IS method of mix design steps

The IS method (IS 10262:2009, revised in 2019) designs a mix for a required grade from the target strength, w/c ratio, water content and aggregate proportions by absolute volume, and is used with IS 456.

Steps

1. Design data: grade, maximum aggregate size, workability, exposure, cement type, specific gravities, absorption.
2. Target mean strength: $f'_{ck} = f_{ck} + 1.65 s$ (s from Table 1 of IS 10262: M20 4.0, M25 4.0, M30 5.0, M35 5.0 N/mm$^2$; add 1 N/mm$^2$ if site control is only fair). The 2019 revision also requires $f'_{ck} \geq f_{ck} + X$ (X = 5.0, 5.5, 6.5 for M10-15, M20-25, M30 and above); take the higher value.
3. w/c ratio from the strength vs w/c relationship for the materials (in the 2019 revision, the curves of Fig 1 for 33/43/53 grade cement) for the target strength; check against the IS 456 Table 5 limits for the exposure; adopt the lower.
4. Water content from Table 2 (186 kg/m$^3$ for 20 mm angular aggregate at 50 mm slump); add 3% for every 25 mm of extra slump; reduce when a superplasticiser is used (by 20% or more).
5. Cement content = water / (w/c); check the minimum from IS 456 Table 5 and the maximum of 450 kg/m$^3$.
6. Volume of coarse aggregate per unit volume of total aggregate from Table 3 (maximum size and zone of fine aggregate, for w/c 0.5); adjust by $\pm$ 0.01 for each $\mp$ 0.05 change in w/c.
7. Mix calculations per m$^3$ using absolute volumes:

$$V_{agg} = 1 - \left[\frac{C}{\rho_c 1000} + \frac{W}{1000} + \frac{\text{admix}}{\rho_a 1000} + \text{air}\right]$$

Mass of CA = $V_{agg} \times V_{CA} \times SG_{CA} \times 1000$; FA = $V_{agg} (1 - V_{CA}) \times SG_{FA} \times 1000$ (entrapped air taken as 2% for 20 mm). 8. Trial mixes: three trials with the w/c varied; check the strength and slump; select the best. 9. Apply moisture and absorption corrections.

Sample table (SG of CA 2.74, FA 2.65)

Graph

 f_target (MPa)
  50|\
  40| `.\
  38|---`-`.   <- read w/c = 0.43
  30|       `-.
    +-----------> w/c
     0.35  0.5