ANS:D - thermal co-efficient of expansion.

Steel rods are normally used for concrete reinforcement because concrete and steel have almost equal thermal coefficient of expansion. Explanation:

• The thermal coefficient of expansion refers to how much a material expands or contracts when subjected to changes in temperature.
• Concrete and steel have similar coefficients of thermal expansion, meaning they expand and contract at approximately the same rate when exposed to temperature variations.
• This similarity in thermal behavior helps prevent the development of internal stresses and cracking within reinforced concrete structures due to temperature changes.
• While concrete is strong in compression, it is relatively weak in tension. Steel, on the other hand, has high tensile strength, making it well-suited for reinforcing concrete to resist tensile forces.
• By combining concrete's compressive strength with steel's tensile strength, reinforced concrete structures can efficiently withstand a wide range of loading conditions.
• Although concrete and steel have different mechanical properties such as tensile strength, compressive strength, and Young's modulus, their similar thermal expansion characteristics contribute to the compatibility and durability of reinforced concrete construction.