Why Do Highways Alternate Concrete and Asphalt Every 50 Feet?

Highways are designed to be resilient and withstand the test of time, but they face unique challenges due to the variations in temperature and the need for flexible and rigid materials. One interesting aspect of highway construction is the alternating pattern of concrete and asphalt. This practice is not arbitrary but is based on the fundamental properties of these materials. In this article, we will explore the reasons why highways sometimes alternate concrete and asphalt every 50 feet, focusing on the properties of concrete and asphalt, and the solution they provide to the problem of thermal expansion and contraction.

Understanding the Properties of Concrete and Asphalt

Concrete is a rigid and durable material commonly used in highway construction due to its compressive strength and longevity. However, one significant drawback of concrete is its limited ability to expand and contract with temperature changes. When the temperature rises, concrete expands, and when it falls, it contracts. This expansion and contraction can put immense pressure on the structure, leading to cracks and potential failure if there is no room for movement.

Conversely, asphalt, another common material used in highway construction, is a flexible pavement that can better accommodate these temperature changes. The ability of asphalt to deform under pressure and then return to its original shape makes it a more adaptable material for highway surfaces. This flexibility is crucial in areas with extreme temperature fluctuations, as it prevents the concrete from being compressed to the point of failure.

The Problem of Expansion Joints

Traditional concrete pavements are constructed with expansion joints placed every 100 feet. These joints are designed to allow concrete to expand and contract without causing damage to the structure. However, over time, these joints can become filled with small pieces of gravel, leading to a buildup of debris. This debris can prevent the concrete from moving freely, causing it to buckle and creating further structural problems.

The issue arises when the concrete is unable to expand and contract freely. As the temperature increases, the concrete tries to expand, but if it is constrained, it can put enormous pressure on the structure. This pressure can cause the concrete to buckle, leading to significant structural damage. Conversely, when the temperature drops, the concrete contracts, and if it is not allowed to do so, it can create tension that can also lead to cracks.

Solution: Alternating Between Concrete and Asphalt

To mitigate these issues, highway crews have adopted a practice of alternating between concrete and asphalt every 50 feet. This approach leverages the strengths of both materials while addressing their limitations. In areas where long stretches of concrete would be problematic, narrower sections of asphalt are used, providing the necessary flexibility.

By alternating every 50 feet, the highway is not just dividing the structural load but also ensuring that the rigid nature of concrete can be balanced by the flexible nature of asphalt. This variant construction method allows for a more adaptable and resilient highway surface, capable of withstanding the challenges of varying temperatures more effectively.

The method of alternating every 50 feet is a practical solution to the problem of expansion and contraction. It ensures that the concrete and asphalt sections do not interfere with each other excessively, and that the highway can maintain its integrity over long distances. This alternating pattern helps in preventing the buildup of debris in expansion joints and allows both materials to perform optimally within their designated areas.

Conclusion

The alternating pattern of concrete and asphalt every 50 feet in highway construction is not just a design choice but a strategic solution to the complex problem of thermal expansion and contraction. By leveraging the rigidity of concrete and the flexibility of asphalt, highway engineers can create surfaces that are both durable and adaptable. This approach not only extends the lifespan of the highway but also enhances its safety and functionality.

As the field of highway construction continues to evolve, understanding and utilizing the unique properties of different materials will remain crucial. The practice of alternating between concrete and asphalt every 50 feet is a testament to the ingenuity and adaptability of engineers and highlights the importance of considering the full lifecycle of a highway when making construction decisions.