Water cement ratio plays an important role in deciding the quantity of water to be added to cement so that strength can be achieved and the concrete can be kept workable.
Water cement ratio gives a comparison of how much water is present per unit quantity of cement. Lower value indicates higher strength but it is difficult to carry out concreting operations.
Another parameter to measure strength of concrete is gel space ratio.
What is Water Cement Ratio?
Water cement ratio is the ratio of amount of water to the amount of cement.
Water Cement Ratio Formula
Effective Water Cement Ratio
Aggregates are believed to use up some of the water to achieve saturated and surface-dry conditions from the water added for hydration. Hence, the net water available for hydration is the water added minus water absorbed by aggregates. Effective water cement ratio is calculated using this net water.
Net water available for hydration = water added – water absorbed by aggregates
Effective water cement ratio = net water available/ wt of cement
It is the effective water cement ratio that governs the strength of concrete and other concrete parameters.
Strength of Concrete & W/C Ratio
Strength of concrete mainly depends on the strength of cement paste.
Strength of cement paste can be improved by-
- Increasing cement content
- Decreasing water content
- Decreasing air content
In ordinary construction, not all the air is expelled from the concrete even if full compaction is exercised on the site.
At a given hydration, w/c ratio becomes the deciding factor of strength in cement paste.
If the water content is high, then the porosity of the cement paste will be increased. Total volume of voids in gel pores and capillary pores as well as entrapped air will reduce the strength of concrete.
With the age of concrete, the degree of hydration increases, therefore strength also increases.
Graph between Strength of Concrete vs W/C Ratio
As seen above, the graph between strength of concrete and w/c ratio is approximately hyperbolic in shape.
The intermediate values in the graph are difficult to interpolate. Hence, the graph between strength and cement water ratio is sometimes plotted.
As seen from the above graph, the graph between strength and cement water ratio has an approximate linear relation. Thus, it is easier to find the intermediate relation as the graph is almost a straight line.
Duff Abrahms’ Law
In 1918, Abrahm postulated that water cement ratio influences the desirable qualities of concrete. He passed a classic law that withstood test of time and is held true even today though modifications are made to it:
Statement:
For plastic mixtures using neat and clean aggregates the strength of concrete under specified condition is governed by the net quantity of water mixed per bag of cement.
S = A/Bx
S = strength of concrete
A, B = constants
X = w/c ratio by volume
For results of 28 days the values of constants are:
A = 1400 lbs/sq in
B = 7
Abrahm concluded that strength of concrete only depended on w/c ratio for workable mixes.
Abrahm’s theory is valid even today as the fundamental truth in concrete technology.
Feret’s Take on Water Cement Ratio
Feret stated as a rule defining the strength of cement paste in terms of volume fractions of constituents even before Abrahm in 1897.
S = strength of concrete
c = volume of cement
e = volume of water
a = volume of air
K = constant
As you can see in the above equation, Feret included the volume of air along with the volume of water.
Besides w/c ratio, degree of compaction also plays a major role in imparting strength to the concrete. Degree of compaction indirectly relates to the volume of air filled in voids of concrete.
Functions of Water in Concrete
- Water reacts chemically with cement resulting in setting of cement and ultimately hardening of cement takes place.
- Water acts as a lubricant for aggregates. Thus, passage of cement through voids between the aggregates is facilitated imparting workability to concrete.
To carry out the above two processes, the water requirement is about 0.5-0.6 times the weight of cement.
Water Requirement for Concrete
Minimum water providing reasonable workability should be added to concrete.
For proper workability, a w/c ratio of about 0.4 to 0.6 is required. As expected, maximum strength is attained at a w/c ratio of 0.4
As we know, for complete hydration of concrete, 38 % of water by weight of cement is required.
To know how 38 % water is required for complete hydrations as well as breakage of 38 % in 15 % and 23 %, refer Water Requirement for Hydration of Cement.
