Concrete Mix Design Steps as per IS 10262 with an M25 Worked Example
Mix design is the process of choosing the proportions of cement, water, fine aggregate, coarse aggregate and admixture to produce concrete of the required strength, workability and durability at the lowest reasonable cost. In India, the method is laid down in IS 10262 (the 2019 revision is current), read together with the durability requirements of IS 456. This article walks through every step for an M25 mix. The numbers are illustrative: a real design uses tested material properties and is confirmed by trial batches.
Design Stipulations and Material Data
| Parameter | Value assumed |
|---|---|
| Grade | M25 |
| Cement | OPC 53, specific gravity 3.15 |
| Maximum aggregate size | 20 mm crushed angular |
| Fine aggregate | River sand, Zone II, specific gravity 2.60 |
| Coarse aggregate specific gravity | 2.74 |
| Exposure | Moderate (RCC) |
| Workability | 75 mm slump |
| Placing | Without pumping, no chemical admixture |
| Aggregates | Saturated surface dry (SSD) for this calculation |
Step 1: Target Mean Strength
IS 10262 requires the target strength to be the higher of two expressions: one based on standard deviation and one based on a factor X that depends on the grade. For M20 and M25, the assumed standard deviation is 4.0 N/mm² and the factor X is 5.5 when no site data is available.
f'ck = fck + 1.65 s = 25 + 1.65 x 4.0 = 31.6 N/mm² f'ck = fck + X = 25 + 5.5 = 30.5 N/mm² Adopt the higher value: f'ck = 31.6 N/mm²
Step 2: Water-Cement Ratio
The standard provides curves relating 28-day strength to free water-cement ratio for different cement strengths. Alternatively, a relationship established from the plant's own trial data is used. For this example, assume the curve gives a w/c ratio of 0.45 for 31.6 N/mm² with OPC 53.
w/c from strength requirement = 0.45 (assumed from curve) Max w/c for moderate exposure, IS 456 = 0.50 Adopt w/c = 0.45 (lower of the two)
Step 3: Water Content
For 20 mm angular aggregate and a slump of 50 mm, IS 10262 gives a maximum water content of 186 kg per cubic metre of concrete. Water is increased by about 3 percent for every 25 mm increase in slump.
Required slump = 75 mm, i.e. 25 mm more than 50 mm Water = 186 x (1 + 0.03) = 191.6 kg/m³
Step 4: Cement Content
Cement = Water / (w/c) = 191.6 / 0.45 = 425.8 kg/m³ Check with IS 456: Minimum for moderate exposure = 300 kg/m³ -> OK Maximum OPC content = 450 kg/m³ -> OK
Step 5: Proportion of Coarse and Fine Aggregate
IS 10262 tabulates the volume of coarse aggregate per unit volume of total aggregate for a w/c ratio of 0.50, depending on maximum size and sand zone. For 20 mm aggregate with Zone II sand, the value is 0.62. The table value is increased by 0.01 for every 0.05 decrease in w/c ratio, because a richer mix has more paste to provide workability.
Table value at w/c 0.50 = 0.62 w/c is 0.45, i.e. 0.05 lower -> +0.01 Volume fraction of coarse aggregate = 0.63 Volume fraction of fine aggregate = 1 - 0.63 = 0.37
Step 6: Mix Calculation per Cubic Metre
Work in absolute volumes. The volume of each ingredient is its mass divided by its specific gravity and the density of water. Entrapped air for 20 mm aggregate is taken as 1 percent.
a) Total volume = 1.000 m³ b) Entrapped air = 0.010 m³ c) Volume of cement = 425.8 / (3.15 x 1000) = 0.1352 m³ d) Volume of water = 191.6 / (1.00 x 1000) = 0.1916 m³ e) Volume of all aggregate = 1 - 0.010 - 0.1352 - 0.1916 = 0.6632 m³ f) Mass of coarse aggregate = 0.6632 x 0.63 x 2.74 x 1000 = 1144.8 kg g) Mass of fine aggregate = 0.6632 x 0.37 x 2.60 x 1000 = 638.0 kg
| Ingredient | Mass (kg/m³) | Ratio by mass |
|---|---|---|
| Cement | 425.8 | 1.00 |
| Water | 191.6 | 0.45 |
| Fine aggregate (SSD) | 638.0 | 1.50 |
| Coarse aggregate 20 mm (SSD) | 1144.8 | 2.69 |
| Total (wet density check) | 2400.2 | - |
The total mass of about 2400 kg/m³ is a sensible wet density for normal concrete, which is a quick check that no arithmetic error has crept in. The mix ratio is 1 : 1.50 : 2.69 with w/c 0.45.
Step 7: Field Corrections and Trial Mixes
Aggregates on site are rarely in SSD condition. If the sand has 2 percent free surface moisture, it brings 638 x 0.02 = 12.8 kg of extra water, so the mixing water is reduced by 12.8 kg and the sand mass increased by the same amount. If the coarse aggregate absorbs 0.5 percent water and is dry, add 1144.8 x 0.005 = 5.7 kg of water to the batch.
- Trial mix 1: cast the calculated mix, measure slump, and cast cubes.
- Trial mix 2: adjust water, if needed, to get the specified slump while keeping w/c constant.
- Trial mixes 3 and 4: vary w/c by about ±10 percent with the same water content to bracket the target strength.
- Select the proportion whose 28-day cube strength meets f'ck, and record it as the approved mix.
Common Mistakes
- Designing for fck instead of the target mean strength f'ck.
- Ignoring the durability limits of IS 456, which may require a lower w/c than the strength curve gives.
- Using aggregate masses from the calculation without moisture correction on site.
- Skipping trial mixes and going straight to production.
Summary
The IS 10262 method fixes the target strength, chooses a water-cement ratio that satisfies both strength and durability, finds water content from aggregate size and slump, and derives cement content from them. Aggregate masses come from absolute volume after subtracting cement, water and air. For the illustrative M25 mix, the result is about 426 kg cement, 192 kg water, 638 kg sand and 1145 kg coarse aggregate per cubic metre, to be confirmed by trial batches.