What material is commonly used for ducts running from manhole to manhole?

Concrete is commonly used for ducts running from manhole to manhole primarily due to its durability, strength, and resistance to environmental factors. Concrete ducts can withstand the heavy loads and pressures typically encountered underground, making them ideal for the installation of utilities such as electrical and telecommunications cables, as well as drainage systems.

Additionally, concrete has excellent fire-resistance properties, which is crucial in preventing the spread of fire through utility systems. Its ability to be molded into various shapes and sizes allows for customizable designs that meet specific project requirements.

In contrast, while steel is strong and durable, it is prone to corrosion, especially in moisture-rich environments, which can compromise its integrity over time. Plastic, although lightweight and resistant to corrosion, may not possess the required strength for heavy loads or harsh underground conditions. Wood, on the other hand, is not suitable for underground use as it can rot and degrade when exposed to moisture and insects. Thus, concrete's combination of characteristics makes it the most practical choice for underground duct systems between manholes.