Mix Design of Concrete:
The process of selecting
suitable ingredients of concrete and determining their relative amounts with
the objective of producing a concrete of the required, strength, durability and
workability as economically as possible, is termed the concrete mix design.
Ø Types of Mixes:
1. Nominal Mixes
In the past specifications
for concrete prescribed the proportions of cement, fine and coarse aggregates.
These mixes of fixed cement aggregate ratio which ensures adequate strength are
termed nominal mixes.
2. Standard Mixes
The nominal mixes of fixed
cement –aggregates ratio (by volume) vary widely in strength and may result in
under or over rich mixes. For this reason, the minimum compressive strength has
been included in many specifications. These mixes are termed as standard mixes.
IS:456-2000 has designated
the concrete mixes into a number of grades as M10,M15,M20,M25,M30,M35 &
M40.In this designation the letter ‘ M ‘ refers to the mix and the number to
the specified 28 days cube strength of mix in N/mm .
3.Designed Mix
In these mixes the
performance of the concrete is specified by the designer but the mix
proportions are determined by the producer of concrete, except that the minimum
cement content can be laid down. This is most rational approach to the
selection of mix proportions with specific materials in mind possessing more or
less unique characteristics. The approach results in the production of concrete
with the appropriate properties most economically.
Ø Design of M30
concrete mix as per IS: 10262-2009, Concrete mix proportioning-guidelines:
a) Grade
designation : M30
b) Type
of
cement : OPC
43 grade confirming to IS 8112
c) Maximum
nominal size of aggregates : 20 mm
d) Minimum
cement
content : 320
kg/m
e) Maximum
water cement ratio : 0.50
f) Exposure
condition : Moderate
g) Maximum
cement
content : 450
kg/m
· TEST DATA FOR MATERIALS
i) Cement
used : OPC 43 grade
confirming to IS 8112
ii) Specific gravity of
cement : 3.15
iii) Specific gravity of
Coarse
aggregate : 2.68
Fine
aggregate : 2.65
iv) Water absorption
Coarse
aggregate : 0.6 percent
Fine
aggregate : 1.0 percent
v) Free (surface) moisture
Coarse
aggregate : Nil (absorbed moisture full)
Fine
aggregate : Nil
vi) Sieve analysis
Coarse
aggregate : Conforming to Table 2 of IS: 383
Fine
aggregate : Conforming to
Zone I of IS: 383
· TARGET STRENGTH FOR MIX
PROPORTIONING
f’ck = fck + 1.65 s
Where
f’ck = Target average
compressive strength at 28 days,
fck = Characteristic
compressive strength at 28 days,
s= Standard deviation
From Table 1 standard
deviation, s = 5 N/mm2
Therefore target strength =
30 + 1.65 x 5 = 38.25 N/mm
· SELECTION OF WATER CEMENT RATIO
From Table 5 of IS:
456-2000, maximum water cement ratio = 0.50 (Moderate exposure)
· SELECTION OF WATER CONTENT
From Table-2, maximum water
content = 186 liters (for 25mm – 50mm slump range and for
20 mm aggregates)
Estimated water content for
25-50 mm slump = 186 liters
· CALCULATION OF CEMENT CONTENT
Water cement ratio = 0.45
Cement content = 186/0.45 =
413 kg/m3 >320 kg/m3(given)
From Table 5 of IS: 456,
minimum cement content for moderate exposure condition = 300 kg/m3
Hence OK
· MIX CALCULATIONS
The mix calculations per
unit volume of concrete shall be as follows
i)Volume of
concrete = 1 m3
ii) Volume of
cement
=
[413/3.15] x [1/1000]
=
0.131 m3
iii)Volume of water =
[186/1] x [1/1000]
=
0.186 m3
iv) Volume of all in
aggregates (e) =a – (b + c)
=
1 – (0.131 + 0.186)
=
0.683 m3
v) Volume of coarse
aggregates = e x Volume of CA x specific gravity of
CA
=
0.683 x 0.61 x 2.68 x 1000
=
1117 kg
l) Volume of fine
aggregates = e
x Volume of FA x specific gravity of FA
=
0.683 x 0.39 x 2.65 x 1000
=
706 kg
· MIX PROPORTIONS FOR TRIAL NUMBER
Cement = 413 kg/m3
Water = 186 kg/m3
Fine aggregate = 706 kg/m3
Coarse aggregates = 1117
kg/m3
Water cement ratio = 0.45
