Why do roads get cracks and holes in them?

Question Answered by Tyler Smith, Parks and Recreation Foreman, City of Idaho Falls

Asphalt Paving
Photo: US Department of Transportation

Generally, cracking of roads happens over time from settling or shifting of grade, sub grade or sub soil material. Settling can be expedited if the materials beneath the road are not compacted enough or there is a subsurface water issue, such as a broken water line. Shifting happens naturally because the Earth’s crust moves. Other factors that will increase the presence of road cracks are utility maintenance that requires cutting into the road to get to buried lines and then patching the area, flood events, and sometimes below standard construction techniques.

How do you build a road?
The first part of road construction is one of the most important: a solid foundation. Without a solid foundation, any road that is built will fail long before its expected lifespan.

Bulldozers and graders move around dirt to create a level surface. Gravel is added in layers and machines roll over the surface to compact and flatten it further. Drains and storm sewers are also installed at this early stage.

Once the foundation is finished and has been inspected, it’s time to pave the road. One of the most common materials used for paving roads is asphalt.

Asphalt uses an oil-based substance called bitumen to make sand and crushed rock stick together like glue. After the asphalt is heated to about 300° F, construction crews spread and compact it onto the foundation already in place.

Asphalt Cookies
Materials: 1/3 cup cocoa powder, 1/2 cup milk, 1/4 pound butter, 2 cups sugar, chopped walnuts, shredded coconut, old fashioned and quick oats, saucepan, stove-top, mixing bowl, stirring spoon, sealed cans, wax paper.

  1. Pour 1/8 cup old fashion oats, 1/8 cup quick oats, 1 tablespoon walnuts and 1 tablespoon coconut into a mixing bowl. These ingredients will simulate road construction materials.
  2. In the saucepan, combine the cocoa powder, milk, butter and sugar.
  3. Heat, stirring frequently until the mixture boils for 2 minutes. This chocolate mixture is like asphalt. When it is heated to 300° F, it is also a liquid.
  4. Pour 1/4 cup of the liquid chocolate mixture into your mixing bowl. For asphalt, a drum mixer mixes all the ingredients together until they are covered with the asphalt binder. Use the spoon to mix all of the ingredients, making sure to completely cover the nuts, oats, and coconut with the chocolate binder. Note that the mixture cools while it is being stirred and becomes stiffer and starts to stick together. Asphalt behaves in a similar manner.
  5. When everything is mixed together, pour and mound the mixture onto a square of wax paper. Cover with another piece of wax paper.
  6. When creating a road, the asphalt is spread with a paver and then rolled into a thin mat with a roller. The roller is very heavy and squishes the air out of the pavement, which makes it stronger. Use a can to roll your chocolate asphalt mixture out to a 1/4” to 1/3” thick cookie.

Can you still identify the different materials you put in your cookie? When you examine an asphalt road, you can see the rocks and gravel added to the asphalt mixture.

Feel the heat coming off the top of the cookie. When asphalt is first rolled out, it is still very hot. It hardens as it cools. When your cookie is cool, you can peel off the wax paper and eat it.

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