air pores and the more compact the concrete structure, which translates into higher strength. The high amount of water will reduce the compressive strength of the concrete because it will increase the gap between the cement.
2. What are the advantages of a low water cement ratio ?
There is reduced drying shrinkage and cracking if the water cement ratio is low. There is lower permeability and it creates a better bond between concrete and reinforcement.
3. How can we reduce the water cement ratio in concrete ?
To reduce the amount of cement you must first reduce the amount of water. Use an admixture, optimize the combined aggregate grading, add fly ash, or obtain aggregate that has a better particle shape.
