design mix of concrete

1. Concrete Mix Design

2. Concrete Mix Design
• Concrete mix design may be defines as the art of selecting
suitable ingredients of concrete and determining their relative
proportions with the object of producing concrete of certain
minimum strength & durability as economically as possible.

3. Objectives of Mix Design
The purpose of concrete mix design is to ensure the most optimum
proportions of the constituent materials to fulfill the requirement of the
structure being built. Mix design should ensure following objectives:
• To achieve the desired workability in the plastic stage
• To achieve the desired minimum strength in the hardened stage
• To achieve the desired durability in the given environment conditions
• To produce concrete as economically as possible.

4. Basic Considerations
• The following point must be considered while designing concrete
mixes
• Cost
• Specification
• Workability
• Strength and Durability

5. Basic Considerations
Cost
• The cost of concrete is made up of
• Material Cost
• Equipment Cost
• Labor Cost
• The variation in the cost of materials arises from the fact that cement is
several times costlier than aggregates. So it is natural in mix design to aim
at as lean a mix as possible. Therefore, all possible steps should be taken to
reduce the cement content of a concrete mixtures without sacrificing the
desirable properties of concrete such as strength and durability.

6. Basic Considerations
Specifications
• The following point may be kept in mind while designing concrete
mixes
• Minimum Compressive Strength required
• Minimum water/ cement ratio
• Maximum cement content to avoid shrinkage cracks
• Maximum aggregate / cement ratio
• Maximum density of concrete in case of gravity dams

7. Basic Considerations
Workability
• The following points related to workability shall be kept in mind while
designing concrete mixes.
• The consistency of concrete should no more than that necessary for placing,
compacting and finishing.
• For concrete mixes required high consistency at the time of placing, the use
of water-reducing and set-retarding admixtures should be used rather than
the addition of more water
• Wherever possible, the cohesiveness and finishibility of concrete should be
improved by increasing sand/ aggregate ratio than by increasing the
proportion of the fine particles in the sand.

8. Workability

9. Strength and Durability
Strength and durability
• Strength and durability require lower w/c ratio. It is usually achieved
not by increasing the cement content, but by lowering the water at
given cement content. Water demand can by lowered by throughout
control of the aggregate grading and by using water reducing
admixtures.

10. Strength and Durability

11. Grade of Concrete
• The concrete shall be in grades designated
Group Grade designation Characteristics compressive
strength of 150 mm cube at 28 days,
N/mm2
Ordinary Concrete M10
M15
M20
10
15
20
Standard Concrete M25
M30
M35
M40
M45
M50
M55
25
30
35
40
45
50
55
High Strength Concrete M60
M65
M70
M75
M80
60
65
70
75
80

12. What is M 20 ?
• M refers to Mix
• 20 refers to characteristic compressive strength of 150 mm
cube at 28 days in N/mm2
• The minimum Grade of Plain Concrete (PCC) shall be 15
N/mm2
• The minimum grade of reinforced Concrete ( RCC) shall be
20 N/mm2

13. Nominal Concrete Mixes
and
Design mix concrete
Nominal Mix Concrete
• The wide use of concrete as construction materials has led to
the use of mixes of fixed proportion, which ensures adequate
strength. These mixes are called nominal mixes.
• They offer simplicity and under normal circumstances, has
margin of strength above that specified.
• Nominal mix concrete may be used for concrete of grades
M5, M 7.5, M10, M15 and M20.

14. Nominal Concrete Mixes
and
Design mix concrete

15. Proportions of Ingredients in Nominal
Mixes
• The proportions of materials for nominal mix shall be in accordance
Grade Proportions
C: FA: CA
M5 1: 5:10
M 7.5 1:4:8
M 10 1:3:6
M 15 1:2:4
M 20 1:1.5:3

16. Mix Proportion designations
• The common method of expressing the proportions of
ingredients of a concrete mix is in the terms of parts or
ratios of cement, fine and coarse aggregates. For e.g., a
concrete mix of proportions 1:2:4 means that cement, fine
and coarse aggregate are in the ratio 1:2:4 or the mix
contains one part of cement, two parts of fine aggregate and
four parts of coarse aggregate. The proportions are either by
volume or by mass. The water-cement ratio is usually
expressed in mass.

