A. Concrete Mix Design Stipulation
- Grade designation: — M30
- Type of cement: — OPC 43 grade
- Maximum nominal size of aggregate: — 20mm
- Minimum cement content: — 320 Kg ……….…. (From Table 5 of IS 456:2000)
- Maximum water-cement ratio: — 0.45 ………… (From Table 5 of IS 456:2000)
- Degree of Workability: — 100mm slump / 0.92 compacting factor
- Exposure condition: — Sever
- Method of concrete placing: — Pumping
- Degree of supervision: — Good
- Type of aggregate: — Crushed angular aggregate
- Maximum cement content: — 450 kg/m^{3}
- Chemical admixture type: — Superplasticizer
B. Test Data of Material (to be Determined in the Laboratory)
- Specific gravity of cement: — 3.15
- Compressive strength of cement at 7 days: — Satisfies the requirement of IS: 269–1989
- (i). Specific gravity of coarse aggregates: — 2.60 (ii). Specific gravity of fine aggregates: — 2.60
- Water absorption (i). Coarse aggregate: — 0.50% (ii). Fine aggregate: — 1%
- Sieve analysis: — Conforming to grading Zone III (Table 2 & 4, IS: 383–1970)
- Free (surface) moisture
1. Coarse aggregate: — Null
2. Fine aggregate: — 2%
Now Start calculation procedure —
Step 1 — Determination of Target Strength—
The target mean strength for specified characteristic cube strength is
Target mean strength f’_{ck}= f_{ck} + kσ
Where,
f_{ck} = characteristic strength below which certain percentage of test results are expected to fall.
k = Himsworth constant for 5% risk factor is 1.65_{. }
σ = Standard deviation is 5.0 M30 (from table 8 of IS:456-2000)
Target mean strength f’_{ck}= 30 + 1.65×5 = 38.25MPa
Step 2 — Selection of water / cement ratio—
Based on experience, water-cement ratio required for the target mean strength of 38.25 MPa is 0.40
Maximum water-cement ratio for Mild exposure condition = 0.45 (From Table 5 of IS 456 -2000)
0.40<0.45, hence OK.
Step 3 — Selection of Water and sand Content—
From Table 2 of IS:10262 – 2009, for 20 mm maximum size aggregate and sand grading for zone II –
maximum water content =186 litre (for 25 to 50 mm slump range)
sand content = 35% of total aggregate.
Table for Correction required in water and sand content
Change in Parameters (per Standard reference condition) | Departure | Correction in Water Content | Correction in Sand Content |
For decrease in water-cement ratio by (0.60–0.40) that is 0.20 | 0.20 | – | –2.0 |
Slump (25-50 mm) to 100 mm | 50 | (3/25) x 50 = +6 | – |
For sand conforming to Zone III of Table 4, IS: 383–1970 | – | – | –1.5 |
Total | – | +6 | –3.5 |
For change in water-cement ratio, workability degree / compacting factor, and sand belonging to Zone III
Estimated water content for 100 mm slump =186+6/100 x 186
= 186 + 11 = 197 litre
As superplasticizer is used as admixture, the water content can be reduced up 20% or more.
Based on trials with superplasticizer water content reduction of 29 percent has been achieved. Hence, the arrived water content =197 x 0.71 =140 litre
Therefore, required sand content as percentage of total aggregate by absolute volume = 35 – 3.5 = 31.5%
Step 4 — Selection of Cement Content
Water-cement ratio = 0.40
Corrected water content = 197kg / m^{3}
Cement content = 140/0.40 =350 kg/m^{3}
From Table 5 of IS 456, minimum cement content for ‘severe’ exposure condition = 320 kg/m3
350 kg/m3 > 320 kg/m^{3}, hence, cement content adopted = 350 kg/m^{3}.
Step 5: Estimation of Coarse Aggregate proportion-
From Table 3 of IS 10262- 2009,
For Nominal maximum size of aggregate = 20 mm,
Volume of coarse aggregate for water-cement ratio of 0.50 =0.64 (Corresponding Zone III)
In this case w/c = 0.4 lower by 0.10. So, 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). For pump or worked around congested reinforcing steel required more workable concrete the coarse aggregate proportion may be reduced up to 10%.
Hence,
Volume of coarse aggregate per unit volume of total aggregate = 0.64 x 90% = 0.576
Volume of fine aggregate = 1 – 0.576 = 0.424
Step 6: Estimation of the mix ingredients
a) Volume of concrete = 1 m^{3}
b) Volume of water
c) Volume of cement
d) Volume of chemical admixture (superplasticizer)
@2.0% by mass of cementitious material = 350 × 2% = 7 kg/m^{3}
e) Volume of total aggregates = a – (b + c)
= 1-(0.111 +0.140+0.007)
= 0.742 m^{3}
f) Mass of coarse aggregates = e × Volume of coarse aggregate × Specific gravity of coarse aggregate × 1000
=0.742 x 0.576 × 2.84 × 1000 = 1213.79 kg/m^{3}
g) Mass of fine aggregates = e x volume of fine aggregate × Specific gravity of fine aggregate × 1000
= 0.742 × 0.424 × 2.64 × 1000 = 830.56 kg/m^{3}
MIX PROPORTIONS FOR TRIAL NUMBER 1
- Cement = 350kg/m^{3}
- Water = 140 kg/m^{3}
- Chemical Admixture = 7 kg/m^{3}
- Fine aggregates = 830.56 kg/m^{3}
- Coarse aggregate = 1215.43 kg/m^{3}
- W/c = 0.4
TRIAL -1 preparing of concrete in lab, to check its properties will satisfy desire strength durability & economy.
Mass of ingredients required for casting of 4 numbers cubes (15cm×15cm×15cm) for trial – 1 will be calculated, assuming 25% wastage.
Volume of concrete required for 4 cubes = 4 x (0.15^{3}^{ }x1.25) = 0.016878 m^{3}
Cement = (350 x 0.016878) kg/m^{3} = 5.91 kg
Water = (140 x 0.016878) kg/m^{3} = 2.36 kg
Coarse aggregate = (1215.43 x 0.016878) kg/m^{3} = 20.51 kg
Fine aggregates = (831.68 x 0.016878) kg/m^{3 =} 14.04 kg
A summary of all the trial mixes is given in the following Table.
Recommended mix proportion of ingredients for grade of concrete M30 obtain from test result –
From obtained test result for target strength 38.25 N/mm^{2} (> 34 N/mm^{2 }, OK) we get,
- Water-cement ratio = 0.38
- Water content = 145 kg/m^{3 }(0.145 m^{3})
- Cement content = (145/0.38) = 381.58 kg/m^{3 }(0.121 m^{3})
- Superplasticizer = 7.63 kg/m^{3 }(0.0067 m^{3})
Volume of total aggregate = 1 – [0.121+ 0.145+0.0067] = 0.7273 m^{3}
A reduction of w/c – 0.02 so, increase of coarse aggregate fraction by 0.004.
Coarse aggregate fraction = 0.576 +.004 = 0.580
Percentage of FA = (1 – 0.580) x 100 = 42%
5. Mass of CA = 0.7273 x 0.580 x 2.84 x 1000 = 1198.00 kg/m^{3}
6. Mass of FA = 0.7273 x 0.420 x 2.64 x 1000 = 806.43 kg/m^{3}