The water-to-cement (W/C) ratio in a concrete mix is defined as the ratio of the weight of water to the weight of cement used. It may also refer to the ratio of water to cement
The water-to-cement (W/C) ratio in a concrete mix is defined as the ratio of the weight of water to the
weight of cement used. It may also refer to the ratio of water to cement plus pozzolan (such as fly ash,
blast furnace slag, etc. which are added to strengthen concrete). The process of hardening of concrete
takes place as a result of the chemical reaction between cement and water. This reaction is known as
hydration, and it produces heat energy which is called the heat of hydration.
The quantity of water required is often expressed in terms of liter per bag of cement. For example, if
water required for one bag of cement is 30 liters, the water-cement ratio is equal to 30/50 = 0.6 (where
1 bag of cement weighs 50kg).
Research has shown that it takes only a small amount of water to turn the dry cement-aggregate mix
into concrete. For example, only 0.35 unit of water is needed for each unit of cement to fully complete
hydration reactions. However, such a mix with a ratio of 0.35 may not flow as desired, which makes it
difficult to work with. Water is further added to the mix for basically two reasons: to provide sufficient
flow in the wet mix for transportation and finishing, and to ensure that the entire mix is sufficiently
moistened. Hence, water-cement ratios of 0.45 to 0.6 are more commonly used.
Factors determining W/C ratio:
While selecting the water-cement ratio, various factors need to be considered in order to obtain the
maximum possible strength from a particular mix. Some of them are:
The type of cement used
The type of structure to be constructed
Properties of aggregates (like bulk density and moisture absorption)
The durability and strength of concrete mainly depends on the water consumption. Thus the quantity of
water to be added is a very important factor and needs to be carefully selected while mixing concrete.
Generally, the lower the ratio, the greater is the strength of concrete. One simple way of understanding
the W/C ratio is that greater the amount of water in a concrete mix, the more dilute the cement paste
will be, and consequently, weaker the strength.
Low W/C ratio:
A lower water-cement ratio leads to higher strength and durability, but at the same time, it makes the
mix difficult to work with and apply. The resultant concrete will be nearly dry, hard to place in the form
and may also create difficulties in compaction. When high-strength concrete is required, lower ratios are
usually used along with a plasticizer or super-plasticizer to increase the flowability. Also known as water-
reducers, plasticizers help to reduce the total water-to-cement ratio, by giving it a more liquid
consistency without the need to dilute the mix with water. Plasticizers are usually added during the
mixing process. They also have a temporary dispersing effect, which allows for a thorough hydration of
each cement particle.
High W/C ratio:
Too much water leads to segregation of the sand and other aggregate components from the cement
paste. Microscopic pores also form on the surface when water that is not consumed by the hydration
reaction leaves the concrete as it hardens. This is known as bleeding and it reduces the final strength of
concrete. The amount of shrinkage also increases as the amount of water added to the mix increases.
When additional water is added to the mix than is required, it begins to leave a void space as it hydrates
and evaporates, thus leaving a fugitive volume. The micro channels and passages that were created
inside the concrete to allow that water to flow become weak zones and micro-cracks. Thus concrete
with high W/C ratio may become more susceptible to cracking and shrinkage.
For a designed mix, water cement ratio is considered based on the strength and workability
requirement. Different types of concrete construction and different structural members require
different workability for proper mixing, transportation, placement and compaction of concrete. Thus,
the water-cement ratio chosen should be such that it imparts a reasonable degree of workability.