Penulis
Concrete mix design is the process of finding the right proportions of cement, sand, and aggregates for concrete to achieve target strength in structures. So, the concrete mix design can be stated as Concrete Mix = Cement/Sand/Aggregates.
The concrete mix
design involves various steps, calculations and laboratory testing to find
right mix proportions. This process is usually adopted for structures which
requires higher grades of concrete such as M25 and above and large construction
projects where the quantity of concrete consumption is huge.
Benefits of concrete
mix design is that it provides the right proportions of materials, thus making
the concrete construction economical in achieving the required strength of
structural members. As the quantity of concrete required for large
constructions are huge, the economy in quantity of materials such as cement makes
the project construction economical.
Concrete Mix Design
of M20, M25, M30 and higher grade of concrete can be calculated from example
below.
Concrete Mix Design
Data Required for Concrete Mix Design
(i) Concrete Mix Design Stipulation
(a) Characteristic
compressive strength required in the field at 28 days grade designation — M 25
(b) Nominal maximum
size of aggregate — 20 mm
(c) The shape of CA —
Angular
(d) Degree of
workability required at the site — 50-75 mm (slump)
(e) Degree of quality
control available at the site — As per IS: 456
(f) Type of exposure
the structure will be subjected to (as defined in IS: 456) — Mild
(g) Type of cement:
PSC conforming IS: 455
(h) Method of
concrete placing: pumpable concrete
(ii) Test data of material (to be determined in the laboratory)
(a) Specific gravity
of cement — 3.15
(b) Specific gravity
of FA — 2.64
(c) Specific gravity
of CA — 2.84
(d) Aggregate is
assumed to be in saturated surface dry condition.
(e) Fine aggregates
confirm to Zone II of IS – 383
Procedure for Concrete Mix Design of M25 Concrete
Step 1 — Determination of Target Strength
Harmsworth constant
for a 5% risk factor is 1.65. In this case, the standard deviation is taken from IS:
456 against M 20 is 4.0.
ftarget = fck + 1.65 x S
=
25 + 1.65 x 4.0 = 31.6 N/mm2
Where,
S
= standard deviation in N/mm2 = 4 (as per table -1 of IS 10262-
2009)
Step 2 — Selection of water/cement ratio:-
From Table 5 of IS
456, (page no 20)
Maximum water-cement
ratio for Mild exposure condition = 0.55
Based on experience,
adopt the water-cement ratio as 0.5.
0.5<0.55, hence
OK.
Step 3 — Selection of Water Content
From Table 2 of IS
10262- 2009,
Maximum water content
= 186 Kg (for Nominal maximum size of aggregate — 20 mm)
Table for Correction in water content
Parameters
|
Values as per Standard
reference condition
|
Values as per Present Problem
|
Departure
|
Correction in Water Content
|
Slump
|
25-50 mm
|
50-75
|
25
|
(+3/25) x 25 = +3
|
Shape of Aggregate
|
Angular
|
Angular
|
Nil
|
–
|
Total
|
+3
|
Estimated
water content = 186+ (3/100) x 186 = 191.6 kg /m3
Step 4 — Selection of Cement Content
Water-cement ratio =
0.5
Corrected
water content = 191.6 kg /m3
Cement content =
From Table 5 of IS
456,
Minimum
cement Content for mild exposure condition = 300 kg/m3
383.2
kg/m3 > 300 kg/m3, hence, OK.
This value is to be
checked for the durability requirement from IS: 456.
In
the present example against mild exposure and for the case of reinforced
concrete the minimum cement content is 300 kg/m3 which is less than 383.2
kg/m3. Hence cement
content adopted = 383.2 kg/m3.
As per clause 8.2.4.2
of IS: 456
Maximum
cement content = 450 kg/m3.
Step 5: Estimation of Coarse Aggregate proportion:-
From Table 3 of IS
10262- 2009,
For Nominal maximum
size of aggregate = 20 mm,
Zone of fine
aggregate = Zone II
And For w/c = 0.5
Volume of coarse
aggregate per unit volume of total aggregate = 0.62
Table for correction in the estimation of the coarse aggregate proportion
Parameter
|
Values as per Standard
reference condition
|
Values as per the present problem
|
Departure
|
Correction in Coarse Aggregate
proportion
|
Remarks
|
W/c
|
0.5
|
0.5
|
Nil
|
–
|
See Note 1
|
Workability
|
–
|
pumpable concrete
|
–
|
-10%
|
See Note 2
|
Total
|
-10%
|
Note 1: For every ±0.05
change in w/c, the coarse aggregate proportion is to be changed by 0.01. If the
w/c is less than 0.5 (standard value), the volume of coarse aggregate is required
to be increased to reduce the fine aggregate content. If the w/c is more than
0.5, the volume of coarse aggregate is to be reduced to increase the fine aggregate
content. If the coarse aggregate is not angular, the volume of coarse aggregate maybe
required to be increased suitably, based on experience.
