NOMINAL MIX DESIGN OF CONCRETE
NOMINAL MIX
The nominal mix proportions varies based on the zones of sand and size of the course aggregates, so its batter to get with the design mix, But the mix design of concrete cannot be used for any reason on the work for grades of M-20 or lower than the M-20. Generally we will go with the nominal mix where there is relatively unimportant works and simpler concrete works.In the nominal mix,all the ingredients which are prescribed and their proportions are specified.
Therefore there is no scope for any deviation by the designer when the size of course aggregates and zone of sand is fixed. This Nominal mix concrete may be used for concrete of M-20 or lower than the M-20. IS: 456-2000 has recommended that the minimum grade of concrete shall not be less than M-20 in reinforced concrete work. The Nominal mixes can be used with the permission of engineer-in-charge for the important concrete works. Which however is likely to involve a higher cement content.
When the size of aggregate is 20mm
and sand zone II is used than the mix proportions for the nominal mix are
tabled below as per IS 456-2000
The table shown below is based
on the sand zone II and the size of aggregates is 20mm
Grade
|
Max.
quantity of dry Aggregates per 50 kg of cement
|
Fine
Aggregate to Coarse Aggregate Ratio, by mass
|
Max.
Quantity of water in litres
|
mix
proportions
|
M-5
|
800
|
Generally
1:2 but may varies from 1:1.5 to 1:2.5
|
60
|
1:5:10
|
M-7.5
|
625
|
45
|
1:4:8
|
|
M-10
|
480
|
34
|
1:3:6
|
|
M-15
|
330
|
32
|
1:2:4
|
|
M-20
|
250
|
30
|
1:1.5:3
|
The Nominal mixes when the course
aggregate is used with various sizes as per IS: 456-2000 and the fine aggregate
is of Zone II as per IS: 383-1970.
Grade of Concrete
|
As per IS:383-1970 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
|
||||
M-20
|
10
|
1 : 1.8 : 2.7
|
0.60
|
1.5
|
||
M-20
|
20
|
1 : 1.5 : 3.0
|
0.60
|
1.5
|
||
M-20
|
40
|
1 : 1.3 : 3.2
|
0.60
|
1.5
|
||
The above nominal mixes are worked
out for Zone II of fine aggregate and the various sizes of course aggregate. As
per IS: 383-1970 there are three more zones of sand. Therefore, the total
nominal mixes shall be 10 for 12, 20 & 40 mm maximum size of coarse
aggregate.
Thus,
it seems to be that the nominal mixes doesn’t have a fix conventional
proportions such as 1:1.5:3 or 1:2:4, but may vary according to maximum size of
coarse aggregate and grading of fine aggregate and their zones. Hence the
nominal mixes are also needed to be designed according to the sizes of
aggregates available at site and the site conditions. However, the ultimate aim
is to get the specified properties of concrete.
The exposures of Indian Construction sites at most of the places are Moderate
for which IS: 456-2000 specified the minimum grade of concrete for reinforced
concrete should be M-25. Accordingly for durability aspects the structural
concrete must not be below M-25 grade. The high strength benefits are obtained
from M-25 should be taken into account in the design consideration of the
concrete structure.
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As per IS 456, Nominal concrete mix may be used for…
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