Last reviewed 16 Sept 2026 · 9 min read
Concrete mix design
Mix design (mix proportioning) is the process of selecting suitable ingredients and determining their relative proportions to produce concrete of required strength, workability and durability as economically as possible.
Requirements
- Fresh concrete: workability suited to placing and compaction; no segregation or excessive bleeding.
- Hardened concrete: specified characteristic compressive strength; durability for the exposure condition (maximum w/c, minimum cement content, cover).
- Economy: minimum cement content consistent with the above (lower cost, less heat and shrinkage).
Nominal versus design mixes
| Nominal mix | Design mix |
|---|---|
| Fixed proportions (e.g. 1 : 1.5 : 3) irrespective of materials | Proportions determined from properties of actual materials |
| May be uneconomical and variable in strength | Economical and reliable |
| Permitted for concrete up to M20 | Required for M25 and above |
Factors governing mix design
Grade (characteristic strength) and degree of quality control (standard deviation); type and grade of cement; exposure conditions; maximum nominal size, shape and grading of aggregates; workability (slump); type of mixing and placing; use of admixtures; maximum and minimum cement content.
IS 10262:2019 method
Step 1 — Target strength
whichever is greater.
| Grade | Assumed standard deviation (N/mm²) | Factor (N/mm²) |
|---|---|---|
| M10, M15 | 3.5 | 5.0 |
| M20, M25 | 4.0 | 5.5 |
| M30 to M60 | 5.0 | 6.5 |
| M65 and above | 6.0 | 8.0 |
(Actual standard deviation from site data is used where available.)
Step 2 — Water–cement ratio
- Selected from established relationships (curves in the code or the producer's experience) between free w/c ratio and 28-day compressive strength for the cement used.
- Checked against durability — must not exceed the maximum w/c ratio for the exposure condition in IS 456; the lower value governs.
Step 3 — Water content
- For 20 mm nominal maximum size (angular coarse aggregate) and 50 mm slump, the base free water content is about 186 kg/m³ (about 208 kg/m³ for 10 mm and 165 kg/m³ for 40 mm aggregate).
- Increase about 3% for every additional 25 mm slump above 50 mm.
- Reduce for rounded/sub-angular aggregates and for water-reducing admixtures (plasticisers about 5–10%, superplasticisers about 20% or more, as established by trials).
Step 4 — Cement (binder) content
Check against the minimum cement content for exposure (IS 456) and the maximum (commonly 450 kg/m³ OPC); if below the minimum, increase cement and recompute w/c. With fly ash or slag, the total cementitious content and replacement limits are considered.
Step 5 — Proportion of coarse aggregate
The volume of coarse aggregate per unit volume of total aggregate is taken from the code table for the maximum size of coarse aggregate and the grading zone of fine aggregate, for a w/c ratio of 0.50 — e.g. for 20 mm aggregate: about 0.60 (Zone I), 0.62 (Zone II), 0.64 (Zone III) and 0.66 (Zone IV).
- Adjust by ±0.01 for every ∓0.05 change in w/c ratio (lower w/c → more coarse aggregate).
- Reduce (e.g. by about 10%) for pumpable concrete.
Step 6 — Mix calculations (absolute volume method)
Per cubic metre of concrete:
- Volume of entrapped air: about 1.0% for 20 mm aggregate (about 1.5% for 10 mm, 0.8% for 40 mm)
- Volume of cement
- Volume of water
- Volume of admixture
- Volume of all-in aggregate
- Mass of coarse aggregate volume of aggregate × CA fraction × × 1000
- Mass of fine aggregate volume of aggregate × FA fraction × × 1000
Step 7 — Trial mixes and adjustments
- Trial mix 1 checked for workability (slump) and absence of segregation; water/admixture adjusted keeping w/c constant.
- Further trials at w/c ratios varying by about ±10% of the chosen value; cubes tested at 7 and 28 days.
- The final mix is selected from the strength–w/c relationship.
- Field adjustments for moisture content and absorption of aggregates (aggregate masses in design are in SSD condition).