Note 2: For pumpable
concrete or congested reinforcement the coarse aggregate proportion maybe
reduced up to 10%.
Hence,
Volume of coarse
aggregate per unit volume of total aggregate = 0.62 x 90% = 0.558
Volume of fine
aggregate = 1 – 0.558 = 0.442
Step 6: Estimation of the mix ingredients
a)
Volume of concrete = 1 m3
b) Volume of cement =
(Mass of cement / Specific gravity of cement) x (1/100)
=
(383.2/3.15) x (1/1000) = 0.122 m3
c) Volume of water =
(Mass of water / Specific gravity of water) x (1/1000)
=
(191.6/1) x (1/1000) = 0.1916 m3
d)
Volume of total aggregates = a – (b + c) = 1 – (0.122 + 0.1916) = 0.6864 m3
e)
Mass of coarse aggregates = 0.6864 x 0.558 x 2.84 x 1000 = 1087.75 kg/m3
f)
Mass of fine aggregates = 0.6864 x 0.442 x 2.64 x 1000 = 800.94 kg/m3
Concrete Mix proportions for Trial Mix 1
Cement
= 383.2 kg/m3
Water
= 191.6 kg/m3
Fine
aggregates = 800.94 kg/m3
Coarse
aggregate = 1087.75 kg/m3
W/c = 0.5
For trial -1 casting
of concrete in lab, to check its properties.
It will satisfy
durability & economy.
For casting trial -1,
mass of ingredients required will be calculated for 4 no’s cube assuming 25%
wastage.
Volume
of concrete required for 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3
Cement
= (383.2 x 0.016878) kg/m3 = 6.47 kg
Water
= (191.6 x 0.016878) kg/m3 =3.23 kg
Coarse
aggregate = (1087.75 x 0.016878) kg/m3 =18.36 kg
Fine
aggregates = (800.94 x 0.016878) kg/m3 = 13.52 kg
Step 7: Correction due to absorbing / moist aggregate:-
Since the aggregate
is saturated surface dry condition hence no correction is required.
Step 8: Concrete Trail Mixes:-
Concrete Trial Mix 1:
The mix proportion as
calculated in Step 6 forms trial mix1. With this proportion, concrete is
manufactured and tested for fresh concrete properties requirement i.e.
workability, bleeding and finishing qualities.
In this case,
Slump value = 25 mm
Compaction Factor =
0.844
So, from slump test
we can say,
The mix is cohesive,
workable and had a true slump of about 25 mm and it is free from segregation
and bleeding.
Desired slump = 50-75
mm
So modifications are
needed in trial mix 1 to arrive at the desired workability.
Concrete Trial Mix 2:
To increase the
workability from 25 mm to 50-75 mm and an increase in water content by +3% is to be
made.
The corrected water
content = 191.6 x 1.03 = 197.4 kg.
As mentioned earlier
to adjust fresh concrete properties the water-cement ratio will not be changed.
Hence
Cement Content =
(197.4/0.5) = 394.8 kg/m3
Which also satisfies
durability requirement.
Volume
of all in aggregate = 1 – [{394.8/ (3.15×1000)} + {197.4/ (1 x 1000)}] = 0.6773
m3
Mass
of coarse aggregate = 0.6773 x 0.558 x 2.84 x 1000 = 1073.33 kg/m3
Mass
of fine aggregate = 0.6773 x 0.442 x 2.64 x 1000 = 790.3 kg/m3
Concrete Mix Proportions for Trial Mix 2
Cement
= 384.8 kg/m3
Water
= 197.4 kg/m3
Fine
aggregate =790.3 kg/m3
Coarse
aggregate = 1073.33 kg/m3
For casting trial -2,
mass of ingredients required will be calculated for 4 no’s cube assuming 25%
wastage.
Volume
of concrete required for 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3
Cement
= (384.8 x 0.016878) kg/m3 = 6.66 kg
Water
= (197.4 x 0.016878) kg/m3 =3.33 kg
Coarse
aggregate = (1073.33 x 0.016878) kg/m3 =18.11 kg
Fine
aggregates = (790.3 x 0.016878) kg/m3 = 13.34 kg
In this case,
Slump value = 60 mm
Compaction Factor =
0.852
So, from slump test
we can say,
Mix is very cohesive,
workable and had a true slump of about 60 mm.
It virtually flowed
during vibration but did not exhibit any segregation and bleeding.
