What are Roads Made Of? Beyond the Surface

Roads connect us, paving the way from point A to point B and beyond. They are the arteries of civilization, vital for the movement of people, goods, and services. Yet, despite their importance in our daily lives, many of us rarely consider what goes into constructing these crucial pathways.

This article peels back the layers of asphalt and concrete to reveal the materials and methods that create the roads under our wheels.

Key Takeaways

  • The foundation of road construction starts with the subgrade, which is the natural soil compacted to create a stable base.
  • The subgrade’s quality, based on its composition and moisture content, is crucial for the road’s strength, requiring stabilization in areas with less ideal soil to support infrastructure.
  • Above the subgrade, the subbase layer of sand, gravel, or crushed stone provides support and enhances drainage, while the base layer of larger aggregate is the main load-bearing layer that distributes the load evenly and prevents water damage.
  • Roads are surfaced with either asphalt or concrete.

The Foundation of Road Construction

Pavement composition

The process of road construction is as complex as it is fascinating, involving more than just laying down a layer of asphalt. It starts with the foundation, the base that ensures the durability and longevity of the road.

Beyond serving as a mere platform for the layers above, the foundation plays a critical role in managing the stresses and strains caused by traffic and environmental conditions.

Proper foundation work is essential for preventing future road deformations and failures, highlighting the importance of meticulous planning and execution in the early stages of road construction.


The subgrade is the ground level or the natural soil upon which the road is built. Engineers compact this soil to create a stable base. The quality of the subgrade affects the road’s strength, requiring careful consideration of the soil’s composition and moisture content.

Imperfections in subgrade preparation can lead to significant maintenance issues down the line, such as potholes and cracks. Thus, assessing and enhancing the load-bearing capacity of the subgrade is a fundamental step in ensuring the longevity of the road.

This often involves the stabilization of the soil with lime or cement, especially in areas where the native soil is not ideally suited for supporting infrastructure.

Subbase and Base Layers

Highway surface components

Above the subgrade lies the subbase layer, typically composed of sand, gravel, or crushed stone. This layer provides a supportive foundation, enhancing drainage and further stabilizing the roadbed. The base layer, sitting directly under the asphalt or concrete surface, is usually made of larger aggregate and serves as the main load-bearing layer.

These layers not only distribute the load evenly but also prevent the upward movement of water by capillary action, thereby protecting the road surface from water damage. The choice of materials for these layers is critical, as it directly impacts the road’s ability to withstand various weather conditions and heavy loads.

Regular testing of these materials ensures that they meet the required specifications for strength and durability.

The Road Surface: Asphalt and Concrete

The surface layer is what most of us recognize as the road itself. This layer can be made of asphalt or concrete, each with its unique advantages. This topmost layer is designed to provide a smooth, durable surface for vehicles, offering the necessary friction for safe travel.

The choice between asphalt and concrete not only depends on the material properties but also on the funding and economic considerations behind these infrastructure projects.

Innovations in surface layer materials continue to improve road safety, noise reduction, and overall driving comfort, making the choice between asphalt and concrete a decision based on local conditions and requirements.


Asphalt, a mixture of bitumen (a sticky, black, and highly viscous liquid) and aggregate (stone, sand, and gravel), is prized for its flexibility and relatively quick installation process. Asphalt roads can be laid down and opened to traffic in a comparatively short period, making them a popular choice for time-sensitive projects.

The material’s ability to withstand extreme temperature changes without cracking adds to its appeal in regions experiencing severe winters or fluctuating weather conditions. Furthermore, asphalt paving can be recycled, making it a more sustainable option by reducing the need for new materials and minimizing waste.


Asphalt road construction

Concrete roads, made from cement, sand, aggregate, and water, are known for their durability and longevity. While the initial cost and construction time are higher than asphalt, concrete roads require less maintenance and have a longer lifespan, making them cost-effective in the long run.

Concrete’s resistance to heavy loads and less frequent need for repairs make it an ideal choice for highways and major urban roads. Additionally, concrete surfaces reflect more light than asphalt, reducing the need for street lighting and offering potential energy savings.

The material’s rigidity, however, requires precise construction techniques to prevent cracking and ensure a smooth ride.


How do engineers determine the appropriate depth for the subgrade and subbase layers?

Engineers determine the depth based on soil testing, traffic load projections, and environmental conditions of the area. They use this data to calculate the necessary thickness to ensure stability and longevity of the road.

Can roads be constructed in areas with poor soil quality?

Yes, roads can be constructed in areas with poor soil quality through soil stabilization techniques, such as mixing in lime, cement, or other binding agents to improve their load-bearing capacity and stability.

How does weather affect the choice between asphalt and concrete for road surfaces?

Weather plays a significant role; asphalt is more flexible and can better accommodate temperature fluctuations without cracking, making it suitable for colder climates. Concrete, being more rigid, is preferred in areas with less severe weather changes and where longevity and lower maintenance are prioritized.

What are the environmental benefits of using recycled materials in road construction?

Using recycled materials, such as rubber from tires or recycled plastic, reduces landfill waste, lowers the demand for new raw materials, and decreases the carbon footprint associated with the production and transportation of new materials.

How does permeable pavement contribute to urban sustainability?

Permeable pavement allows water to pass through the surface into the ground, reducing runoff, mitigating flood risks, and improving groundwater recharge. This can contribute to more sustainable urban drainage systems and water management practices.


In conclusion, we have two main materials for the road surface: asphalt, which is quick to install and flexible, and concrete, known for its strength and long life. It starts with a strong foundation, including the subgrade and base layers, to make sure the road lasts a long time and can handle traffic and weather.

Building roads involves much more than just laying down asphalt or concrete.