ANS:D - 350 kg/cm²

Prestressed concrete is a construction material and technique where internal stresses are introduced into a concrete element before it is subjected to an external load. This process helps to counteract the tensile stresses that occur when the member is under load, enhancing its performance and durability. Here's a more detailed explanation:

1. Purpose of Prestressing: The primary purpose of prestressing is to place the concrete in compression before it is subjected to loads. This compression helps to counteract the tensile stresses that occur when loads are applied to the structure. By placing the concrete in compression from the outset, prestressed concrete structures can achieve longer spans, higher load-carrying capacities, and improved resistance to cracking and deformation.
2. Materials Used: Prestressed concrete typically involves the use of high-strength steel tendons or cables that are tensioned before the concrete is poured. These tendons are usually made of high-strength materials such as steel, and they are placed in the concrete formwork according to the design requirements. Once the concrete has reached sufficient strength, the tendons are tensioned using hydraulic jacks, applying a predetermined amount of force.
3. Concrete Strength: The strength of the concrete used in prestressed members is critical to the performance and safety of the structure. High-strength concrete is typically required to withstand the high stresses induced by the prestressing forces. The minimum cube strength of the concrete used for prestressed members is usually specified by design codes or project specifications.
4. Minimum Cube Strength: The minimum cube strength of concrete used for prestressed members is commonly specified to be 400 kg/cm². This high strength ensures that the concrete can withstand the stresses induced by the prestressing forces and provide the desired level of structural performance and durability. Lower strength concrete may not be able to adequately support the prestressing forces, leading to potential failure or reduced performance of the structure.
5. Quality Control: Achieving the specified concrete strength is crucial for the success of prestressed concrete structures. Quality control measures are implemented during concrete production, transportation, and placement to ensure that the concrete meets the required strength and other performance criteria. Testing of concrete samples is typically conducted to verify the strength of the concrete before it is used in construction.

In summary, the minimum cube strength of concrete used for prestressed members is typically specified to be 400 kg/cm² to ensure the structural integrity, durability, and performance of the prestressed concrete structures. This high strength concrete, combined with prestressing techniques, allows for the construction of efficient and durable structures capable of withstanding significant loads and environmental conditions.