What Is Concrete Mix Design?
Mix design is a method which determines the proportions of cement, water, fine aggregates and coarse aggregates to produce the concrete of required strength, workability and durability with minimum cost.
Principle Of Concrete Mix Design:
Cement Concrete Mix Design:
Cement concrete mix design means, determination of the proportion of the concrete ingredients i.e. cement , water, fine aggregate, coarse aggregate which would produce concrete possessing specified properties such as workability, strength and durability with maximum overall economy.
Basic Considerations :
Design involves determination of the proportions of the given constituents namely cement, water, coarse and fine aggregates and admixtures
This would produce concrete possessing specified properties both in the fresh and hardened states with the maximum overall economy.
Workability is specified as the important property of concrete in the fresh state.
For hardened state compressive strength and durability are important. Proportioning of the ingredients of concrete is an important phase as it ensures the quality. Design of plastic concrete of medium strength can be based on following assumptions:
i. Compressive strength is governed by W/C ratio.
ii. Workability of concrete is governed by water content.
Factors Influencing The Choice Of Mix Design Grade Designation:
Type and grade of cement.
Maximum nominal size of coarse aggregates.
Grading of combined aggregates.
It gives characteristic compressive strength requirements of the concrete.
It’s the major factor influencing mix design.
Grade & Type Of Cement:
Choice of type of cement depends upon the requirements at hand.
For very high compressive strength portland cement of grades 43 and 53 used.
Where early strength is required, rapid hardening portland cement is used.
For mass construction low heat portland cement is used.
Cement content in concrete varies inversely with the strength of cement used in preparation of concrete, i.e. higher the strength lesser will be the cement content.
Fineness of cement increases workability by reducing friction between aggregates.
Max Nominal Size Of Coarse Aggregate:
It is determined by sieve analysis.
Maximum nominal size of aggregate used is governed by size of the section and the spacing of the reinforcement.
For the concrete with higher W/C ratio, the larger max size of aggregate is used.
For high strength concrete, 10-20mm size.
Grading Of Combined Aggregate:
Grading is one of the important factor affecting strength of concrete.
The aggregates can be combined by:
i. Analytical method and
ii. Graphical method.
The ratio of water-to-cement, or w/c, is the single most important parameter with regards to concrete quality.
Theoretically, about 0.22 to 0.25 is required for complete hydration.
Practically, the useful limit is around 0.33.
Reducing the water for a given amount of cement will move the cement particles closer together, which in turn densifies the hydrated cement paste.
This increases strength and reduces permeability.
It also makes the concrete more difficult to work.
In combination, the w/c and degree of hydration control many of the properties of the hardened concrete.
Steps to be followed in selecting the water cement ratio:
i. The strength of cement to be used is determined.
ii. When cement strength data are available, corresponding curve is chosen for the determination of W/C.
The workability of concrete for satisfactory placing and compaction is controlled by:
i. Size and shape of section to be concreted.
ii. Quantity and spacing of reinforcement.
iii. Methods employed for transportation, placing and compaction of concrete.
Durability of concrete is the resistance to deteriorating influences which may reside inside the concrete itself, or to the aggressive environments.
Durability can be achieved by restricting the minimum water cement content and the maximum water cement ratio and the type of cement.
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