The following data is required for concrete mix proportioning of a particular grade of concrete:
- Exposure conditions of the structure under consideration (see table 3 of IS 456:2000).
- Grade designation – The minimum grade of concrete to be designed for the type of exposure condition under consideration (see table 3 and table 5 of IS 456:2000).
- Type of cement (OPC, PPC, PSC).
- Minimum and maximum cement content (see tables 3, 4, 5 and 6 of IS 456:2000).
- Type of aggregate.
- Maximum nominal size of aggregate to be used.
- maximum w/c ratio (tables 3 and 5 of IS 456:2000).
- Desired degree of workability (clause 7 of IS 456).
- Use of admixture , its type and condition of use.
- Maximum temperature of concrete at the time of placing.
- Method of transporting and placing.
- Early age strength requirements.
The step by step mix proportioning procedure.
Step 1: Calculate the target mean compressive strength for mix proportioning. The 28 day target mean compressive strength as per clause 3.2 of IS 456.
fck’ = fck + 1.65*s, where fck’ is the characteristic compressive strength.
S = Standard deviation, (M10 to M15 = 3.5, M20 to M25 = 4.0, M30 to M55 = 5.0)
Step 2: Select the w/c ratio, the maximum w/c ratio for various environmental exposure conditions are given in table 5 of IS 456 and may be taken as a starting point.
Step 3: Select the water content. The quality of water considered per cubic meter of concrete decides the workability of the mix. The use of water reducing admixtures in the mix helps to achieve increased workability at lower water contents. The water content given in table 2 of IS 10262 is the maximum value for a particular nominal maximum size of aggregate.The water content per unit volume of concrete can be reduced when increased size of aggregate or rounded aggregates are used. On the other hand, the water content has to be increased when there is increased temperature, cement content and fine aggregate content.
In the following cases, a reduction of water content is suggested,
- For sub-angular aggregates, a reduction of 10kg.
- For gravel with crushed particles, a reduction of 20kg.
- For rounded gravel, a reduction of 25kg.
Step 4: Calculate the content of cementitious material, the cement and cementitious material can be calculated from the free w/c ratio as in step 2. The total cement content should be checked against the minimum cement content for the requirements of durability and the greater of the two values adopted. The maximum cement content should not exceed 450kg/m³.
Step 5: Estimate the proportion of coarse aggregate, table 3 of IS 10262, gives the volume of coarse aggregate for unit volume of total aggregate for different zones of fine aggregate for a w/c of 0.5, which requires to be suitably adjusted for other w/c.
Step 6: Identify the combination of different sizes of coarse aggregate fractions, coarse aggregate from stone crushers are normally available in two sizes, i.e., 20mm and 12.5mm. Coarse aggregates of different sizes can be mixed to attain proper grading.
Step 7: Estimate the proportion of fine aggregates, the absolute volume of cementitious material, water, and the chemical admixture is found by dividing their mass by their respective specific gravity and multiplying by 1/1000. The volume of all aggregates is obtained by subtracting the summation of volumes of these materials from the unit volume.
Step 8: Perform trial mixes: The calculated mix proportions should always be checked by means of trial batches. The concrete for trial mixes shall be produced by means of actual materials and production methods. The trial mixes may be made by varying the free w/c ratio by ±10%.