At w/c ratio less than 0.4
As concluded from above, a minimum w/c ratio of 0.38 is required for hydration. But, as per considerations of workability, when w/c ratio is less than 0.4, improper consistency and workability of concrete renders the handling of concrete difficult. It may even promote honeycombing in the structure.
But if the concrete is compacted by a vibrator, more strength can be achieved even at the w/c ratio of 0.3- as shown by the dotted line in the graph.
At w/c ratio more than 0.6
Volume of hydrated products increases and they won’t be able to comply with the space filled with water. This increases porosity and strength reduces drastically.
Effect of Excess Water in Concrete
For 1 kg cement, about 0.22 to 0.25 kg of water is required for hydration. But for hydration, only free water is available.
Excess water available beyond that required by hydration is evaporable water or water absorbed by the aggregates.
Still, excess water beyond hydration is added to the cement mix to improve the workability of concrete.
Harmful Effects of Excess Water:
- But this excess water causes development of capillary voids in concrete.
- Also, it occupies space in concrete. When this water is evaporated, voids are left in concrete.
- The voids increase porosity and permeability of concrete.
- At the same time, strength and durability of concrete is reduced.
Addition of 1 litre extra water to one bag of cement reduces the strength of the concrete by about 15 kg/cm2.
W/C Ratio and Permeability Relationship
As the water content increases, the permeability of concrete increases. This can be seen from the graph between w/c ratio and permeability below:
With that being said, permeability of concrete increases exponentially beyond w/c ratio of 0.5.
Excess water can even result into Segregation of Concrete. The constituents of concrete are segregated and cause reduction in strength. To know how segregation occurs and what points can be kept in mind to avoid segregation, Read Segregation of Concrete.
Importance of Water Cement Ratio
Significance of water cement ratio is discussed below:
- Higher water cement ratio leads to cracks in concrete.
- It also causes shrinkage in concrete which then results in micro cracks forming zones of weakness.
- Excess water may squeezed out of the concrete owing to the weight of aggregates and cause bleeding of concrete
- Excess water may cause the cement slurry to separate out from the aggregates causing segregation of concrete
- If w/c ratio is too low, the concrete would not be workable and stiff concrete may be formed
- Stiff concrete results into honeycombed structure
Thus, w/c ratio is a crucial factor to be taken into consideration.
Concrete Quality Parameters Governed by W/C ratio
- Strength: Strength increases with decrease in w/c ratio
- Resistance to weathering: As the strength is increased, resistance to weathering increases with decrease in w/c ratio
- Durability: Durability increases as resistance to weathering increases with decrease in w/c ratio
- Bond between two successive concrete layers: Increases with decrease in w/c ratio
- Bond between concrete and reinforcement: Increases with decrease in w/c ratio
- Volume changes due to wetting and drying: Decreases with decrease in w/c ratio
Thumb Rules for Deciding W/C Ratio
By experience and practice, two thumb rules for deciding the quantity of water are formulated. There are some assumptions while using this rules as listed below:
- Ordinary concrete is used
- The materials used for making concrete are dry and non-absorbent
Weight of Water = 28 % wt of cement + 4 % total wt of aggregates
OR
Weight of Water = 30 % wt of cement + 5 % total wt of aggregates
Working with Concrete having Low W/C Ratio
As discussed above, concrete having low water cement ratio gives high strength but the workability is too low.
To work with such concrete, admixtures like plasticizers or superplasticizers are added. These admixtures impart flowability to the concrete. In this way, workability of the concrete can be enhanced without increasing w/c ratio.
Water cement Ratio for Structures Exposed to Different Criteria
For structures exposed to weather, extra care should be taken while deciding the w/c ratio.
