CONCRETE MIX
DESIGN PROCEDURE & EXAMPLE IS456
DESIGN PROCEDURE & EXAMPLE IS456
Concrete mix
design is the process of finding the proportions of concrete mix in terms of
ratios of cement, sand 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 concrete mix design proportions are either
by volume or by mass. The water-cement ratio is usually expressed in mass.
design is the process of finding the proportions of concrete mix in terms of
ratios of cement, sand 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 concrete mix design proportions are either
by volume or by mass. The water-cement ratio is usually expressed in mass.
Requirements
for concrete mix design:
for concrete mix design:
The grade
designation giving the characteristic strength requirement of concrete.
designation giving the characteristic strength requirement of concrete.
The type of
cement influences the rate of development of compressive strength of concrete.
cement influences the rate of development of compressive strength of concrete.
Maximum
nominal size of aggregates to be used in concrete may be as large as possible
within the limits prescribed by IS 456:2000.
nominal size of aggregates to be used in concrete may be as large as possible
within the limits prescribed by IS 456:2000.
The cement
content is to be limited from shrinkage, cracking and creep.
content is to be limited from shrinkage, cracking and creep.
The
workability of concrete for satisfactory placing and compaction is related to
the size and shape of section, quantity and spacing of reinforcement and
technique used for transportation, placing and compaction.
workability of concrete for satisfactory placing and compaction is related to
the size and shape of section, quantity and spacing of reinforcement and
technique used for transportation, placing and compaction.
Procedure for Concrete Mix
Design as per IS 456 : 2000 :
Design as per IS 456 : 2000 :
1. Determine the mean
target strength ft from the specified characteristic compressive strength at
28-day fck and the level of quality control.
target strength ft from the specified characteristic compressive strength at
28-day fck and the level of quality control.
ft = fck + 1.65 S
where S is the
standard deviation obtained from the Table of approximate contents given after the
design mix.
standard deviation obtained from the Table of approximate contents given after the
design mix.
2. Obtain the water
cement ratio for the desired mean target using the empirical relationship
between compressive strength and water cement ratio so chosen is checked
against the limiting water cement ratio. The water cement ratio so chosen is
checked against the limiting water cement ratio for the requirements of
durability given in table and adopts the lower of the two values.
cement ratio for the desired mean target using the empirical relationship
between compressive strength and water cement ratio so chosen is checked
against the limiting water cement ratio. The water cement ratio so chosen is
checked against the limiting water cement ratio for the requirements of
durability given in table and adopts the lower of the two values.
3. Estimate the amount
of entrapped air for maximum nominal size of the aggregate from the table.
of entrapped air for maximum nominal size of the aggregate from the table.
4. Select the water
content, for the required workability and maximum size of aggregates (for
aggregates in saturated surface dry condition) from table.
content, for the required workability and maximum size of aggregates (for
aggregates in saturated surface dry condition) from table.
5. Determine the
percentage of fine aggregate in total aggregate by absolute volume from table
for the concrete using crushed coarse aggregate.
percentage of fine aggregate in total aggregate by absolute volume from table
for the concrete using crushed coarse aggregate.
6. Adjust the values
of water content and percentage of sand as provided in the table for any
difference in workability, water cement ratio, grading of fine aggregate and
for rounded aggregate the values are given in table.
of water content and percentage of sand as provided in the table for any
difference in workability, water cement ratio, grading of fine aggregate and
for rounded aggregate the values are given in table.
7. Calculate the
cement content form the water-cement ratio and the final water content as
arrived after adjustment. Check the cement against the minimum cement content
from the requirements of the durability, and greater of the two values is
adopted.
cement content form the water-cement ratio and the final water content as
arrived after adjustment. Check the cement against the minimum cement content
from the requirements of the durability, and greater of the two values is
adopted.
