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Care in Concrete Mix Design

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By
KAUSHAL KISHORE
Materials Engineer, Roorkee

While carrying out Concrete Mix Design, there are chances that we do not consider some technical points resulting the production of poor quality concrete. This is best illustrated by the following two examples. The Ist mix design is carried out carelessly, where as the 2nd mix design is worked out taking into consideration all technical points. In both mix design examples the materials and requirements are same as given below:

1. Cement, PPC, 7-days strength 36.5 N/mm2 and 28-days strength 46.8 N/mm2. Specific gravity 3.0
2. Aggregates grading are as given below:

 

I.S. Sieve size

Percentage passing by mass
River sand Crushed aggregate
10 mm 20 mm
40 mm 100 100 100
20 mm 100 100 68
12.5 mm 100 83 X
10 mm 94 59 0
4.75 mm 79 2  
2.36 mm 60    
1.18 mm 43    
600 Micron 29    
300 Micron 15    
150 Micron 3    
Specific gravity 2.65 2.65 2.65
Water absorption 0.8 0.6 0.5
Bulk density kg/lit. 1.72 1.40 1.38

3. (a) Grade of concrete M-25
(b) Maximum free W/C Ratio = 0.50 Minimum cement content = 300 kg/m3

4. Laboratory design target strength =
25 + 1.65 x 4 = 31.6 N/mm2 at 28 days age

5. Mix required for pumped concrete, slump = 100 mm

6. Superplasticizer 1.2 % b.w.c. which will give a reduction of 15% in the mixing water.

7. Air = 1.5%

8. The grading of aggregates are not according to IS: 383-1970, but the site has no choice and want the mix design to be worked out with the given site materials.

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Ist Mix Design which is carried out and reported as per general practice with out mentioning the mix is on the basis of room dry aggregates or on the basis of saturated and surface dry aggregates.

Water = 180 kg/m3
Cement, PPC = 382 kg/m3
Sand = 716 kg/m3
10 mm crushed aggregate = 710 kg/m3
20 mm crushed aggregate = 470 kg/m3
Superplasticizer = 4.584 kg/m3

Density (Excluding Superplasticizer) = 2458 kg/m3

OBSERVATION OF ABOVE MIX DESIGN
1. The density is not correct it should be 2360 kg/m3
2. The combination of sand and aggregates are also not correct as shown in the following table:

COMBINE AGGREGATES GRADING IN THE Ist Mix Design

 

 

I.S. Sieve size

Percentage passing by mass
River sand Crushed Agg. Combined grading of mix Suitable combined grading for pumped concrete
10 mm 20 mm
38% 37% 25%
40 mm 100 100 100 100 100
20 mm 100 100 68 92 100
12.5 mm X 83 X X X
10 mm 94 59 0 58 60-73
4.75 mm 79 2   31 40-58
2.36 mm 60 0   23 28-46
1.18 mm 43     16 18-35
600 Micron 29     11 12-25
300 Micron 15     6 7-15
150 Micron 3     1 2-6

REMARKS
The Ist Mix Design concrete will be harsh and segregate. This concrete will be of poor quality and not durable.

The 2nd Concrete Mix Design is carried out with due care and reported below:

Water = 180 kg/m3

Cement, PPC = 382 kg/m3

CALCULATED DENSITY

Um = 10 Ga (100-A) +Cm (1-Ga/Gc)-Wm (Ga-1)
= 10 x 2.65 (100-1.5) + 382 (1 – 2.65/3.0) – 180 (2.65-1)
= 2358, Density say 2360 kg/m3

Total aggregates = 2360 – 180 – 382 = 1798 kg/m3

The combination of aggregates in the 2nd Mix Design is given below:

 

 

I.S. Sieve size

Percentage passing by mass
River sand Crushed Agg. Combined grading of mix Suitable combined grading for pumped concrete
10 mm 20 mm
50% 30% 20%
40 mm 100 100 100 100 100
20 mm 100 100 68 94 100
12.5 mm X 83 X X X
10 mm 94 59 0 65 60-73
4.75 mm 79 2   40 40-58
2.36 mm 60 0   30 28-46
1.18 mm 43     22 18-35
600 Micron 29     15 12-25
300 Micron 15     8 7-15
150 Micron 3     2 2-6

M-25 Grade of concrete, 2nd Mix Design on the basis of saturated and surface dry aggregates.

Water = 180 kg/m3

Cement, PPC = 382 kg/m3

Sand = 899 kg/m3

10 mm crushed aggregate = 539 kg/m3

20 mm crushed aggregate = 360 kg/m3

Superplasticizer = 4.584 kg/m3

Density (Excluding Superplasticizer) = 2360 kg/m3

REFERENCES
1. “Civil Engineering Materials and Testing” by Kaushal Kishore Standard Publishers Distributors, 1705-B, Nai Sarak, Delhi-110006.

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We at engineeringcivil.com are thankful to Er. Kaushal Kishore for submitting this paper to us. We are sure this would be of great help to all those seeking information on concrete mix.

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