Durability of Concrete and Its Proportions

Introduction:
Durability of concrete refers to its ability to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. The durability of concrete is dependent on various factors such as water-cement ratio, cement-aggregate ratio, aggregate-cement ratio, sand-cement ratio, and others. In this question, we have to identify which proportion has a direct relationship with concrete durability.

Explanation:
The correct answer is option C, which is cement-aggregate ratio. The following points explain why cement-aggregate ratio is directly proportional to the durability of concrete.

1. Cement-Aggregate Ratio:
The cement-aggregate ratio is the proportion of cement to the aggregate used in the concrete mix. It is a critical factor that affects the strength and durability of concrete. Cement provides the binding material that holds the aggregate together, while the aggregate provides the structural support to the concrete.

2. Strength and Durability:
The strength and durability of concrete are directly related to the amount of cement in the mix. The more cement used, the stronger and more durable the concrete will be. This is because cement provides the necessary strength to resist the compressive and tensile forces applied to the concrete.

3. Water-Cement Ratio:
The water-cement ratio is also a critical factor that affects the durability of concrete. The more water added to the mix, the weaker the concrete will be. This is because excess water reduces the strength of the cement paste, which weakens the bond between the cement and the aggregate.

4. Aggregate-Cement Ratio:
The aggregate-cement ratio is the proportion of aggregate to cement used in the concrete mix. It is another critical factor that affects the durability of concrete. The more aggregate used, the weaker the concrete will be. This is because excess aggregate reduces the amount of cement paste available to bond with the aggregate.

Conclusion:
In conclusion, the durability of concrete is directly proportional to the cement-aggregate ratio. This is because the amount of cement used in the mix affects the strength and durability of the concrete. The more cement used, the stronger and more durable the concrete will be. Therefore, it is essential to maintain the correct proportion of cement to aggregate in the concrete mix to ensure its durability.