Understanding Nominal and Design Mixes
Posted in Mix Design  Email This Post By
Kaushal Kishore
Materials Engineer, Roorkee
Cement concrete in India on large scale is being used since the last about 70 years. In the early days the following nominal ratio by volume for concrete were specified.
Cement 
: 
Sand 
: 
Aggregate 

1 
: 
2 
: 
4 
Correspond to M15 Grade 
1 
: 
1.5 
: 
3 
Correspond to M20 Grade 
1 
: 
1 
: 
2 
Correspond to M25 Grade 
IS : 4562000 has recommended that minimum grade of concrete shall be not less than M20 in reinforced concrete work. Design mix concrete is preferred to nominal mix. If design mix concrete cannot be used for any reason on the work for grades of M20 or lower, nominal mixes may be used with the permission of engineerincharge, which however is likely to involve a higher cement content.
Accordingly all concrete of above M20 Grade for RCC work must be of design mixes. The code allows nominal mix for RCC work of M20 Grade, but what shall be the nominal mix, the reader will find from the following table that it is better to adopt design mix, rather than to go for M20 nominal mix which is too cumbersome to determine a fixed nominal mix value.
Nominal mixes as per IS : 4562000 if fine aggregate is of Zone II as per IS : 3831970.
Grade of Concrete 
As per IS:3831970 Maximum size of graded coarse aggregate 
Mix Ratio by Weight 
Max W/C Ratio 
Max cement: Aggregate ratio by mass 

Cement 
Fine Aggregate 
Coarse Aggregate 

M20 
10 
1 : 1.8 : 2.7 
0.60 
1.5 

M20 
20 
1 : 1.5 : 3.0 
0.60 
1.5 

M20 
40 
1 : 1.3 : 3.2 
0.60 
1.5 
Proportions by weight can be converted to proportions by volume, by dividing with the bulk density of the materials available for use at site. The bulk density of cement may be taken 1.44 kg/lit.
The above nominal mixes are worked out for Zone II fine aggregate. As per IS: 3831970 there are three more zone of sands. Therefore, the total nominal mixes shall be 12 for 10, 20 & 40 mm maximum size of coarse aggregate.
Thus, it could be seen that nominal mixes cannot have a fix conventional proportions such as 1:2:4 or 1:1.5:3, but may vary according to maximum size of coarse aggregate and grading of fine aggregate. Hence nominal mixes are also needed to be designed according to the sizes of aggregates available at site. However, the ultimate aim must be to get the specified properties of concrete.
As per IS: 4562000, volume batching may be allowed only where weight batching is not practical and provided accurate bulk densities of materials to be actually used in concrete have earlier been established. Allowance for bulking shall be made in accordance with IS: 2386(Part 3). The mass volume relationship should be checked as frequently as necessary.
The exposures of Indian Construction sites at most places are Moderate for which IS: 4562000 specified that minimum grade of concrete for reinforced concrete should be M25. Accordingly for durability consideration the structural concrete must not be below M25 grade. The high strength benefits obtained should be taken into account in the design consideration of the concrete structure.
If for practical purpose, we go deeper than we will find that for all reinforced concrete structures we must have concrete from design mixes.
In the IS: 4562000 there is nothing mentioned of 1:1:2 ration for M25 grade of concrete. Concrete of above M20 must be design mixes. If one takes 1:1:2 ratio then the cement content comes to 528 kg/m^{3}. Where as IS: 4562000 on page 19 clause 8.2.4.2 mentioned that OPC in excess of 450 kg/m^{3} should not be used.
The concrete surfaces of the structure exposed to severe rain, alternate wetting and drying such as RCC OH water tank comes to severe exposure environment for which the minimum grade of concrete shall be M30, minimum cement content 320 kg/m^{3} and maximum free W/C ration 0.45. The following table will show the compression of nominal and design mixes for RCC work.
Materials : OPC 43grade, River sand of Zone II and 20 mm graded crushed stone aggregate. Specific gravity of sand and aggregate 2.65. Workability of design mixes 50±10mm slump.
Grade of Concrete 
Mix. Free W/C ratio 
Min. Cement content kg/m^{3} 
Nominal mixes by weight C:S:A 
Design mixes by weight C:S:A 
Saving in cement 
M20 
0.55 
300 
1:1.5:3 Cement= 392kg/m^{3} 
1:2.22:3.48 Cement= 327kg/m^{3} 
65 kg/m^{3} 
M25 
0.50 
300 
1:1:2 Cement= 528kg/m^{3} 
1:1.93:3.17 Cement= 360kg/m^{3} 
168 kg/m^{3} 
M30 
0.45 
320 
— 
1:1.67:2.84 Cement= 400kg/m^{3} 
— 
Note: For high strength concrete plasticizer/superplasticizer should be used which will reduce water and with the same W/C ratio reduction in cement content.
From the above table it can be calculated in nominal mixes of M20 and M25 how much extra cement is used in the construction, its total cost and how much CO2 is emitted in the production of this extra cement.
When a mix is referred for designing, it is design for target strength. For example M30 (by Vol. ratio) is design for:
30 + 1.65 x 6 = 39.9 N/mm^{2} at 28 days age
The above is design target strength of the consultant Laboratory. When this mix is used at construction site, its concrete shall have strength as per table 11 of IS : 4562000.
30 + 4 = 34 N/mm^{2} at 28 days age
For starting the work a construction site cannot weight for 28 days. Therefore according to various literatures, if at 7 days its strength is about 65% (22 N/mm^{2} ) the work may be started. However in all the cases 28 days cube compressive strength shall alone be the criterion for acceptance and rejection of the concrete.
We have received many emails and comments seeking clarification on this topic and we requested Sir Kaushal Kishore to write on this topic. We at engineeringcivil.com are thankful to Sir Kaushal Kishore for accepting our request and helping civil engineers understand Nominal and Design mixes.
what is the cost saving per CUM of concrete between nominal mix and design mix.