8. From the quantities of water and cement per unit volume of
concrete and the percentage of sand already determined in steps 6 and 7 above,
calculate the content of coarse and fine aggregates per unit volume of concrete
from the following relations:
concrete and the percentage of sand already determined in steps 6 and 7 above,
calculate the content of coarse and fine aggregates per unit volume of concrete
from the following relations:
where V = absolute volume of concrete = gross volume (1m3) minus the volume of
entrapped air
entrapped air
Sc = specific gravity
of cement
of cement
W = Mass of water per
cubic meter of concrete, kg
cubic meter of concrete, kg
C = mass of cement per
cubic meter of concrete, kg
cubic meter of concrete, kg
p = ratio of fine
aggregate to total aggregate by absolute volume
aggregate to total aggregate by absolute volume
fa, Ca = total masses
of fine and coarse aggregates, per cubic meter of concrete, respectively, kg,
and
of fine and coarse aggregates, per cubic meter of concrete, respectively, kg,
and
Sfa, Sca = specific gravity of saturated surface dry fine and coarse
aggregates, respectively
aggregates, respectively
9. Determine the
concrete mix proportions for the first trial mix.
concrete mix proportions for the first trial mix.
10. Prepare the
concrete using the calculated proportions and cast three cubes of 150 mm size
and test them wet after 28-days moist curing and check for the strength.
concrete using the calculated proportions and cast three cubes of 150 mm size
and test them wet after 28-days moist curing and check for the strength.
11. Prepare trial mixes
with suitable adjustments till the final mix proportions are arrived at.
with suitable adjustments till the final mix proportions are arrived at.
CONCRETE MIX DESIGN EXAMPLE –
M50 GRADE CONCRETE
M50 GRADE CONCRETE
Grade Designation = M-50
Type of cement = O.P.C-43 grade
Brand of cement = Vikram ( Grasim )
Admixture = Sika [Sikament 170 ( H ) ]
Fine Aggregate = Zone-II
Type of cement = O.P.C-43 grade
Brand of cement = Vikram ( Grasim )
Admixture = Sika [Sikament 170 ( H ) ]
Fine Aggregate = Zone-II
Sp. Gravity
Cement = 3.15
Fine Aggregate = 2.61
Coarse Aggregate (20mm) = 2.65
Coarse Aggregate (10mm) = 2.66
Cement = 3.15
Fine Aggregate = 2.61
Coarse Aggregate (20mm) = 2.65
Coarse Aggregate (10mm) = 2.66
Minimum Cement (As per contract) =400 kg / m3
Maximum water cement ratio (As per contract) = 0.45
Maximum water cement ratio (As per contract) = 0.45
Concrete Mix Design Calculation: –
1.
Target Mean Strength = 50 + ( 5 X 1.65 ) = 58.25 Mpa
Target Mean Strength = 50 + ( 5 X 1.65 ) = 58.25 Mpa
2.
Selection of water cement ratio:
Assume water cement
ratio = 0.35
ratio = 0.35
3.
Calculation of water content:
Approximate
water content for 20mm max. Size of aggregate = 180 kg /m3 (As per Table No. 5 , IS : 10262 ). As plasticizer is proposed
we can reduce water content by 20%.
water content for 20mm max. Size of aggregate = 180 kg /m3 (As per Table No. 5 , IS : 10262 ). As plasticizer is proposed
we can reduce water content by 20%.
Now
water content = 180 X 0.8 = 144 kg /m3
water content = 180 X 0.8 = 144 kg /m3
4. Calculation of cement content:
Water
cement ratio = 0.35
Water content per m3 of concrete = 144 kg
Cement content = 144/0.35 = 411.4 kg / m3
Say cement content = 412 kg / m3 (As per contract
Minimum cement content 400 kg / m3 )
Hence O.K.
cement ratio = 0.35
Water content per m3 of concrete = 144 kg
Cement content = 144/0.35 = 411.4 kg / m3
Say cement content = 412 kg / m3 (As per contract
Minimum cement content 400 kg / m3 )
Hence O.K.
