What materials are commonly used for base layers in highway construction?

The correct choice identifies crushed stone, gravel, and recycled asphalt as commonly used materials for base layers in highway construction. These materials are favored because they provide essential structural support and stability to the highway surface.

Crushed stone is known for its strength and permeability, allowing water to drain through while maintaining load-bearing capacity. Gravel, similarly, offers good drainage and compacts well, creating a solid foundation. Recycled asphalt has gained popularity as an environmentally friendly option, as it can be reused to create a strong and durable base layer that also promotes sustainability in construction practices.

In contrast, the other options include materials that do not offer the same structural integrity or suitability for use as base layers in highway construction. For instance, sand, clay, and loam may not provide adequate load-bearing support; they are more often used for subgrade stabilization or landscaping rather than being a robust foundation layer. Concrete, bricks, and steel are typically used in the surface layers or structural components rather than as base layers. Finally, pebbles, boulders, and soil lack the necessary compactible and consistent qualities required to create a reliable highway base. Thus, the selection of crushed stone, gravel, and recycled asphalt is optimal for ensuring durability and performance in highway construction