Concrete Mix Design (M20 Grade)

Concrete mix design is a method of calculating the right proportions of cement, sand, and aggregates for concrete to achieve target strength in structures. The Concrete mix design Concrete Mix = Cement:Sand:Aggregates.

There are two Methods

1. Volume Batching
2. Weigh Batching

For a mix design of concrete, we have to follow various steps, calculations, and laboratory testing to find the right mix proportions.

M20 (1 cement :1.5 sand :3 stone/brick aggregate). To determine the proportions you have to perform mix design, you have to find out the Specific Gravity of cement, CA, FA, and water-cement ratio, type of exposure, Maximum size of aggregate, etc. Once the W/C ratio is determined use 186 kg water (for 20mm CA) to find out the qty. of cement per cum of concrete. Then determine the qty. of FA and CA with the help of the formula given in IS:10262-1982, and then divide all the ingredients by cement, then you will get the proportion similar to you mentioned which are in kg. Or you can follow IS:10262 -2009, where the calculations are easier.

1. REQUIREMENTS

a) Specified minimum strength = 20 N/mm2

b) Durability requirements
I)  Exposure Moderate
ii) Minimum Cement Content = 300 Kgs/m3

c) Cement (Refer Table No. 5 of IS:456-2000)
I)  Make Chetak (Birla)
ii) Type OPC
iii) Grade 43

d) Workability
i) Compacting Factor = 0.7

e) Degree of Quality Control: Good

2. TEST DATA FOR MATERIALS SUPPLIED

a) CEMENT
i) Specific gravity = 3.05
ii) Avg. comp. strength 7 days = 46.5 more than 33.0 OK
28 days = 55.0 more than 43.0 OK

b) COARSE AGGREGATE
i) 20mm Graded
Type Crushed stone aggregate
Specific gravity = 2.68
Water absorption = 1.46
Free (surface) moisture = 0

c) FINE AGGREGATE (Coarse sand)
i) Type Natural (Ghaggar)
Specific gravity = 2.6
Water absorption = 0.5
Free (surface) moisture = 1.4

3. TARGET MEAN STRENGTH (TMS)
a) Statistical constant K = 1.65
b) Standard deviation S = 4.6
Thus, TMS = 27.59 N/Sqmm

4. SELECTION OF W/C RATIO
a) As required for TMS = 0.5
b) As required for ‘Moderate’ Exposure = 0.55
Assume W/c ratio of 0.5

5. DETERMINATION OF WATER & SAND CONTENT
For W/C = 0.6
C.F. = 0.8
Max. Agg. Size of 20 mm
a) Water content = 186 Kg/cum
b) Sand as a percentage of total aggregate by absolute volume = 35 %

Thus,
Net water content = 180.42 Kg/cum
Net sand percentage = 33 %

6. DETERMINATION OF CEMENT CONTENT
W/c ratio = 0.5
Water content = 180.42 Kg/cum
Thus, Cement content = 360.84 Kg/cum Adequate for moderate exposure Say 360 Kg/cum

7. DETERMINATION OF COARSE AND FINE AGGREGATE CONTENT
Assume entrapped air as 2 %
Thus,
0.98 cum = [180.42+360/3.05 + {1/0.33}*{fa/2.6}]/1000
& 0.98 cum = [180.42+360/3.05 + {1/0.67}*{Ca/2.68}]/1000
Hence,
fa = 584 Kg/cum
Ca = 1223.8 Kg/cum

The final mix proportions of M-20 grade of concrete become:-

Detailed analysis on M20  = 1:1.5:3 (Ratio)

As we know that  during concreting when we place wet concrete , it gets harden after certain standard time( 30 mins IST & 10hrs FST), considering same it had be decided upon by Civil design Engineers to take a factor of safety ranging from 1.54 to 1.57 to counter that shrinkage.

i.e volume of dry Concrete = 1.54 to 1.57 times Volume of wet concrete.

Now calculations is as follows for 1cum(assumed) of Concrete work

ratio Sum = 1+1.5+3=5.5 

Shrinkage or safety Factor =1.57 (you can take 1.54 also)

So Total volume of wet concrete required is :- 1.57cum

Volume of broken stone Require = (3/5.5) x 1.57 = 0.856 m3

Volume of sand Require = (1.5/5.5) x 1.57 = 0.471 m3

Volume of cement = (1/5.5) x 1.57 = 0.285 m3 

= 0.285 x1440 = 411 kg 

For 1m3 of M20 (1:1.5:3) 

Broken stone = 0.856 m3 

Sand = 0.472 m3

Cement = 8.22 bag.

Some important conclusions from above 

8 bag of Cement is required for 1cum of concrete work in M20.

4.4 bag of cement is required for 1cum of concrete work in M10.

the volume of dry concrete =1.54 times the volume of wet concrete.

Thank You, Folks!