Examples of concrete mix proportioning
1 Problems 1 - M40 pumpable concrete
A-1 Design stipulations for proportioning
a) Grade designation : M40
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/m3
e) Maximum water cement ratio : 0.45
f) Workability : 100 mm (slump)
g) Exposure condition : Severe (for reinforced concrete)
h) Method of concrete placing : Pumping
i) Degree of supervision : Good
j) Type of aggregate : Crushed angular aggregate
k) Maximum cement content : 450 kg/m3
l) Chemical admixture type : Superplsticiser
A-2 TEST DATA FOR MATERIALS
a) Cement used : OPC 43 grade confirming to IS 8112
b) Specific gravity of cement : 3.15
c) Chemical admixture : Superplasticiser conforming to IS 9103
d) Specific gravity of Coarse aggregate : 2.74 Fine aggregate : 2.74
e) Water absorption Coarse aggregate : 0.5 percent Fine aggregate : 1.0 percent
f) Free (surface) moisture Coarse aggregate : Nil (absorbed moisture also nil) Fine aggregate : Nil
g) Sieve analysis Coarse aggregate : Conforming to Table 2 of IS: 383 Fine aggregate : Conforming to Zone I of IS: 383
A-3 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 = 40 + 1.65 x 5 = 48.25 N/mm2
A-4 SELECTION OF WATER CEMENT RATIO
From Table 5 of IS:456-2000, maximum water cement ratio = 0.45 Based on experience adopt water cement ratio as 0.40 0.4 < 0.45, hence ok
A-5 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 100 mm slump = 186 + 6/100 x186 = 197 liters
As superplsticiser is used, the water content can be reduced up to 20 percent and above
Based on trials with SP water content reduction of 29 percent has been achieved. Hence the water content arrived = 19 x 0.71 =140 liters
A-6 CALCULATION OF CEMENT CONTENT
Water cement ratio = 0.40 Cement content = 140/0.40 = 350 kg/m3 From Table 5 of IS: 456, minimum cement content for severe exposure condition = 320 kg/m3 350 kg/m3 > 320 kg/m3, hence OK
A-7 PROPORTION OF VOLUME OF COARSE AGGREGATE AND FINE AGGREGATE CONTENT
From Table 3, volume of coarse aggregate corresponding to 20 mm size aggregate and fine aggregate (Zone I) for water-cement ratio of 0.50 =0.60
In the present case w/c= 0.40.
The volume of coarse aggregate is required to be increased to decrease the fine aggregate content. As w/c ratio is lower by 0.10, increase the coarse aggregate volume by 0.02 ( at the rate of -/+ 0.01 for every +/- 0.05 change in water cement ratio).
Therefore corrected volume of coarse aggregate for w/c of 0.40 =0.62
Note: In case the coarse aggregate is not angular, then also the volume of CA may be required to be increased suitably based on experience.
For pumpable concrete these values should be reduced by 10 percent Therefore volume of coarse aggregate = 0.62 x 0.9 = 0.56 Volume of fine aggregate content = 1- 0.56 = 0.44
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 = = [350/3.15] x [1/1000] = 0.111 m3
c) Volume of water = [140/1] x [1/1000] = 0.140 m3
d) Volume of chemical admixture = [7/1.145] x [1/1000] = 0.006 m3 ( SP 2%by mass of cement) e) Volume of all in aggregates
(e) =a – (b + c + d) = 1 – (0.111 + 0.140 + 0.006) = 0.743 m3
f) Volume of coarse aggregates = e x Volume of CA x specific gravity of CA = 0.743 x 0.56 x 2.74 x 1000 = 11140 kg
g) Volume of fine aggregates = e x Volume of FA x specific gravity of FA = 0.743 x 0.44 x 2.74 x 1000 = 896 kg
A-9 MIX PROPORTIONS FOR TRIAL NUMBER
Cement = 350 kg/m3
Water = 140 kg/m3
Fine aggregate = 896 kg/m3
Coarse aggregates = 1140 kg/m3
Chemical admixture = 7 kg/m3
Water cement ratio = 0.40
Aggregates are assumed to be in SSD.
Otherwise corrections are to be applied while calculating the water content. Necessary corrections are also required to be made in mass of aggregates.
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