Thursday, December 10, 2015

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:

fc = k1/k2w/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. 


Manufactured Sand

What is M – Sand?


          M Sand or manufactured sand, is the substitute in place of natural sand which is being used in the production of concrete, worldwide. The extraction of natural sand and gravel has become a heavily taxes activity due to the emerging scarcity and tendency to introduce environmental hazards. However, with its various advantageous characteristics such as compliance with local regulations and product consistency, M sand has emerged as a product of high demand.


Why it’s better?

          Upon closer observations, it is found that the shape of M sand, effect of micro – fines on concrete, characteristics such as modulus of elasticity, shrinkage and creep are of vital importance.
However, when compared to quarry dust, crusher operated M sand is processed commodity obtained under well controlled production of aggregates with fine particles size distribution, improved size and surface texture.

          On the other hand, quarry dust is the byproduct formed during the production of crusher aggregates and it is often angular, sub angular and flaky in shape, unstable in surface texture and likely to be quite improve.

               Moreover, concrete produced with M sand bares higher flexural strength, improved abrasion resistance, higher unit weight and low permeability due to the fine fitting of pores in between concrete with micro fines.


Comparison between M Sand, Natural Sand and Quarry Dust

Properties
M Sand
Natural sand
Quarry Dust
Colour
Grey
Tan
Grey
Particle shapes
Rounded, oval, octagonal, cubical
Rounded, oval, cubical
Flaky
Product
Manufactured as per BS, ASTM etc. standards
Naturally occurring granular material composed of finely divided rocks and minerals
Elongated (shapeless)
1)      Fractured dust of jaw crusher
2)      Waste product of stone crushing
Manufactured process
A controlled crushing process with international technology and imported machines
Extracted from naturally occurring sources such as rivers etc.
No controlled manufacturing process since it is the by-product of a stone crusher
Clay content
0 – 0.75%
1 – 10%
0.5 – 6.0%
Flowability
Good
Good
Poor
Shape index
16.06
23
26.7
Gradation
Complies with BS, ASTM etc. standards
Complies with BS, ASTM etc. standards
Does not comply with any standards
Strength
Recommended for use in construction industry due to high compressive and tensile strength. Manufactured to requirement
Recommended for use in concrete and masonry work worldwide
Not recommended for use in industry since it has poor quality
Environmental impact
None
High
None

 

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