5. Calculation of Sand & Coarse Aggregate Quantities:
Volume
of concrete = 1 m3
Volume of cement = 412 / ( 3.15 X 1000 ) = 0.1308 m3
Volume of water = 144 / ( 1 X 1000 ) = 0.1440 m3
Volume of Admixture = 4.994 / (1.145 X 1000 ) = 0.0043 m3
Total weight of other materials except coarse aggregate = 0.1308 + 0.1440
+0.0043 = 0.2791 m3
of concrete = 1 m3
Volume of cement = 412 / ( 3.15 X 1000 ) = 0.1308 m3
Volume of water = 144 / ( 1 X 1000 ) = 0.1440 m3
Volume of Admixture = 4.994 / (1.145 X 1000 ) = 0.0043 m3
Total weight of other materials except coarse aggregate = 0.1308 + 0.1440
+0.0043 = 0.2791 m3
Volume
of coarse and fine aggregate = 1 – 0.2791 = 0.7209 m3
Volume of F.A. = 0.7209 X 0.33 = 0.2379 m3 (Assuming 33% by volume of total aggregate )
of coarse and fine aggregate = 1 – 0.2791 = 0.7209 m3
Volume of F.A. = 0.7209 X 0.33 = 0.2379 m3 (Assuming 33% by volume of total aggregate )
Volume of C.A. = 0.7209 – 0.2379 = 0.4830 m3
Therefore
weight of F.A. = 0.2379 X 2.61 X 1000 = 620.919 kg/ m3
weight of F.A. = 0.2379 X 2.61 X 1000 = 620.919 kg/ m3
Say
weight of F.A. = 621 kg/ m3
weight of F.A. = 621 kg/ m3
Therefore
weight of C.A. = 0.4830 X 2.655 X 1000 = 1282.365 kg/ m3
weight of C.A. = 0.4830 X 2.655 X 1000 = 1282.365 kg/ m3
Say
weight of C.A. = 1284 kg/ m3
weight of C.A. = 1284 kg/ m3
Considering
20 mm: 10mm = 0.55: 0.45
20mm = 706 kg .
10mm = 578 kg .
Hence Mix details per m3
Increasing cement, water, admixture by 2.5% for this trial
20 mm: 10mm = 0.55: 0.45
20mm = 706 kg .
10mm = 578 kg .
Hence Mix details per m3
Increasing cement, water, admixture by 2.5% for this trial
Cement = 412 X 1.025 = 422 kg
Water = 144 X 1.025 = 147.6 kg
Fine aggregate = 621 kg
Coarse aggregate 20 mm = 706 kg
Coarse aggregate 10 mm = 578 kg
Admixture = 1.2 % by weight of cement = 5.064 kg.
Water = 144 X 1.025 = 147.6 kg
Fine aggregate = 621 kg
Coarse aggregate 20 mm = 706 kg
Coarse aggregate 10 mm = 578 kg
Admixture = 1.2 % by weight of cement = 5.064 kg.
Water: cement: F.A.: C.A. = 0.35: 1: 1.472: 3.043
Observations from Concrete Mix
Design: –
Design: –
A. Mix was cohesive and homogeneous.
B. Slump = 120 mm
C. No. of cube casted = 9 Nos.
7 days average compressive strength = 52.07 MPa.
28 days average compressive strength = 62.52 MPa which is greater than 58.25MPa
Hence the mix accepted.
B. Slump = 120 mm
C. No. of cube casted = 9 Nos.
7 days average compressive strength = 52.07 MPa.
28 days average compressive strength = 62.52 MPa which is greater than 58.25MPa
Hence the mix accepted.
Percentage strength of concrete
at various ages:
The strength of
concrete increases with age. Table shows the strength of concrete different
ages in comparison with the strength at 28 days.
concrete increases with age. Table shows the strength of concrete different
ages in comparison with the strength at 28 days.
|
Age
|
Strength per cent
|
|
1 day
|
16%
|
|
3 days
|
40%
|
|
7 days
|
65%
|
|
14 days
|
90%
|
|
28 days
|
99%
|
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