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What is mix design of concrete and different steps followed in mix design as per IS 10262 code?
Mix design of the concrete is the process or procedure to determine the mix ratios of the materials required to make the concrete. As per standards there are two types of mix design exists for concrete in that one is nominal mix and second one is design mix. If the values of mix ratio or mix proportion values directly considered from IS code provisions it is called as nominal mix, if the mix proportions are calculated as per the material properties it is called as design mix. The mix ratios in the design mix are different from nominal mix ratios because of the different material properties values.
The following ratio is the mix proportion for M20 grade concrete as per nominal condition
M20 1:1.5:3 W/C 0.5
Where the factor 1 indicates the ratio of cement, 1.5 indicates the ratio of fine aggregates or sand, 3 indicates the ratio of coarse aggregates and W/C is the water cement ratio value.
Code books used in calculation
There are two code books required in order to calculate the mix ratio of the certain specified grade concrete. They are IS 10262 and IS 456 standards.
Different steps followed in mix design calculation
For determining the mix design or mix ratio of the concrete the following steps are considered as per the IS 10262 code standards
- Calculation of target strength
- Choosing of water cement ratio
- water content selection
- Cement content calculation
- Proportion of volume of coarse aggregates and fine aggregates
- Mix calculation
- Mix proportion
Calculation of target strength
As per the IS 10262 standards the target strength is calculated by using formulae
Target strength = (characteristic strength)X1.65(S)
It is also written as σ’ck = σck+1.65S
Where S = Standard deviation
The value of the standard deviation is considered from table number 1 of IS 10262 code
Choosing of water cement ratio
The selection of water cement ratio is the second step in the mix calculation. The table number 5 of IS 456 code is considered in this step. As per the table the water cement ratio value is contained in two categories under plain concrete and reinforced concrete. The complete cement quantity value is depends upon this water cement ratio in mix design calculation.
Water content selection
The water content selection is the third step in the mix design calculation in this step the water content value is considered from table 2 of IS 10262 code standards. As peer the table 2 the value of water content is exists for the size of aggregates. For 10mm size maximum aggregates size the water content is considered as 208, for 20mm size maximum aggregates size the water content is considered as 186 and finally for 40mm size maximum aggregates size the water content is considered as 165.
The water content value is also depends upon the slump value. The adopted water content for 10mm, 20mm and 40mm size maximum aggregates size is only applicable for the 25mm to 50mm slump value for every increase in the 25mm slump value the water content is also increased by 3%.
Cement content calculation
From the available values of w/c ratio and water content values in step 2 and step 3 we can easily determined the cement content quantity. After calculation is done cross check the values of maximum cement content which is specified in table 5 of IS 456 2000 code standards. If the value is within the limits the assumed water content is correct otherwise repeat the calculation from step number 2.
Proportion of volume of coarse aggregates and fine aggregates
Initially the proportion of volume of coarse aggregates per unit value of coarse aggregates is considered from Table 3 of IS 10262. As per the specifications the considered value is only applicable for the 0.5 water cement ratio. For every increase in 0.05 water cement ratio the volume is reduced by 0.01, in the same process for every decrease in 0.05 water cement ratio the volume is increase by 0.01. After determining the volume of coarse aggregate proportion deduct the volume from 1 factor to get fine aggregates proportion.
Mix calculation
The mix calculation is the process of determining the quantity of materials which are required to make 1 cubic meter of concrete. In this initially the volume of concrete is assumed as 1m3, then the volume of cement and volume of water is calculated separately by considering quantity of cement and water content which are already available from step 3 and step 4.
Now deduct the sum of volume of cement and volume of water from 1 factor (total volume) in order to get volume of aggregates (Vfa+Vca) which is sum of volume of coarse aggregates and fine aggregates.
Finally the quantity of the coarse aggregates and fine aggregates are determined in the last stages with the help of the specific gravity values, volume of coarse aggregates and fine aggregates and volume of aggregates.
Mix proportion
The mix proportion or trial calculation is the final stage to determine the final mix as per the specified grade of concrete. Initially write all the quantities of materials like cement, fine aggregates, coarse aggregates and water and divide the entire values of quantities with cement to obtain final mix ratio.
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Conclusion of mix design of concrete and different steps followed in mix design
Well now the above explained concepts are related to the mix design of concrete and different steps followed in mix design calculation as per IS 10262 code standards. They are classified in to 7 steps related to the Calculation of target strength choosing of water cement ratio, water content selection, cement content calculation, proportion of volume of coarse aggregates and fine aggregates, mix calculation and mix proportion.
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