Factors in mix design; Indian method

Factors influencing concrete mix design

1. Characteristic strength required (grade) and the target mean strength; level of quality control (standard deviation).
2. Workability required (slump, compaction factor) for the type of placing and compaction.
3. Durability / exposure condition: maximum w/c ratio and minimum cement content (IS 456 Table 5).
4. Maximum size, shape and grading of aggregate, and the zone of the fine aggregate.
5. Type and grade of cement, and its specific gravity.
6. Water-cement ratio and water content.
7. Admixtures and mineral additives.
8. Properties of materials: specific gravity, absorption, moisture and bulking.
9. Placing conditions: reinforcement density, section size, temperature, and method of transport.
10. Economy and the materials available.

Indian method (IS 10262:2009, revised 2019)

The mix is designed in these steps:

1. Collect the data: grade, aggregate size, workability, exposure, cement, specific gravities and so on.
2. Target strength $f'_{ck} = f_{ck} + 1.65 s$ (Table 1: s = 4 for M20/M25, 5 for M30 and above, for good site control); the 2019 revision also requires at least $f_{ck} + X$, whichever is higher.
3. w/c ratio from the strength-w/c relationship (Fig 1 of the 2019 revision: 28-day strength vs w/c for 33/43/53 grade cement), checked against IS 456 Table 5; adopt the lower.
4. Water content from Table 2 (186 kg/m$^3$ for 20 mm angular aggregate at 50 mm slump; $\pm 3\%$ for each 25 mm change in slump; reduce by about 10 kg for sub-angular, 20 kg for gravel with some crushed particles and 25 kg for rounded gravel; reduce by 20% or more with a superplasticiser).
5. Cement content = water / (w/c); check the minimum (IS 456 Table 5) and the maximum limit of 450 kg/m$^3$.
6. Volume of coarse aggregate from Table 3 (size, fine aggregate zone, for w/c 0.5; change by 0.01 for each 0.05 change in w/c).
7. Absolute volume calculation: the volume of concrete minus the volumes of water, cement, admixture and air gives the aggregate volume, which is split into FA and CA by the CA fraction.
8. Trial mixes to check workability and strength, then adjust.