What type of reinforcement is only acceptable for structural use in spiral reinforcing?

The correct answer highlights that plain reinforcement is specified for use in spiral reinforcing due to its adequate performance in handling the specific stresses and requirements of these applications.

Spiral reinforcement, typically used in circular columns and other similar structural elements, needs to effectively confine the concrete. Plain reinforcement, made from a smooth steel bar without any surface deformations, is capable of providing the necessary confinement without being overly complex or expensive. It can also be easily placed in a spiral configuration, ensuring the tight and uniform spacing needed for structural integrity.

Other types of reinforcement, such as deformed or high-strength reinforcement, may provide additional mechanical interlock and higher strength characteristics. However, they are not suitable for spiral reinforcement primarily because their design and surface characteristics can hinder effective spiraling and can potentially lead to complications in confinement relationships. Composite reinforcement is not standard in this context and would not meet the specific requirements typically outlined for spiral configurations in structural applications.