Which of the following materials may not be imbedded in concrete?

The correct answer is that flex, or flexible metal conduit, should not be imbedded in concrete. This is primarily because flexible conduits are not designed to withstand the alkaline environment of concrete over time, which can lead to deterioration or degradation of the conduit.

Flexible conduits are typically used for applications requiring movement or where the conduit may need to bend around obstacles. When these materials are buried in concrete, they may not provide the necessary protection from corrosive elements or physical stresses that concrete can introduce, leading to potential electrical hazards or equipment failures in the future.

On the other hand, materials such as PVC, metal conduit, and fiberglass are generally suited for embedding in concrete. PVC is resistant to corrosion and does not react negatively with concrete, making it a common choice for underground installations. Metal conduit, when properly rated and treated for concrete use, provides a strong pathway for wiring and can protect against physical damage and electromagnetic interference. Fiberglass is also resistant to corrosion and can withstand harsh environments, making it suitable for embedding as well.