W/C Ratio for Structures Exposed to Different Weather Condition of Structure Criteria is tabulated below:
W/C Ratio for Structures Exposed to Different Weather Condition of Structure Criteria
| Weather Condition of Structure | w/c ratio | |
| Thin Section | Mass Concreting | |
| Regularly wetting and drying | 0.45 | 0.55 |
| Concreting underwater | 0.55 | 0.65 |
IS : 10262 – 2009 (Table 5) gives the values of maximum w/c ratio along with other minimum cement content, minimum grade of concrete for plain concrete and reinforced concrete under different exposure conditions:
W/c Ratio @ Diff Exposure Conditions
W/c Ratio @ Diff Exposure Conditions
| Exposure | Maximum Free W/C Ratio | |
| Plain Concrete | Reinforced Concrete | |
| Mild | 0.5 | 0.55 |
| Moderate | 0.6 | 0.5 |
| Severe | 0.5 | 0.45 |
| Very Severe | 0.45 | 0.45 |
| Extreme | 0.4 | 0.4 |
How to Calculate Water Quantity
For a given grade of concrete, water quantity is calculated based on water cement ratio, which is decided based on the exposure condition of the structure from the above table.
Required cement quantity is first calculated as per design mix procedure. After that, the quantity of water is calculated so that the required w/c ratio is achieved.
w/c ratio is selected after carrying out workability tests on the site.
Key Take Away
Water Cement Ratio is a major factor affecting strength development of cement paste but not the only one.
Definition:
Water cement ratio can be defined as the amount of water used per unit weight of cement.
Abrams Law:
Abram concluded that the strength of a fully compacted concrete with sound and clean aggregates can be improved by reducing the water content per unit weight of cement. This concept is known as Abram’s law.
Feret’s Rule:
Feret included the volume of air too in deciding the strength of concrete even before Abrahm. The volume of air accounts for compaction of concrete.
W/c Ratio and Strength of Concrete:
With increase in w/c ratio, permeability increases and strength decreases. For w/c ratio above 0.5, permeability increases exponentially, hence w/c ratio should be limited to 0.5.
The graph plotted between strength of concrete and w/c ratio is hyperbola. For ease in calculation, a graph between strength and cement/water ratio is plotted which is a straight line.
Role of Water in Concrete:
In concrete, there are two main functions of water- reacting with cement for hydration reaction and lubricating the aggregates.
Importance of W/c Ratio:
Very low w/c ratio makes the concrete stiff and not workable. Honeycombing can also be observed in the structure afterwards. While working with concrete having low w/c ratio, plasticizers or superplasticizers are added.
On the other hand, an increase in w/c ratio decreases the strength of concrete. Besides strength, w/c ratio also affects durability, resistance to weathering, bond strength of concrete, etc.
Ideal w/c ratio:
W/c ratio below 0.4 does not give workable concrete. Thus, w/c ratio is kept between 0.4 to 0.6. However, for concrete compacted with vibration, w/c ratio of 0.3 can also be used. Excess water increases voids in concrete and this reduces the strength of concrete.
Thumb Rule–
Weight of Water = 28 % wt of cement + 4 % total wt of aggregates
OR
Weight of Water = 30 % wt of cement + 5 % total wt of aggregates
Above thumb rules are applicable only when ordinary concrete is used and the materials are dry and non-absorbent.
FAQ
What is the standard water cement ratio?
Water cement ratios between 0.4 to 0.6 are used. Water cement ratio above 0.6 makes concrete too flowy and strength is reduced drastically. Water cement ratio below 0.4 gives stiff concrete which has very low workability to handle the concreting operations. In such cases, admixtures enhancing flowability can be added to produce high strength concrete.
What is the minimum water cement ratio?
For concrete, the minimum water cement ratio is 0.4 and not 0.38. For cement mortar, the minimum water cement ratio is less than 0.4.
Why is water cement ratio important?
Water cement ratio decides the quantity of water to be added per unit weight of cement. Higher water content reduces strength and durability of concrete while lower water cement ratio forms stiff concrete with less workability. Many other qualities like bond strength, resistance to weathering, volume changes due to wetting and drying are also affected by water cement ratio.
What is the effective water cement ratio?
Effective water cement ratio is the ratio of quantity of effective water to the quantity of cement. Effective water is the water mixed minus the water absorbed by the aggregates. In actual, effective water cement ratio governs the concrete properties.