Concrete mix design is the process of determining the right proportions of cement, aggregates, water, and admixtures to achieve the desired strength and durability at an economical rate. It is a critical step in ensuring the structural integrity, performance, and cost-efficiency of concrete structures.
This article provides a step-by-step explanation of the concrete mix design procedure as per IS 10262:2019 (latest code in India), which is used widely for general and structural concrete.
Objectives of Concrete Mix Design
- To achieve the desired workability in fresh concrete.
- To attain the required compressive strength.
- To ensure durability based on environmental exposure.
- To optimise cost by minimising the use of cement and admixtures.
- To maintain consistent quality during mass concrete production.
Step-by-Step Concrete Mix Design Procedure
Step 1: Collection of Data
Before beginning the mix design, gather the following basic information:
- Grade of concrete (e.g., M20, M25, M30, etc.)
- Workability requirement (slump value)
- Maximum size of aggregate
- Type of exposure condition (mild, moderate, severe, etc.)
- Type of cement (OPC 43, OPC 53, PPC, PSC, etc.)
- Source and specific gravity of materials
- Method of placing concrete (manual or pump)
- Type and quantity of admixtures (if any)
Step 2: Target Mean Strength Calculation
The target mean strength (fck) is higher than the characteristic strength to account for variations:
fck(target)=fck+1.65×S
Where:
- fck = characteristic compressive strength at 28 days
- S = standard deviation (from IS 10262 or past test results)
Example: For M25 grade with standard deviation 4 MPa:
fck(target)=25+1.65×4=31.6 MPa
Step 3: Selection of Water-Cement Ratio
Select the w/c ratio based on durability (Table 5 of IS 456:2000) and strength requirement. Choose the lower value to satisfy both.
For example:
- For severe exposure: Max w/c = 0.45
- From strength graph: Required w/c = 0.40
Use 0.40.
Step 4: Selection of Water Content
From Table 7 of IS 10262:2019, get the water content for the desired slump and aggregate size.
For 20 mm aggregate and 50 mm slump:
- Water = 186 kg/m³ (adjust if higher slump or admixtures are used)
Adjustments:
- Increase the water for a higher slump
- Reduce water if superplasticiser is used (e.g., reduce by 20%)
Step 5: Calculation of Cement Content
Cement content=Water content / (w/c ratio)
Example:
186/0.40=465 kg/m3
Check that this value is:
- Not less than the minimum cement content for durability (from IS 456:2000)
- Not more than the maximum cement content (usually 450 kg/m³ for shrinkage control)
Step 6: Proportion of Fine Aggregate and Coarse Aggregate
From Table 8 of IS 10262, select the volume of coarse aggregate per unit volume of total aggregate based on:
- Nominal max aggregate size
- Zone of fine aggregate
- w/c ratio
Example:
- For 20 mm aggregate and Zone II sand, w/c = 0.40: Volume of CA = 0.62
Then:
- Volume of FA = 1 – Volume of CA = 0.38
Adjust if sand is Zone I or Zone III.
Step 7: Calculation of Mix Ingredients
Use the absolute volume method to calculate mix quantities:
🔍 What is the Absolute Volume Method?
In concrete mix design, we assume that the total volume of concrete = 1 m³ (for design purposes). Then we calculate how much space is occupied by each ingredient: cement, water, admixture, fine aggregate, and coarse aggregate, based on their specific gravities.
This method ensures the mix is balanced by volume, not just weight.
🧪 Formula for Volume of Each Ingredient
Volume=Mass/(Specific Gravity×1000)
Where:
- Mass = weight of ingredient (kg/m³)
- Specific Gravity (SG) = of that material (unitless)
- 1000 = conversion factor (1 m³ = 1000 liters)
✅ Step-by-Step Example (for M25 Mix)
Let’s assume:
- Water = 186 kg
- Cement = 465 kg
- Admixture = 0.5% of cement = 2.3 kg
- SG of cement = 3.15
- SG of water = 1.00
- SG of admixture = 1.10
- Total volume = 1 m³
1️. Volume of Cement
Vcement=465/3.15×1000=0.1476 m3
2️. Volume of Water
Vwater=186/1.00×1000=0.186 m3
3️. Volume of Admixture
Vadmix=2.3/1.10×1000=0.00209 m3
4️. Volume of Aggregates
Now subtract the sum of volumes of cement, water, and admixture from the total concrete volume (1 m³):
Vaggregates=1−(0.1476+0.186+0.00209)=0.6643 m3
This is the combined volume of fine and coarse aggregates.
