Question

Mix Design -2: Grade : M30 Workability : 120mm Specific gravity of Cement: 3.05 Specific gravity of CA: 2.75% Specific gravity of FA = 2.62 Water absorption of CA = 0.75% & FA = 1.25% Assume any necessary data suitably

Answer 1

Mix proportioning for a concrete of M30 grade
A·I STIPULATIONS FOR PROPORTIONING

a) Grade designation : M30
b) Type of cement : OPC 53 Grade conforming IS 12269
c) Maximum nominal size of aggregate : 20mm
d) Minimum cement content : 320 kg/m3
e) Maximum water-cement ratio : 0.45
f) Workability : 120mm slump
g) Exposure condition : Moderate (For Reinforced Concrete)
h) Method of concrete placing : Pumping
j) Degree of supervision : Good
k) Type of aggregate : Crushed Angular Aggregates
m) Maximum cement content : 360 kg/m3
n) Chemical admixture type : Super Plasticizer ECMAS HP 890

A-2 TEST DATA FOR MATERIALS
a) Cement used : OPC 53 Grade conforming IS 12269

b) Specific gravity of cement : 3.05

c) Chemical admixture : Super Plasticizer conforming to IS 9103 (ECMAS HP 890)

d) Specific gravity of

1) Coarse aggregate 20mm : 2.67
2) Fine aggregate : 2.65
3) GGBS : 2.84 (JSW)

e) Water absorption:

1) Coarse aggregate : 0.75%
2) Fine aggregate (M.sand) : 1.25%

A-3 TARGET STRENGTH FOR MIX PROPORTIONING
f’ck =fck + 1.65 s
where
f’ck = target average compressive strength at 28 days,
fck = characteristics compressive strength at 28 days, and
s = standard deviation.

Standard Deviation, s = 5 N/mm2. Therefore, target strength = 30 + 1.65 x 5 = 38.25 N/mm2.

A-4 SELECTION OF WATER•CEMENT RATIO
Adopted maximum water-cement ratio = 0.44. From the Table 5 of IS 456 for Very severe Exposure maximum Water Cement Ratio is 0.45 0.44 < 0.45 Hence ok.

A-5 SELECTION OF WATER CONTENT
Maximum water content for 20 mm aggregate = 186 litre (for 25 to 50 mm slump range) Estimated water content for 100 mm slump = 186+ (6/186) = 197 litre.

(Note: If Super plasticizer is used, the water content can be reduced upto 20% and above.)

Based on trials with Super plasticizer water content reduction of 20% has been achieved, Hence the arrived water content = 197-[197 x (20/100)] = 158 litre.

A-6 CALCULATION OF CEMENT CONTENT
Adopted w/c Ratio = 0.44
Cement Content = 158/0.44 = 359 kg/m3
From Table 5 of IS 456, Minimum cement content for ‘Moderate’ exposure conditions 320kg/m3
= 359 kg/m3 > 340 kg/m3 hence ok.

A-7 PROPORTIO0N OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT
Volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate (Zone II) for water-cement ratio of 0.50 =0.62 .

In the present case water-cement ratio is 0.44. Therefore, volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As the water-cement ratio is lower by 0.06. The proportion of volume of coarse aggregate is increased by 0.02 (at the rate of -/+ 0.01 for every ± 0.05 change in water-cement ratio).

Therefore, corrected proportion of volume of coarse aggregate for the water-cement ratio of 0.44 = 0.64

NOTE – In case the coarse aggregate is not angular one, then also volume of coarse aggregate may be required to be increased suitably based on experience & Site conditions.

For pumpable concrete these values should be reduced up to 10%. Therefore, volume of coarse aggregate =0.64 x 0.9 =0.576.

Volume of fine aggregate content = 1 – 0.576= 0.424.

A-8 MIX CALCULATIONS
The mix calculations per unit volume of concrete shall be as follows:
a) Volume of concrete = 1 m3

b) Volume of cement = [Mass of cement] / {[Specific Gravity of Cement] x 1000}
= 359/{3.05 x 1000}
= 0.117m3

c) Volume of water = [Mass of water] / {[Specific Gravity of water] x 1000}
= 158/{1 x 1000}
= 0.158m3

d) Volume of chemical admixture = 1.75 litres/ m3 (By Trial and Error Method used 0.4% by the weight cement)

e) Volume of all in aggregate = [a-(b+c+d)]
= [1-(0.114+0.158+0.004)]
= 0.724m3

f) Mass of coarse aggregate= e x Volume of Coarse Aggregate x Specific Gravity of Fine Aggregate x 1000
= 0.724x 0.576 x 2.67 x 1000
= 1113 kg/m3

g) Mass of fine aggregate= e x Volume of Fine Aggregate x Specific Gravity of Fine Aggregate x 1000
= 0.724x 0.576x 2.60 x 1000
= 798 kg/m3

A-9 MIX PROPORTIONS
Cement = 359 kg/m3
GGBS = 72 kg/m3 (20% By Total weight of Cement)
Water = 158 l/m3
Fine aggregate = 798 kg/m3 Coarse aggregate 20mm = 882 kg/m3
12mm = 223 kg/m3 (20% By Total weight of Coarse Aggregate)
Chemical admixture = 1.34 kg/m3 (0.4% by the weight of cement)
Density of concrete = 2430 kg/m3
Water-cement ratio = 0.47
Mix Proportion By weight = 1:2.21:3.09

NOTE – Aggregates should be used in saturated surface dry condition. If otherwise, when computing the requirement of mixing water, allowance shall be made for the free (surface) moisture contributed by the fine and coarse aggregates. On the other hand, if the aggregates are dry the amount of mixing water should be increased by an amount equal to the moisture likely to be absorbed by the aggregates. Necessary adjustments are also required to be made In mass of aggregates. The surface water and percent water absorption of aggregates shall be determined

A-10 The slump shall he measured and the water content and dosage of admixture shall be adjusted for achieving the required s lump based on trial , if required. The mix proportions shall he reworked for the actual water content and checked for durability requirements.