Sometime around early spring, many roads develop deep divots and pockmarks called potholes, and certain cities are even said to have two seasons: winter and pothole repair. The reasons behind this road damage involves both nature and the limitations of road construction.
Most roadways are built in layers, starting with compacted earth and gravel for drainage. Some older city streets may even have a subsurface of bricks. All of these layers are covered with asphalt, which is a gooey blend of bitumen, oil byproducts, curatives and aggregate gravel. In an ideal setting, this layer of asphalt repels rainfall and snow, forcing it into drains or the shoulder of the road.
Potholes form because asphalt road surfaces eventually crack under the heat of the day and the constant stresses of traffic. These cracks allow snow and rainwater to seep into the underlying dirt and gravel. During cold nights, the water freezes and expands, pushing out some of the dirt and gravel, leaving a hole when the water eventually melts. Drivers continue to drive over these unseen holes, putting even more stress on the thin asphalt layer covering them.
Eventually, the asphalt layer over these divots collapses, leaving holes in the roadway. Potholes can cause significant damage to a car's suspension system or tires if the driver fails to avoid them. They can also fill with water, obscuring any other hazards they may contain. Even in places where the air temperature rarely falls below freezing, excessive rainfall or flooding can eat away at the road.
Road maintenance crews have two different way of fixing the damage. These repairs are roughly similar to a dentist using either a temporary or permanent filling material for cavities. During the winter months, potholes receive what is known as a cold winter mix. This is a temporary fix consisting of a soft asphalt poured into the holes after they have been cleared of debris. A layer of gravel may be added to increase strength and stability, but the damage is often expected to reappear by spring.
A more permanent fix is called a hot summer mix. This combination of roadgrade asphalt and aggregate is designed to last for years, but it can only be applied during dry, warm weather. When road crews use a hot summer mix, they often reroute traffic around the worksite and spend more time preparing the road surface for the patch. The finished layer of new asphalt is usually compacted to match the level of the road, making it nearly invisible.