18. Minimum Cement content Maximum Water-Cement ratio and Minimum
Grade of Concrete for different exposures with normal weight of
aggregate of 20mm nominal maximum size.
Sl.
No.
Exposure
Plain Concrete Reinforced Concrete
Minimum
Cement
Content
kg/m3
Maximum
Free
Water
Cement
Ratio
Minimum
Grade of
Concrete
Minimum
Cement
Content
kg/m3
Maximum
Free
Water
Cement
Ratio
Minimum
Grade of
Concrete
i) Mild 220 0.60 - 300 0.55 M20
ii) Moderate 240 0.60 M15 300 0.50 M25
iii) Severe 250 0.50 M20 320 0.45 M30
iv) Very
Severe
260 0.45 M20 340 0.45 M35
v) Extreme 280 0.40 M25 360 0.40 M40

19. Design Mix Concrete
• The concrete mix produced under quality control keeping in view the
strength, durability, and workability is called the design Mix.
• Others factors like compaction equipment's available, curing method
adopted, type of cement, quality of fine and coarse aggregate etc. have
to be kept in mind before arriving at the mix proportion.
• The design mix or controlled mix is being used more and more in
variety of important structures, because of better strength, reduced
variability, leaner mixed with consequent economy, as well as greater
assurance of the resultant quality.

20. Design Mix Concrete

21. Factors Influencing Choice of Mix Design
• According to IS 456:2000 and IS 1343:1980 the important influencing the
design of concrete mix are
• Grade of Concrete
• Type of Cement
• Maximum nominal Size of Aggregate
• Grading of Combined aggregate
• Maximum Water/ Cement Ratio
• Workability
• Durability
• Quality Control.

22. Factors Influencing Choice of Mix Design
Grade of Concrete
• The grade of concrete gives characteristic compressive
strength of concrete. It is one of the important factor
influencing the mix design
• The grade M 20 denotes characteristic compressive strength
fck of 20 N/mm2. Depending upon the degree of control
available at site, the concrete mix is to be designed for a
target mean compressive strength (fck) applying suitable
standard deviation.

23. Factors Influencing Choice of Mix Design

24. Factors Influencing Choice of Mix Design
Type of Cement
• The rate of development of strength of concrete is influenced
by the type of cement.
• The higher the strength of cement used in concrete, lesser
will be the cement content. The use of 43 grade and 53 grade
of cement, gives saving in cement consumption as much as
15 % and 25 % respectively, as compared to 33 grade of
cement. For concrete of grade M25 it is advisable to use 43
and 53 grade of cement.

25. Types of Cement

26. Factors Influencing Choice of Mix Design
Maximum Nominal Size of Aggregates
• The workability of concrete increases with an increase in the maximum size of
aggregate. But the smaller size of aggregates provide larger surface area for
bonding with the mortar matrix which gives higher strength.

27. Factors Influencing Choice of Mix Design
Grading of Combined Aggregates
• The relative proportions of the fine and coarse aggregate in a
concrete mix is one of the important factors affecting the
strength of concrete.
• For dense concrete, it is essential that the fine and coarse
aggregate be well graded. In the case when the aggregate
available from natural sources do not confirm to the specified
grading, the proportioning of two or more aggregate become
essential.

28. Factors Influencing Choice of Mix Design
Maximum Water/ Cement Ratio
• Abram’s water/Cement ratio states that for any given condition of test,
the strength of a workability concrete mix is dependent only on
water/cement ratio. The lower the water/Cement ratio, the greater is
the compressive strength.
Workability
• Workability of fresh concrete determines the case with which a
concrete mixture can be mixed, transported, placed, compacted and
finished without harmful segregation and bleeding.

29. Factors Influencing Choice of Mix Design
Durability
• Durability require low water/Cement ratio. It is usually
achieved not by increasing the cement content, but by
lowering the water demand at a given cement content.
• Water demand can be lowered by through control of the
aggregate grading and by using water reducing admixtures.