_____ piping shall not be placed in direct contact with concrete walls or floors.

Copper piping should not be placed in direct contact with concrete walls or floors primarily due to the potential for corrosion. Concrete can have high levels of alkalinity, which can create an environment that promotes the oxidation of copper. When copper is in direct contact with alkaline materials like concrete, it can suffer from a type of corrosion known as "concrete corrosion," which leads to pitting and eventual failure of the piping.

To ensure the longevity and integrity of copper plumbing systems, it is important to use protective measures such as insulation or sleeves that maintain a barrier between the piping and the concrete surfaces. This helps to minimize the risk of corrosion and contributes to the overall durability of the plumbing installation.

In contrast, ductile iron, cast iron, and galvanized steel have different properties and corroding behaviors. Ductile iron and cast iron can generally withstand direct contact with concrete due to their protective coatings and resistance to certain types of moisture. Galvanized steel, while also more resilient than copper in this situation, can still be prone to corrosion without protective measures but does not suffer as readily from alkaline environments as copper does.