Why is only iron or steel used as reinforcement in concrete structures, while others metals ,duralumin for example , are never employed ?

Answer

### Step-by-Step Solution:

1. **Understanding the Role of Reinforcement in Concrete:**
- Concrete is strong in compression but weak in tension. To enhance its tensile strength, materials that can withstand tensile forces are added as reinforcement.

2. **Common Reinforcement Materials:**
- The most commonly used materials for reinforcement in concrete structures are iron and steel. This is due to their favorable mechanical properties and compatibility with concrete.

3. **Thermal Expansion Coefficient:**
- One of the critical factors in choosing reinforcement materials is the coefficient of thermal expansion. This coefficient indicates how much a material expands or contracts with temperature changes.

4. **Comparison of Thermal Expansion Coefficients:**
- The thermal expansion coefficient of steel is approximately \(12 \times 10^{-6} \, \text{°C}^{-1}\), and for iron, it is about \(13 \times 10^{-6} \, \text{°C}^{-1}\).
- In contrast, the thermal expansion coefficient of concrete is around \(10 \times 10^{-6} \, \text{°C}^{-1}\).

5. **Compatibility of Materials:**
- The thermal expansion coefficients of steel and iron are very close to that of concrete. This similarity ensures that when temperature changes occur, the expansion or contraction of the reinforcement and concrete occurs at similar rates.

6. **Consequences of Using Other Metals:**
- If metals with significantly different thermal expansion coefficients (like Duralumin) were used, they would expand or contract at different rates compared to concrete.
- This discrepancy would lead to the development of friction between the concrete and the reinforcement, which could cause cracking, reduce the overall strength of the structure, and compromise its integrity.

7. **Conclusion:**
- Therefore, iron and steel are preferred for reinforcement in concrete structures due to their similar thermal expansion properties, which help maintain the structural integrity and prevent issues related to thermal stress.