How to calculate quantity of water in concrete

Quantity of water required to be added in concrete is of major concern while preparing a concrete mix. An optimum amount of water is required to prepare a good concrete mix. Both addition of excess water or shortage of water in concrete mix is not desirable, as in both cases certain properties of concrete get modified and in turn results in a bad concrete mix. So, here properties of concrete depending upon water content of concrete mix and calculation of quantity of water required for concrete mix is discussed.

Properties depending upon water content of concrete

Basically, two properties of concrete have direct dependency over amount of water in concrete namely workability of concrete and strength of concrete.

Workability: It is the ease with which concrete can be handled, transported and placed without any loss inhomogeneity of concrete. More the water in concrete, it will be easier to work with it. So, it’s obvious that a greater amount of water in mix will result in a more workable concrete mix. Thus workability is directly proportional to water content in concrete mix.

Strength: It is the load carrying capacity of concrete (generally 28 day compressive strength). If water is added beyond a limit in concrete mix, then evaporation of water results in lots of voids, which in turn decreases the strength of concrete. Thus, addition of more and more water will make concrete weaker and leads to lesser strength. So, it’s clear that strength of concrete is indirectly proportional to the amount of water in concrete mix.

Upon considering above two properties i.e. workability and strength of concrete it is found that workability increases with increase in water and strength decrease with increase in water and vice versa. So, we have to decide such amount of water which satisfies our both requirements: i.e. good workability as well as high strength. 

To decide the optimum amount of water a term called water-cement ratio is defined and a relation of water-cement ratio is established with both workability as well as strength of concrete. First let us understand what is water-cement ratio? Then its relation with both workability as well as strength will be discussed.

Water-cement ratio: As we know a concrete mix consists of cement, fine aggregates and coarse aggregates, and water. Water-cement ratio (W/C) is defined as the ratio of weight of water to weight of cement content in the given concrete mix. 

Relation between Water-cement ratio and workability:

Workability of concrete mix increases with increase in water-cement ratio. Generally workability is measured in terms of slump value. Graph below shows the variation of workability (Slump) with water-cement ratio.

Here, Mix A: M10, Mix B: M15, Mix C: M20 (M means Mix, Value after M shows 28 days compressive strength of concrete)

Relation between Water-cement ratio and strength

Strength of concrete (generally 28 day compressive strength) decreases with the increase in water-cement ratio as shown in graph:

Calculation of quantity of water in concrete:

Before calculation of water requirement of concrete, one have to define water-cement ratio for the concrete mix. Water-cement ratio is decided as per Indian Standard code IS 456:2000, Table number 5, which have values defined for minimum cement content, maximum water-cement ratio for different grades of concrete for different environmental conditions as shown in figure:

After selection of water-cement ratio one can calculate quantity of water required for concrete mix by following relation:

Water required= water-cement ratio x weight of cement

We will learn this with help of one example.

Example: Calculation of water required for M20 grade concrete

We will here calculate water requirement of 1 cubic meter concrete of grade M20 with water cement ratio 0.5 ( water-cement ratio can be decided with the help of IS code, here we have assumed it 0.5 ) and density of cement 1440 kg/m3.

Proportion of M20 grade concrete is 1:1.5:3, here 1 part is cement, 1.5 parts is fine aggregates & 3 parts is coarse aggregates. 

Now, Dry volume of 1 cubic meter of M20 concrete= 1x1.54=1.54 cubic meter.

(Dry volume of concrete= 1.54 x wet volume of concrete)

Part of cement out of 1.54 cubic meter of dry concrete= (1/5.5)x1.54=0.28 cubic meter

Weight of 0.28 cubic meter cement= 0.28x1440=403 kg

Water required= water-cement ratio x weight of cement=0.5x403=201.5 kg or 201.5 liters.

(1 kg of water= 1 litre of water)

Alternatively, 

Water requirement of 1 bag of cement= water-cement ratio x weight of cement

= 0.5x50= 25 kg or 25 liters.

Number of bags of cement in 403 kg is 403/50=8 (approximately)

(Weight of 1 bag of cement is 50 kg)

Water requirement of 8 bags of cement= 8x25=200 kg or 200 liters (approximately)

Thus, water requirement of 1 cubic meter M20 grade concrete is 200 litres approximately. Similarly, we can calculate water requirement for other grades of concrete.