Factors Affecting The Choice of Mix Proportions

The various factors that affect the choice of mix proportions are:

• Compressive strength
• Workability
• Durability
• Maximum Nominal Size Aggregate
• Grading and Type of Aggregate
• Quality Control

Compressive Strength

It is one of the most important Properties of concrete and influences many other desirable properties of the hardened concrete. The mean compressive strength that is required at a specific age usually 28 days, will determine the nominal water-cement ratio of the mix. The other factor affecting the strength of the concrete at a given age and cured at a prescribed temperature is the degree of compaction. According to Abraham's law, the strength of fully compacted concrete is inversely proportional to the water-cement ratio.

Workability 

The degree of workability required depends on the below mentioned three factors:

i. Size of the section to be concreted

ii. Amount of reinforcement

iii. Method of compaction that is used

Durability 

The durability of concrete is its resistance to the aggressive environmental conditions. The high strength concrete is generally more durable than the low strength concrete.

Maximum Nominal Size of Aggregate 

Larger the maximum size aggregate, smaller is the cement requirement for a particular water-cement ratio. This is because the workability of the concrete increases with the increase in the maximum size of the aggregate. However, the compressive strength tends to increase with the decrease in the size of the aggregate.

IS 456:2000, Indian Code For Concrete Design, recommend that the nominal size of the aggregate should be as large as possible.

Grading and Type of Aggregate 

Coarser the grading, leaner will be the mix. A very clean mix is not desirable since it does not contain enough finer material to make the concrete cohesive. The type of aggregate influences strongly the aggregate-cement ratio for the desired workability and stipulated water-cement ratio. An important feature of a satisfactory aggregate is the uniformity of the grading which is achieved by mixing different size fractions.

Quality Control

 The degree of control can be estimated statistically by the variations in the test results. The variation in the strength results from the variations in the properties of the mix ingredients and lack of control of accuracy in batching, mixing, placing, curing and testing. The lower the difference between the mean and the minimum strength of the mix, lower will be the cement-content required. The factor controlling this difference is termed as quality control.