CE 6002 CONCRETE TECHNOLOGY UNIT III

DEPARTMENT OF CIVIL ENGINEERING
G.GUNA SRVEC

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.
Objectives :
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 designed/ 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.
G.GUNA SRVEC

The following point must be considered while designing concrete mixes
Cost
Specification
Workability
Strength and Durability
Cost
The cost of concrete is made up of
1. Material Cost
2. Equipment Cost
3. Labour 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.
G.GUNA SRVEC

Specifications
The following point may be kept in mind while designing concrete mixes
1. Minimum Compressive Strength required
2. Minimum water/ cement ratio
3. Maximum cement content to avoid shrinkage cracks
4. Maximum aggregate / cement ratio
5. Maximum density of concrete in case of gravity dams
G.GUNA SRVEC

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 finishiability of concrete should be improved by increasing sand/
aggregate ratio than by increasing the proportion of the fine particles in the sand.
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.
G.GUNA SRVEC

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
Grade of Concrete
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
G.GUNA SRVEC

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.
Proportions of Ingredients in Nominal Mixes
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
G.GUNA SRVEC

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.
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 Quality Control.
Maximum Water/ Cement Ratio
Workability Durability
G.GUNA SRVEC

• The proportions of materials for nominal mix shall be in accordance
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.
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.
G.GUNA SRVEC

Maximum Nominal Size of Aggregates
•The maximum size of C.A is determined by sieve analysis. It is designated by the sieve size higher than
larger size on which 15 % or more of the aggregate is retained. The maximum nominal size of C.A. should
not be more than one-forth of minimum thickness of the member.
•For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of
the aggregate should usually be restricted to sum less than the minimum clear distance between the main
bars or 5 mm less the minimum cover to the reinforcement, whoever is smaller.
•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.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.
G.GUNA SRVEC

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
Method of Concrete Mix Design
Some of the commonly used mix design methods are
I.S. Method
A.C.I method
Road Note 4 method ( U.K. Method)
IRC 44 method
Arbitrary method
Maximum Density method
Fineness modulus method
Surface area Method
Nix design for high strength Concrete
Mix design for pumpable Concrete
DOE (British) Mix design method
G.GUNA SRVEC

IS Method of Mix Design
The Bureau of Indian Standards, recommended a set of procedure for design of concrete mix. The procedure is
based on the research work carried out at national laboratories.
Data for mix design
The following basic data are required to be specified for design a concrete mix
Characteristic Compressive strength only a few specified proportions of test results are expected to fall of
concrete at 28 days (fck)
Degree of workability desired
Limitation on water/Cement Ratio with the minimum cement to ensure adequate durability
Type and maximum size of aggregate to be used.
Standard deviations of compressive strength of concrete.
Target Strength for Mix Design
The target average compressive strength (fck) of concrete at 28 days is given by
Fck= f ck + t.s
Where,
Fck= target average compressive strength at 28 days
F ck= characteristics compressive strength at 28 days
s= Standard deviation
t= a stastical value, depending upon the accepted proportion of low results and the number of tests.
G.GUNA SRVEC

The ACI Method of Mix Design
This method assumes that the optimum ratio of the bulk volume of coarse aggregates and on the
grading of fineness aggregates regardless of shape of particles. This method also assumes that
even after complete compaction is done, a definite percentage of air remains which is inversely
proportional to the maximum size of aggregate.
G.GUNA SRVEC

CE 6002 CONCRETE TECHNOLOGY UNIT III

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CE 6002 CONCRETE TECHNOLOGY UNIT III