Pavement is the durable surface covering that makes modern transportation possible, providing a smooth, safe, and wear-resistant surface for travel. Its primary function is to effectively distribute concentrated vehicle loads over a wider area of the native soil beneath. This load spreading ensures the foundational earth, or subgrade, is not overloaded, preventing deformation and pavement failure.
Asphalt Concrete Pavement
Asphalt concrete, often called “blacktop” or simply “asphalt,” is a composite material used for the majority of the world’s road surfaces. It is a blend of two main components: aggregate (crushed stone, gravel, and sand) and a petroleum-based binder. The aggregate constitutes 90% to 97% by weight, providing the structural strength through its interlocking framework. The binder is a sticky, black, viscous substance called bitumen, or asphalt cement. Bitumen coats the aggregate particles, holding them together and providing water-resistant and flexible properties.
The most common form is Hot Mix Asphalt (HMA), where the aggregate is dried and heated to temperatures around 300°F (150°C), and the bitumen is also heated to reduce its viscosity. These hot materials are mixed at a central plant to ensure uniform coating before being transported and compacted while still hot. The pavement gains its final strength and hardness as the material cools and the bitumen solidifies.
Portland Cement Concrete Pavement
Portland cement concrete is a rigid pavement material selected for high-traffic highways, airports, and city intersections where strength and durability are paramount. Unlike asphalt, concrete hardens through a chemical reaction. The primary components are Portland cement, aggregates, and water. Portland cement is the fine, powdery binding agent that reacts with water to create a hardened matrix. It is made from materials rich in calcium, silica, alumina, and iron, typically sourced from limestone and clay, which are heated to high temperatures in a kiln.
Aggregates (sand and gravel) provide the bulk and structural stability. The hardening process is called hydration, an exothermic chemical reaction that occurs when water is introduced to the cement powder. This reaction forms new crystalline structures, such as calcium silicate hydrate (C-S-H) gel, which binds the aggregates together. This chemical set means the pavement cures over a period of days to weeks to reach its full load-bearing capacity.
The Underlying Pavement Structure
The visible surface layer of asphalt or concrete is only the top portion of a multi-layered system designed to manage traffic loads and environmental factors. Beneath the surface is the subgrade, the prepared, compacted native soil or engineered fill that forms the foundation of the road. The subgrade must be stable, as weakness here can lead to premature pavement failure.
Above the subgrade sit the subbase and base course layers, made of compacted granular materials like crushed stone or processed gravel. The base course is immediately beneath the surface and provides the main structural support, distributing the load uniformly. The subbase, when used, sits between the base and the subgrade, offering additional load spreading, drainage, and frost protection in certain climates. These underlying layers are essential because they ensure the heavy traffic loads are gradually dispersed, reducing the stress on the subgrade soil to an acceptable level. They also serve a hydraulic function, allowing water that infiltrates the pavement structure to drain away.