Project on Concrete Mix Design for various grades of Concrete

The study carried out at national laboratories served as the primary foundation for the Bureau of Indian Standards' recommendation of a standard approach for designing concrete mix. The 2019 edition of IS 10262 covers the Concrete Mix Design process. The techniques are applicable to both medium- and high-strength concrete.

The following data is required for Concrete Mix Design Calculations of a particular grade of concrete.

1. Grade designation
2. Type of cement, and grade of cement (if applicable)
3. Maximum nominal size of aggregate.
4. Minimum cement/cementitious materials content and maximum water-cement/cementitious materials ratio to be adopted; (Exposure conditions as per Table 3 and Table 5 of IS456)
5. Workability required at the time of placement
6. Transportation time
7. Method of Placing
8. Degree of site control (good/fair) or value of established standard deviation, if any;
9. Type of coarse aggregate (angular/sub-angular/ gravel with some crushed particles/rounded gravel/manufactured coarse aggregate)
10. Type of fine aggregate (natural sand/ crushed stone or gravel sand/manufactured sand/ mixed sand);
11. Maximum cement content
12. Whether a chemical admixture shall or shall not be used and the type of chemical admixture and the extent of use;
13. Whether a mineral admixture shall or shall not be used and the type of mineral admixture and the extent of use; and
14. Any other specific requirements like early age strength requirements.

The steps to be followed for concrete mix design are:

Step 1: Determine the concrete's target mean strength

The concrete mix must be proportioned for a higher target mean compressive strength f'ck in order to ensure that no more test results than the prescribed percentage are likely to fall below the characteristic strength. The connection below determines the Target Mean Characteristic Strength:

f’ck = fck + 1.65 S

or

f’ck = fck + X

whichever is higher.

where,

f’ck = Target mean strength (compressive) at end of 28 days, in N/mm2;

fck = characteristic strength (compressive) at end of 28 days, in N/mm2;

S = standard deviation, in N/mm2 (see 4.2.1); Table 2 as Per IS Code 10262 – 2019

X = Factor as per Grade of Concrete, as per Table 1 as Per IS Code 10262 – 2019.

Always select a higher value of the two.

Step-2: Selection of Water-Cement Ratio

In general, different types of cement, aggregates with different maximum sizes, supplementary cementitious materials, grading, surface texture, shape, and other characteristics may result in concrete with a different compressive strength for the same free water-cement ratio, according to the code. Consequently, it is preferable to develop a good link between the water-cement ratio and the strength of concrete for the materials that will actually be utilized. In the absence of such information, the relationship depicted in Fig. 1 which is the Water-Cement ratio Graph for Concrete Mix Design as Per IS Code 10262 – 2019 corresponds to the compressive strength at 28 days may be used for the anticipated 28-day strength of cement.

If extra cementitious materials, such as mineral admixtures, are used, the water cementitious material (w/cm) ratio must be determined in accordance with Table 5 of IS 456, and this w/cm must be as defined in Tables 3 and Table 5 of IS 456.

Always choose a greater water-to-cement ratio for your concrete since tests and studies required by the IS code are carried out under controlled conditions in labs, and such care and process are not feasible on building sites. In light of this, I advise choosing a greater water-to-cement ratio for your concrete grade.

Step 3: Calculating the Air Content
Table 3 provides an estimate of the amount of trapped air to be expected in typical (non-air-entrained) concrete (Clause 5.2).

Step-4: Selections of Water Content and Admixture Content
The water content of concrete is often influenced by a number of variables, including cement, cement, and other supplemental cementitious materials type and content, chemical admixture, ambient conditions, workability, aggregate size, shape, and texture. The water need will decrease when aggregate maximum size rises, water-cement ratio and slump are reduced, rounded aggregate is used, and water-reducing admixture is used. Table 4 may be used to calculate the concrete's water content per unit volume. To achieve the same workability, the water content listed in Table 4 can be decreased by around 10 kg for sub-angular aggregates, 15 kg for gravel that contains some crushed particles, and 20 kg for rounded gravel.

Hence, if additional workability is required but the slump value is greater than 50 mm, the required water content may be adjusted by adding or subtracting 3 percent for every 25 mm increase or reduction in a slump, or it may be determined by trial and error. The notional maximum aggregate size that we allow is 20 mm. According to the preceding data, the aggregate's nominal maximum size has a water content of 186 kg (Liter) Slump 50 mm. The droop must be raised twice by 25 mm if we want to expand it to 100 mm. This is an anticipated water content for a 100 mm slump since according to the IS rule, water must be increased by around 3% for every extra 25 mm of a slump.

For the 75 mm slump increase by 3%, for the 100 mm increase by 6%, increase by 9% for the 125 mm slump, increase by 12% for the 150 mm slump & increase by 15% for the 175 mm slump.

Step 5: Calculating the Cement Content
We have the Water-Cement Ratio and the Water Content. So, we can determine the cement content of concrete using this data. from IS 456 Table 5.

Step-6: Estimation of Coarse Aggregate Proportion
When a certain volume of coarse aggregate is utilized per unit volume of total aggregate, concrete of adequate workability will be produced when the specified nominal maximum size, type, and grading are employed.
For a water-cement/water cementitious materials ratio of 0.5, approximations for this aggregate volume are shown in Table 5, which may be appropriately modified for various ratios. For every 0.05 drop in the ratio of the water-cement/cementitious material and for every 0.05 rise in the water-cement ratio, the volume of coarse aggregates as a percentage of total aggregates increases by 0.01 and decreases by 0.01 respectively.

Occasionally more workable concrete is needed, such as when placing concrete with a pump or while working around crowded reinforcing steel. In this situation, it would be preferable to lower the projected coarse aggregate content, which was calculated using Table 5, by up to 10%. As the water-cement ratio is higher by 0.12. the proportion of the volume of coarse aggregate is decreased by 0.014 (at the rate of -/+ 0.01 for every ± 0.05 change in the water-cement ratio). or pumpable concrete these values should be reduced up to 10%.

Step-7: Calculate the mix for 1m³.

Material quantity calculation for concrete mix design as per IS Code – 10262: 2019,

A) Volume of concrete = 1 m³

B) Volume of Air Content wet concrete = 0.01m³

C) Volume of cement = [Mass of cement] / {[Specific Gravity of Cement] x 1000}

D) Volume of water = [Mass of water] / {[Specific Gravity of water] x 1000}

F) Volume of all in aggregate = [(A-B)-(C+D)]

G) Weight of coarse aggregate= E x Coarse Aggregate Volume x Specific Gravity of coarse Aggregate x 1000

H) Weight of fine aggregate= E x Volume of Fine Aggregate x Specific Gravity of Fine Aggregate x 1000