Potholes form due to cracks in the asphalt layer, allowing water to seep into the underlying layers and freeze, causing the material to push out. Road maintenance crews use temporary cold winter mix or permanent hot summer mix to repair them.
In early spring, many roads develop deep pits and pockmarks called potholes, and some cities are said to have two seasons: winter and pothole repair. The reasons behind this road damage relate to both the nature and limitations of road construction.
Most roads are built in layers, starting with compacted earth and gravel for drainage. Some streets in the older city may even have a brick subsoil. All of these layers are covered with asphalt, which is a sticky mix of bitumen, oil by-products, curatives, and aggregate gravel. In an ideal environment, this layer of asphalt repels rain and snow, forcing it into sewers or the side of the road.
Potholes form because paved 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 earth and gravel. During cold nights, the water freezes and expands, pushing some of the dirt and gravel out, leaving a hole when the water melts. Drivers continue to drive over these unseen potholes, putting even more stress on the thin layer of asphalt covering them.
Eventually, the asphalt layer above these pits 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 up with water, obscuring any other dangers they may contain. Even in places where the air temperature rarely drops below freezing, excessive rainfall or flooding can erode the road.
Road maintenance crews have two different ways to repair the damage. These repairs are much the same as a dentist using a temporary or permanent filling material for cavities. During the winter months, the holes get what is known as a cold winter mix. It is a temporary solution consisting of a soft asphalt poured into potholes after they have been cleared of debris. A layer of gravel can be added to increase strength and stability, but the damage is often expected to reappear by spring.
A more permanent solution is called a hot summer mix. This combination of road asphalt and aggregates is designed to last for years, but can only be applied during dry, hot weather. When road crews use a warm summer mix, they often divert traffic around the construction site 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.
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