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
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