This article describes the** mix design procedure for M25 concrete** by

**. The article has been divided into three parts- data collection, parameters setting, and calculations.**

*IS 10262:2019*Each step contains a reference to the corresponding codes and calculations are done accordingly. Let us start the mix design procedure for concrete of grade M25.

## Mix Design For M25 Concrete

### 1. Data Collection

The data collection part requires us to find out the inputs required for the design mix. Let us decide on the inputs.

Grade | M25 |

Type of Cement | PPC |

Maximum Size of Aggregate | 20 mm |

Exposure Condition | Severe |

Slump | 75 mm |

Method of Concrete placing | Pumpable |

Aggregate Type | Crushed |

Zone of Fine Aggregate | II |

Max. cement content: | 450 kg/cum |

Admixture: | Superplasticizer |

### 2. Parameter Setting

Under this head, we shall set the parameters that will govern our calculations. The parameters shall be based on the input data and relevant IS codes.

#### Target Mean Strength

For M25 grade concrete, 25 N/sqmm is the characteristic strength. However, this value may not be consistent for all the batches of concrete.

The reasons for deviation in the strength of each batch may be multiple and different. Therefore, it is necessary to set the mean strength which is higher than the characteristic strength.

The target mean strength is calculated by adding the characteristic strength and standard deviation.

f’ck = fck + 1.65xS

OR

f’ck = fck + X, whichever is higher

where,

f’ck is target mean strength fck is characteristic strength

S is the standard deviation (for M25 it is 4 as per Table 2)

X is the factor based on the grade of concrete (for M25 it is 5.5 as per Table 1) Therefore, the target mean strength is calculated as-

f’ck = 25 + 1.65×4 = 31.6

OR

f’ck = 25 + 5.5 = 30.5

Thus, **the mean target strength is 31.6 N/sqmm **(higher value of the above two calculations).

#### Air Content

As per Table 3 of IS 10262:2019, for the maximum aggregate size of 20 mm, **the approximate air content entrapped in concrete is 1%**.

#### Water Cement Ratio

To determine the water content, let us refer to the below chart from IS 10262:2019. For 31.6 N/sqmm target strength and 20 mm maximum aggregate size, we shall choose Curve 1. The **free water content as determined from the below graph is 0.45**.

#### Water Content

The water content requirement of concrete depends upon several factors. The aggregate size and shape, desired workability, and use of additives are major influencing factors.

Table 4 of IS 10262:2019 specifies 186 kg of water for 1 cubic meter of concrete with 20 mm maximum size aggregate. The slump for this value is 50 mm.

Estimated water content for 75 mm slump-

= 186 + (3×186)/100 = 191.58 kg

With the 1% dose (by weight of cement) of superplasticizer, the water requirement is reduced by 20- 30%. Say 25%.

Hence, the water content is

= 191.58 x 75% = 143.685 kg = 144 kg.

### 3. Calculation

#### Cement Calculations

Water cement ratio = 0.45

= 144/0.45= 320 kg/m^{3}

From Table 5 of IS456, the minimum cement content for a ‘severe’ exposure condition is 320 kg/m^{3}. Hence our calculation is ok.

#### Proportioning Of Volume Of Coarse & Fine Aggregate

As per Table 5 of IS10262, for the nominal maximum aggregate size of 20 mm & Zone-II of fine aggregate, the coarse aggregate percentage is 62%.

It should be noted that the coarse aggregate is 62% of the total aggregate when the water-cement ratio is 50%. In our case, the water-cement ratio is 45%. Therefore, the ratio of coarse aggregate needs correction.

For every 5% decrease in water, the coarse aggregate is increased by 1%. Our water-cement ratio is lesser by 0.5, therefore, increasing the coarse aggregate by 1%.

Final percentage of coarse aggregate = 62+1 = **63%**

Therefore, percentage of fine aggregate = 100 – 63 = **37%**

#### Mix Calculation

Total concrete volume = 1 m^{3}

The volume of entrapped air in wet concrete = 1% = 0.01

**•** **Volume of cement **= { (Mass of cement) / (Sp. Gravity of cement) } x 1/1000

= (320/2.88) x (1/1000) = **0.11 m ^{3}**

**•** **Volume of Water **= { (Mass of water) / (Sp. Gravity of water} x 1/1000

= (144/1) x (1/1000) = 0.144 m^{3}

**•** **Volume of admixture **= { (Mass of admixture) / (Sp. Gravity of admixture} x 1/1000

= (3.2/1.145) x (1/1000) = 0.0028 m^{3}

**•** **Volume of aggregate **= { (a – b) – (c + d + e)}

= {(1-0.01) – (0.11+0.144+0.0028)} = 0.7332 **m ^{3}**

**•** **Mass of Coarse Aggregate **= f x Volume of Coarse Aggregate x Sp. Gravity x 1000

= 0.7332 x 0.63 x 2.74 x 1000 = 1256.65 kg = 1257 kg

**•** **Mass of Fine Aggregate **= f x Volume of Fine Aggregate x Sp. Gravity x 1000

= 0.7332 x 0.37 x 2.65 x 1000 = 718.90 kg = 719 kg

#### Adjustment On Water & Aggregate if Aggregate is Dry

The underlying assumption in water & aggregate calculation is that the aggregate is used in saturated surface dry (SSD) conditions. However, if the aggregate is dry, the correction needs to be done as follows.

**•** **Fine Aggregate (Dry)**

= Mass of Fine Aggr. In SSD Condition / (1+ Water Absorption/100)

= 719/ (1 + 1/100) = 712 kg/m^{3}

**•** **Coarse Aggregate (Coarse)**

= Mass of Coarse Aggr. In SSD Condition / (1+ Water Absorption/100)

= 1257/ (1+0.5/100) = 1251 kg/m^{3}

The extra water to be added for absorption by the fine and coarse aggregate-

**•** **For Coarse Aggregate**

= Mass of Coarse Aggr in SSD – Mass of Coarse Aggregate in Dry

= 1257 – 1251 = 6 kg/m^{3}

**•** **For Fine Aggregate**

= Mass of Fine Aggr in SSD – Mass of Fine Aggregate in Dry

= 719 – 712 = 7 kg/m^{3}

Total extra water requirement = 6 + 7 = 13 kg/m^{3}

Therefore, the **water requirement for the dry aggregate **is 144 + 13 = **157 kg/m ^{3}.**

### 4. Points to Remember

1. The total mass of aggregate shall be divided into two parts- 10-20 mm & 4.75-10 mm. The ratio of the two parts shall be by Table 7 of IS383.

2. The slump shall be tested for each mix and the water should be adjusted as required.

3. Two more trials of +-10% variation in water-cement ratio shall be carried out and plotted on a graph against their achieved strengths. This shall be worked out to set the ratio to achieve target strength in the field trials.