Water Cement ratio

Role of water in cement

When cement is mixed with water, hydrated cement paste is formed. It consists of 3 parts:

• Hydration products
• Anhydrous cement
• Capillary pores

As cement hydration progresses, the amount of capillary pores decree. However, when the hydration process is completed. Anhydrous cement will disappear and hydrous cement and capillary pores will remain. Therefore, higher the water content added to cement, higher the amount of capillary pores in the cement.

As the capillary pores in cement paste reduces, the strength increases and permeability of the concrete also decreases. Therefore, in order to achieve higher strength, and less permeability, it is very important the amount of water used in cement, i.e. water should be proportionate with cement content.

Water to cement ratio

Water-cement ratio of weight of water to the weight of cement used in a concrete mix. It has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability. The water-cement ratio is independent of the total cement content (and the total water content) of a concrete mix.

The outcome of not maintaining a proper water to cement ratio would be:

• Not achieving required compressive strength of concrete.
• Reduction of concrete durability due to higher permeability.
• Loss of fresh concrete properties.

The relationship between the strength and water cement ratio was established by Duff Abrams in 1918 as a result of extensive testing at the Lewis Institute, University of Illinois. Popularly known as Abram’s water/cement ratio rule, this inverse relation is represented as follows:

f_c = k₁/k₂^w/c

W/C represent the water/cement ratio of the concrete mixture and k1 and k2 are empirical constants.

Understand the role or Water Cement

The water cement ratio helps us determine the strength and durability of the concrete.

Relationship of water/cement ratio with other elements in concreting.

In design a concrete mix, there has to be a reasonable balance between workability, strength, durability and cost consideration. How do we achieve this?

• Workability of concrete can be expressed in terms of consistency and cohesiveness.
• Consistency of the mix is measured in terms or the slump of the mix (i.e. wetness of the mix).
• For given slump, the water requirement generally decreases when.
• The maximum size of a well graded aggregate in increased.
• The content of angular and rough textured particles in aggregates is reduced.
• The amount of entrained air in the concrete mixture is increased,

• Cohesiveness is a measure of compactability which is generally evaluated by trowelability and visual judgment of resistance to segregation.
• If the cohesiveness is poor, the sand/ coarse aggregate proportion or partial replacement of coarse sand with a finer and increase of cement/ aggregate ratio at the given water cement ratio may be done.
• However, past experience and visual judgment supports in deciding the correct water content in the mix.

The W/C ratio and durability

The W/C ratio affects the porosity and thereby the durability of the concrete, the higher the porosity, permeability to many external chemicals and substances is increased. This results in faster deterioration of concrete. 


SUMMARY

• W/C ratio is one of the most important factors in making concrete and plays an important role with cement in concrete. 
• When excess water to cement ratio is used it affects to the concrete strength and durability. 
• As a rule of thumb, the lower the water content, the better the strength and durability. Although water is required for the workability of concrete and for cement hydration reaction, it is should be noted that achieve-ment of workability by adding more water should be avoided as much as possible. Use of correct water to cement ratio will ensure enhanced performance of concrete. 

• In designing a concrete mix, there has to be a reasonable balance between workability, strength, durability and cost consideration.