GRADE OF CONCRETE &
MIX DESIGN
INTRODUCTION
TO GRADE OF CONCRETE
•
Among the many
properties of concrete, compressive strength is found to be most important
properties
•
It is assumed to
represent the overall quality of concrete
•
Many engineering
properties are generally found to be related to compressive strength of
concrete
•
So, concrete is
mostly graded according to its compressive strength.
•
Hence, grade of
concrete is simply the representation of the compressive strength of concrete
Representation
of grade of concrete
•
Grade of concrete
is represented by letter “M”, which represents mix, followed by number, which
is the characteristic compressive strength of 15 cm cube at 28 days, expressed
in N/mm2
Representation
of grade of concrete
•
For example:
Consider grade of concrete is M20
•
M refers to mix
•
20 refers to the
characteristic compressive strength of 15 cm cube at 28 days, expressed in N/mm2
•
Hence, M20 grade
of concrete has characteristics compressive strength of 20 N/mm2
Code
recommendation for grade of concrete
•
As per IS
456:2000, grade of concrete for RCC used shall not be less than M20
•
As per IS
456:2000, grade of concrete for PCC used shall not be less than M15
•
Grades as per
table 5 of IS 456:2000 code is given below:
EXPOSURE CONDITION
|
PCC
|
RCC
|
Mild
|
-
|
M20
|
Moderate
|
M15
|
M25
|
Severe
|
M20
|
M30
|
Very severe
|
M20
|
M35
|
Extreme
|
M25
|
M40
|
Designation
of Grade of concrete as per code
Group
|
Grade designation
|
Fck
|
Ordinary concrete
|
M10
|
10
|
M15
|
15
|
|
M20
|
20
|
|
Standard concrete
|
M25
|
25
|
M30
|
30
|
|
M35
|
35
|
|
M40
|
40
|
|
M45
|
45
|
|
M50
|
50
|
|
M55
|
55
|
|
High strength concrete
|
M60
|
60
|
M65
|
65
|
|
M70
|
70
|
|
M75
|
75
|
|
M80
|
80
|
Characteristic
strength of concrete
•
Cube test result
for determination of compressive strength of concrete follows normal
distribution
•
It means that
value of strength of material below which not more than 5% of test result
results are expected to fall.
•
Suppose 100 cubes
are tested and specified characteristics strength is 30N/mm2
•
Here 5% of cube
means 5% of 100 = 5 and 95% of 100 = 95
•
So, if strength
of more than 95 cubes is more than 30N/mm2 then 30N/mm2
is the characteristic compressive strength of concrete
TARGET MEAN
STRENGTH OF CONCRETE
•
As per IS code,
Target mean strength of concrete is defined as ft = fck +
KS
•
Where, ft
= target mean strength of concrete
•
fck = Characteristic strength of concrete
•
K is a constant
and derived from mathematics of normal distribution
•
Here K is 1.65
for not more than 5% of test cubes fall under characteristics strength
•
S is standard
deviation
Normal
distribution curve for test result
Mix design
of concrete
•
Mix design can be
defined as the process of selecting suitable ingredients of concrete and
determining their relative proportions with the object of producing concrete of
certain minimum strength and durability as economically as possible.
•
Following two methods
can be used
•
Nominal mix
•
Design mix
Nominal mix
•
In this type of
mix, all the ingredients are prescribed and their proportions are specified.
•
Used for
relatively unimportant work and simple concrete work
•
It is used for
concrete of M20 or lower
•
Water cement
ratio is based on durability criteria, experience & practical trials.
•
Water content can
be modified according to slump value (field-based test)
•
There is no scope
for deviation by designer
•
Volumetric
batching is used
•
It is
prescriptive type
•
No quality controls
•
No entrapped air
content is considered.
Design mix
•
In Design mix,
quantity of each ingredients of concrete are calculated based on site
condition, environment, etc. as per the code
•
It is adopted for important concrete work
•
It is used for
higher grade of concrete
•
Water cement
ratio based on concrete grade & 28 days compressive strength of concrete
& durability of concrete.
•
Water content can
be modified according to compaction factor value (laboratory-based test)
•
Weight batching
is used
•
It is performance
based
•
There is quality
control
•
Entrapped air
content considered according nominal maximum size aggregates.
NOMINAL MIX
FOR different GRADE OF CONCRETE
•
Nominal mix for
some grade of concrete is given below in ratio of
Cement: Sand: Coarse Aggregate
Grade of concrete
|
Ratio (Nominal mix)
|
M10
|
1:3:6
|
M15
|
1:2:4
|
M20
|
1:1.5:3
|
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