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## 8 Factors Influencing the Choice of Mix Proportions

According to **IS 456-2000 and IS 1343-1980**, the designs of the concrete mix should be based on the following factors:

### 1. Grade Designation

The grade designation gives characteristic compressive strength requirements of concrete. It is the major factor influencing the **mix design**.

**The concrete mix** has to be designed for a target mean compressive strength which is somewhat higher than the characteristic compressive strength.

### 2. Types and Grade of Cement

The rate of development of compressive strength of concrete depends upon the** type & grade of cement **used. The choice of the **type of cement** depends upon the requirements of performance at hand.

Where very high compressive strength is required, portland cement of grade 43 and 53 will be found suitable.

### 3. Maximum Nominal Size of Aggregates

The workability and compressive strength of concrete greatly depend upon the **maximum size of aggregates**. The workability increases with an increase in the maximum size of aggregate.

On the other hand use of the large maximum size of aggregate, requires a smaller quantity of cement for a particular **water-cement ratio**. However, the smaller size aggregates provide a large surface area for bonding with the mortar matrix which **increases the compressive strength**.

### 4. Grading of Combined Aggregate

The **grading of combined aggregate** i.e, the relative proportion of the fine & coarse aggregates in a concrete mix is one of the important factor affecting the strength and workability of concrete. For dense concrete, it is essential that the **coarse and fine aggregates** be well grade.

### 5. Water-Cement Ratio

At a given age and under normal temperature, the compressive strength of concrete depends primarily on the water-cement ratio. Lower the water-cement ratio, greater is the compressive strength & vice-versa.

### 6. Workability

Insufficient **workability of concrete** may liable for incomplete compaction of concrete which ultimately affects the strength, durability & surface finish of the concrete.

### 7. Durability

The durability of concrete is its ability to resist deterioration due to **weathering action**, **chemical attack**, **abrasion etc**. The requirements of durability are achieved by restricting the minimum cement content & the maximum water-cement ratio.

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From the consideration of permeability, the water-cement ratio is usually restricted to **0.45 to 0.55**. For a given water-cement ratio, the cement content should correspond to the required workability considering the placing conditions and the concentration of reinforcement.

**In addition,** the cement content is chosen to ensure sufficient alkalinity to provide a passive environment against corrosion of steel.

### 8. Quality Control

The strength of concrete may vary from batch to batch over a period of time. The source of variation in the strength of concrete may be considered due to the variation in the **quality of materials**,** mix proportion**, mixing equipment, supervision & workmanship. The factor controlling this variation is **quality control**. The degree of control is ultimately evaluated by the variation in test results.

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