Data for Mix Proportioning :
The following data are required for mix proportioning of a particular grade of concrete:
ft= fck+1.65.s
Where,
ft = target mean compressive strength at 28 days in N/mm2
fck = Characteristic compressive strength of concrete at 28 days in N/mm2
S = standard deviation in N/mm2 ( refer table 1 given below)
Table 1 : Assumed Standard Deviation
Sl No |
Grade of Concrete |
Assumed Standard Deviation (N/mm2 |
1 |
M 10 |
3.5 |
2 |
M 15 |
|
3 |
M 20 |
4.0 |
4 |
M 25 |
|
5 |
M 30 |
5.0 |
6 | M 35 | |
7 | M 40 | |
8 | M 45 | |
9 | M 50 | |
10 | M 55 |
Step II : Selection of Water Cement
(w/c) Ratio
Exposure |
Plain Concrete |
Reinforced Concrete |
||||
Min Cement in Kg/m3 |
Max w/c |
Min Grade |
Min Cement in Kg/m3 |
Max w/c |
Min Grade |
|
Mild |
220 |
0.60 |
-- |
300 |
0.55 |
M 20 |
Moderate |
240 |
0.60 |
M 15 |
300 |
0.50 |
M 25 |
Severe |
250 |
0.50 |
M 20 |
320 |
0.45 |
M 30 |
Very Severe |
260 |
0.45 |
M 20 |
340 |
0.45 |
M 35 |
Extreme |
280 |
0.40 |
M 25 |
360 |
0.40 |
M 40 |
Table 3 : Adjustment to minimum cement content for aggregates other than 20 mm nominal max size aggregates As per IS 456: 2000
Sl No |
Nominal Max Aggregate |
Adjustment to minimum cement content in Table 2 in Kg/m3 |
1 |
10 |
+40 |
2 |
20 |
0 |
3 |
40 |
-30 |
Step III: Selection of water content :
The quantity of maximum mixing water per unit volume of concrete can be determined from table 4 and table 5 given below.
Table 4 Maximum Water content per cubic metre of concrete for nominal maximum size of aggregate. ( For angular coarse aggregate and slump range 25-50 mm )
Sl No |
Nominal Maximum size of Aggregate (in mm) |
Maximum Water Content (in Kg) |
1 |
10 |
208 |
2 |
20 |
186 |
3 |
40 |
165 |
Table 5: Correction to values in table 4 for other site conditions
Sl No |
Condition |
Correction |
1 |
Sub – Angular Aggregates |
-10 kg |
2 |
Gravel+ Crushed Aggregates |
-20 kg |
3 |
Rounded Gravel |
-25 kg |
4 |
For every slump increase of 25 mm |
+ 3 % |
5 |
Use of Water reducing Admixture |
-5 to - 10 % |
6 |
Use of superplasticzing Admixtures |
-20 % |
Step IV : Calculation of Cementitious Material Content
Calculate the cement content from w/c ratio and final water content arrived after adjustment. Check the cement content calculated against the minimum cement content specified in table 2. The greater of the two values shall be adopted.
Step V : Proportion of volume of coarse aggregate
For w/c ratio of 0.5 , table 6 is to be used .
Table 6 Volume of Coarse Aggregate per unit volume of total aggregate for different zones of Fine Aggregate .
Sl No |
Nominal Maximum size of Aggregate (mm) |
Volume of coarse aggregate per unit volume of Total Aggregate for Different Zones of Fine Aggregate |
|||
Zone IV |
Zone III |
Zone II |
Zone I |
||
i) |
10 |
0.50 |
0.48 |
0.46 |
0.44 |
ii) |
20 |
0.66 |
0.64 |
0.62 |
0.60 |
iii) |
40 |
0.75 |
0.73 |
0.71 |
0.69 |
orrection in Coarse aggregates values
Step VI: Final Mix Calculations :
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