5️. Split Between FA & CA
Let’s say from the IS 10262 table, FA/CA ratio = 0.38:0.62
Then:
- FA volume = 0.38 × 0.6643 = 0.2524 m³
- CA volume = 0.62 × 0.6643 = 0.4119 m³
Now convert these volumes into weight:
6️. Convert Aggregate Volumes to Weight
Assume:
- SG of FA = 2.65
- SG of CA = 2.7
WFA=0.2524×2.65×1000=669.86 kg
WCA=0.4119×2.7×1000=1112.13 kg
✅ Final Mix Proportions (kg/m³)
| Material | Quantity (kg) |
| Cement | 465 |
| Water | 186 |
| Fine Aggregate | ~670 |
| Coarse Aggregate | ~1112 |
| Admixture | 2.3 |
🔁 Mix Ratio by Weight (Rounded):
Cement : FA : CA : Water = 1 : 1.44 : 2.39 : 0.40
Step 8: Trial Mix and Adjustments
- Prepare a trial batch with calculated proportions.
- Check workability (slump) and compressive strength.
- Adjust the mix by changing:
- Fine/coarse aggregate ratio for workability
- Water or superplasticiser for the slump
- Cement or w/c ratio for strength
- Finalise the mix after a minimum of 3 successful trials.
Sample Concrete Mix Design (M25 Grade)
Target strength: 31.6 MPa
Water-cement ratio: 0.40
Cement content: 186 / 0.40 = 465 kg/m³
Water: 186 kg
Fine aggregate: 722 kg
Coarse aggregate: 1150 kg
Admixture: 0.5% of cement = 2.3 kg
Slump: 75-100 mm
Mix proportion (by weight):
Cement: FA: CA: Water = 1:1.44:2.39:0.40
You all know the M25 mix ratio is 1:1:2, then how its come to 1:1.44:2.39:0.40? Read below.
🔍 M25 (1:1:2) — What It Means?
The mix ratio 1:1:2 is an approximate nominal mix ratio, used only for small-scale or non-engineered construction (like floors, non-structural elements, etc.).
In a nominal mix, we assume:
- 1 part Cement
- 1 part Fine Aggregate
- 2 parts Coarse Aggregate
(by volume, not by weight)
This does not guarantee strength like M25 — it only roughly achieves around M20 under good conditions.
🧪 M25 — Why We Use Mix Design (like 1:1.44: 2.39: 0.40)?
When we want M25 grade concrete, we must ensure that the concrete achieves 25 MPa compressive strength after 28 days. So, we use the Concrete Mix Design (as per IS 10262) — not just thumb rules.
In a designed mix, proportions are based on:
- Material properties (specific gravity, moisture content)
- Water-cement ratio (for strength & durability)
- Workability requirement (for site conditions)
Hence, we get exact weight-based proportions like:
- Cement : Fine Aggregate : Coarse Aggregate : Water = 1 : 1.44 : 2.39 : 0.40 (by weight)
This mix is accurate, economical, and satisfies all code requirements — unlike the fixed nominal ratio 1:1:2.
🔁 Summary
| Type | Ratio | Use Case |
| Nominal Mix | 1:1:2 | Small works, no lab tests |
| Design Mix | 1:1.44:2.39 | Structural concrete (M25, etc) |
📊 Nominal Mix vs Design Mix Comparison
| Grade of Concrete | Nominal Mix Ratio<br>(Cement:Sand:Aggregate) | Design Mix Ratio<br>(Approx. by weight) | Strength (MPa) | Where Used |
| M20 | 1 : 1.5 : 3 | 1 : 1.6 : 2.9 : 0.50 | 20 | Floors, beams (small buildings) |
| M25 | 1 : 1 : 2 (rarely used) | 1 : 1.44 : 2.39 : 0.40 | 25 | Columns, slabs, beams (standard RCC) |
| M30 | Not permitted as nominal mix | 1 : 1.2 : 2.1 : 0.38 | 30 | Footings, high-rise RCC, foundations |
| M35 and above | — (must be design mix) | Varies (lab-designed) | ≥35 | Bridges, heavy-duty structures, precast |
🔍 Key Notes:
- Nominal mixes are allowed only up to M20, per IS 456:2000 Table 9.
- Design mixes must be done using proper lab procedures and IS 10262 for M25 and above.
- The water-cement ratio in design mixes is adjusted based on:
- Required strength
- Durability (exposure conditions)
- Workability (slump)
- Design mix ratios vary slightly with:
- Type of cement (OPC/PPC)
- Zone of sand (I to IV)
- Admixture usage
Conclusion
Concrete mix design is both a science and an art. By following the IS 10262 procedure, civil engineers can ensure optimal concrete performance concerning strength, durability, and cost. It is essential to conduct trial mixes and adjust based on real-site conditions for the best results.
share now