What Makes Potholes Form?
Freeze Thaw Processes, Weathering, and the Geology of Pavement
Jan 27, 2009
Potholes are an annoying part of winter and spring driving, and they can be expensive to fix. They are bowl-shaped holes in a pavement surface. What causes them? It's geology in action.
What is Pavement Made Of?
Pavement is the surface of a road or a sidewalk. It is supposed to last a long time and sustain the traffic of many people and cars. There are two main sorts of pavement that are used as walkways and roads.
Asphalt pavement is built on a base of gravel, tiny rocks. This stabilizes the road. The asphalt itself is a heavy, dark substance that comes from deposits of asphalt or is a product of the petroleum industry. It has been used for waterproofing in shipbuilding and mortar in building since 6000 BC. Asphalt paving is a mixture of asphalt cement, sand, and rock. To create the road surface, it is heated and placed on the roadway, then smoothed and compacted with a steamroller.
Concrete pavement is made using a mix of Portland cement, gravel, and sand. These materials are mixed together and laid smooth on the ground as a thick, wet layer of concrete. It dries into a smooth surface.
What Happens to Roads in the Winter?
Winter brings two parts to the road geology equation: water and cold. In the wintertime, snow and ice build up on the pavement. There may be small cracks that have already formed in the road over time. As the fall and winter go on, water seeps into these cracks. This is where the trouble begins for the pavement.
Roads, Sidewalks, and Freeze-Thaw Processes
Anyone who has ever filled a glass or a bottle with water and placed it in the freezer knows that if you forget it, the container may explode.
Most liquids condense as they get colder. The molecules in the liquid move more slowly, so they cannot resist the forces that bring the molecules together. As the liquid becomes more solid, it develops crystalline structures that bring molecules even closer together.
Water is different. As water cools, it contracts. However, when it reaches about four degrees, it begins to expand. Why? Water is made of hydrogen and oxygen molecules. Each hydrogen atom has an atom of oxygen on either side. The hydrogen molecules like to bond to each other, and as water cools, there is less energy to move the hydrogen from its natural tendency to bond to each other. The hydrogen bonds to each other, and the crystal structure of the water becomes more open than it was before. This makes the water get up to 9% larger as it cools.
What Water Expansion Does to Roads
As the water slips into the cracks in the road, it freezes and expands. Water may pool under the pavement, forcing the roadway upwards. It may pool in a crack, forcing the pavement apart. Over time, the force of the expanding water breaks the rocks and other parts of the pavement. Vehicles roll over the weakened pavement, putting pressure on it. The road begins to collapse. Pieces of the road move away from each other, and a pothole forms. Over time, repeated freeze-thaw action weakens the road or the sidewalk.
In February and March, the repeated freezing and thawing of the road exacerbates potholes. Ice under and inside the cracks in the road melts from the top down, letting in more water that freezes, thaws, and moves the pavement apart.
This form of weathering or erosion is common in nature. It happens to cliffs, paths, and even statues. For example, the statuary of older churches in cool climates is often affected by weathering. This puts roads in good company.
How to Repair or Prevent Potholes
There are many ways to prevent potholes. A good base road material that contains very little water will reduce the amount of ice that forms under the road. Good road drainage underneath the road and on the road prevents water from seeping into cracks.
When patching potholes, it is important to use high quality patching materials and processes that do not allow water to enter again. Placing material into the hole, packing it, and packing it again helps create a sturdy road patch.
Potholes are the bane of winter and spring drivers, but with some knowledge of geological processes and diligent pavement creation and management, they can be prevented and repaired.
Sources
E-Notes Encyclopedia: Cement and Asphalt
Kuennen, Tom. 2004. Better Roads for the Government / Contractor Project Team
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