Desired slump = 50-75
mm
So, it has achieved
desired workability by satisfying the requirement of 50-75 mm slump value.
Now, we need to go
for trial mix-3.
Concrete Trial Mix 3:
In case of trial mix
3 water-cement ratio is varied by +10% keeping water content constant. In the present example, the water-cement ratio is raised to 0.55 from 0.5.
An increase of 0.05
in the w/c will entail a reduction in the coarse aggregate fraction by 0.01.
Hence the coarse
aggregate as percentage of total aggregate = 0.558 – 0.01 = 0.548
W/c = 0.55
Water content will be
kept constant.
Cement content =
(197.4/0.55) = 358.9 kg/m3
Hence, volume of all
in aggregate
=
1 – [{(358.9/ (3.15 x 1000)} + (197.4/1000)] =0.688 m3
Mass
of coarse aggregate = 0.688 x 0.548 x 2.84 x 1000 = 1070.75 kg/m3
Mass
of fine aggregate = 0.688 x 0.452 x 2.64 x 1000 = 821 kg/m3
Concrete Mix Proportions of Trial Mix 3
Cement
= 358.9 kg/m3
Water
= 197.4 kg/m3
FA
= 821 kg/m3
CA
= 1070.75 kg/m3
For casting trial -3,
mass of ingredients required will be calculated for 4 no’s cube assuming 25%
wastage.
Volume
of concrete required for 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3
Cement
= (358.9 x 0.016878) kg/m3 = 6.06 kg
Water
= (197.4 x 0.016878) kg/m3 =3.33 kg
Coarse
aggregate = (1070.75 x 0.016878) kg/m3 =18.07 kg
Fine
aggregates = (821 x 0.016878) kg/m3 = 13.85 kg
In this case,
Slump value = 75 mm
Compaction Factor =
0.89
So, from slump test
we can say,
Mix is stable, cohesive,
and workable and had a true slump of about 75 mm.
Desired slump = 50-75
mm
So, it has achieved
desired workability by satisfying the requirement of 50-75 mm slump value.
Now, we need to go
for trial mix-4.
Concrete Trial Mix 4:
In this case water /
cement ratio is decreased by 10% keeping water content constant.
W/c = 0.45
A reduction of 0.05
in w/c will entail an increase of coarse aggregate fraction by 0.01.
Coarse aggregate
fraction = 0.558 +.01 =.568
W/c
= 0.45 and water content = 197.4 kg/m3
Cement
content = (197.4/0.45) = 438.7 kg/m3
Volume of all in
aggregate
=
1 – [{438.7/(3.15 x 1000)} + (197.4/1000)] = 0.664 m3
Mass
of coarse aggregate = 0.664 x 0.568 x 2.84 x 1000 = 1071.11 kg/m3
Mass
of fine aggregate = 0.664 x 0.432 x 2.64 x 1000 = 757.28 kg/m3
Concrete Mix Proportions of Trial Mix 4
Cement
= 438.7 kg/m3
Water
= 197.4 kg/m3
FA
= 757.28 kg/m3
CA
= 1071.11 kg/m3
For casting trial -4,
mass of ingredients required will be calculated for 4 no’s cube assuming 25%
wastage.
Volume
of concrete required for 4 cubes = 4 x (0.153 x1.25) = 0.016878 m3
Cement
= (438.7 x 0.016878) kg/m3 = 7.4 kg
Water
= (197.4 x 0.016878) kg/m3 =3.33 kg
Coarse
aggregate = (1071.11 x 0.016878) kg/m3 =18.07 kg
Fine
aggregates = (757.28 x 0.016878) kg/m3 = 12.78 kg
A local correction
due to moisture condition of aggregate is again applied on this proportions.
With corrected proportions, three concrete cubes are cast and tested for 28 days
compressive strength.
A summary of all the
trial mixes are given in the following Table.
Recommended mix proportion of ingredients for grade of concrete M25:
From Compressive
Strength vs. c/w graph for target strength 31.6 MPa we get,
W/c = 0.44
Water
content = 197.4 kg/m3
Cement
content = (197.4/0.44) = 448.6 kg/m3
Volume of all in
aggregate
=
1 – [{448.6/ (3.15 x 1000)} + (197.4/1000)] = 0.660 m3
A reduction of 0.05
in w/c will entail and increase of coarse aggregate fraction by 0.01.
Coarse aggregate
fraction = 0.558 +.01 =.568
Volume of fine
aggregate = 1 – 0.568 = 0.432
Mass
of coarse aggregate = 0.660 x 0.568 x 2.84 x 1000 = 1064.65 kg/m3
Mass
of fine aggregate = 0.660 x 0.432 x 2.64 x 1000 = 752.71 kg/m3