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While Minnesotans typically associate hitting bumps in the road with spring or summer, the science behind what makes a pothole occur begins months prior. 

St. Olaf Professor of Chemistry and Environmental Studies Paul Jackson ‘92 explains why Minnesota’s climate in particular is prone to deep asphalt divots — aka, potholes.

Before fixing a pothole, it would help to know: What is asphalt? 
Asphalt concrete –– often referred to as asphalt, blacktop, or pavement –– is a mixture of mineral aggregate and a bitumen binder, frequently laid in layers. Each layer is composed of aggregate of a particular shape and size and has a designated target thickness depending on the loads it needs to support. 

Bitumen, a complex carbon-based chemical mixture, behaves as both a solid and an extremely viscous material; think of cold, thick molasses syrup. It can be found in natural deposits, such as Pitch Lake in Trinidad or the Athabasca Oil Sands in Canada, but is most frequently generated as a residue product of crude oil distillation. 

When heated to over 300 degrees Fahrenheit, the bitumen will flow and mix sufficiently to coat the sands and rock used as aggregate, which in our conversation becomes the road surface. Asphalt concrete relies on the properties of bitumen to link materials together, resulting in a more flexible surface than the typical concrete derived from silica and clay.

What causes potholes in Minnesota? 
A number of factors may generate a pothole. First, you need some sort of deformation or cracking to occur in the asphalt concrete. This can arise from many factors, including: 

• Residual air pockets not removed during compaction
• Thermal stresses, which are when cold temperatures cause the asphalt aggregate to contract (or pull away) from neighbors
• The flexing of the pavement as vehicles cross its surface
• Poorly prepared subbase and base layers, including softening due to water infiltration. 

Once some degradation occurs, water penetrates the asphalt concrete. In climates with freeze-thaw conditions, water in the asphalt expands as it freezes, pushing the bitumen-coated aggregate apart and upward. Repeated traffic quickly wears away the displaced aggregate, leading to a pothole. Continued traffic combined with additional freeze-thaw cycles expands the pothole quite quickly in depth and diameter. The Minnesota Department of Transportation has a one-minute video showing the most frequent cause of potholes.

How are potholes and other pavement cracks fixed?
Repair strategies vary depending on the severity of the pothole (its width and depth), the season, and whether the moment calls for a short- or long-term fix. If the weather isn’t too cold and the pothole isn’t too deep, a hot mix (300-350 degrees Fahrenheit) containing bitumen and aggregate is packed into the hole. A compactor or heavy roller is then used to compress the patch. 

Deeper or more severe potholes require more substantive repairs. In the most severe cases, about a six foot square of the asphalt layer is removed, the issue is dug out, the base is repacked, and hot mix is added in compacted layers before a final lift and heavy roller. In cold weather, a hot mix cools too fast to adequately adhere to the existing pavement, so cold mix asphalt is used instead. 

Cold mix relies on an emulsion of water and bitumen, which is then used to coat the aggregate. The consistency is similar to your favorite, well-mixed salad dressing. This well-mixed dressing keeps all the lettuce, tomatoes, onions and croutons together. To properly emulsify a cold mix, the consistency needs to be sticky.  Quality compaction, along with evaporation of water, allows the mixture to cure. However, the lower bitumen content and ambient temperatures limit the durability and lifespan of the patch, which may last anywhere from a few months to a few years. 

Pavement cracks, especially those that cut across a surface, are typically patched with a rubberized sealant.

What other common road issues do Minnesotans experience? 
In addition to the stresses mentioned previously, road materials are challenged by Minnesota’s large seasonal temperature changes, heavy use of de-icing salt, increased traffic loads, and chemical reactions within older concrete aggregate mixes. 

For example, cement based concrete roadways may buckle when the air temperature reaches extreme heat levels. Cut segments allow for expansion and contraction; however, during extreme heat, that expansion might not be enough and a buckle or blow out forms. 

Asphalt, with its flexibility, is less inclined to buckle; however, it is susceptible to frost heaves –– water that freezes and expands in the subgrade, pushing up the surface. 

Another challenge, especially with old concrete, is the reaction between alkali in the cement and silica in the aggregate. Add a little bit of moisture, and you produce a gel-like substance that absorbs water and expands. This absorption of water creates stress on the concrete and leads to “map cracking,” spalling, and increased water infiltration into roads. Salt use for de-icing also degrades metal rods (rebar) used inside the roadway, in addition to having a number of negative environmental effects on soil and water.

Why does “construction season” seem to be getting longer and longer?
Regional climate trends influence our perception about road construction season. As winters shorten and the shoulders from spring and fall lengthen, the possible window for outdoor work expands. Moreover, warmer overnight temperatures in winter can increase the frequency of freeze-thaw cycles, further impacting the durability of the materials used for our sidewalks, trails, and roadways. In addition, increasing population, coupled to a reliance on road vehicles to move goods and people, places more demands on our roadways, affecting the frequency of maintenance and repairs.

How does your work at St. Olaf support student learning about material science and the environment? 
As an environmental scientist and chemist, I help connect the properties and behavior of matter to the material culture students experience in daily life. We recognize that materials –– such as water, petroleum, and copper –– exist in fixed amounts and that they are unevenly distributed around the globe. Humanity’s quest to access these materials yields both positive and negative impacts on the Earth, its systems and life within it. We learn how historical changes have occurred, work to understand the continuing material challenges we face, and strive to avoid the mistakes of past policies, scientific practice, and cultural norms. 

My research and student work teams partner with community members to bring a broader systems perspective to agricultural practices that improves surface water quality and soil health. They evaluate the abundance and distribution of microplastics in rural agricultural watersheds and seek to support actions rooted in sustainability and built on thoughtful metrics that account for the environmental footprints of our material and energy flows. 

About Paul Jackson
Paul Jackson, Ph.D., is a professor of chemistry and environmental studies at St. Olaf College. His curiosity about environmental contaminants in surface waters and the ability to measure these materials establish projects of local relevance that remind us of our global connectivity. He enjoys exploring the intersection of science, communities, materiality, ethics and communication. Jackson leads off-campus study courses, including “Environmental Science in Australia and New Zealand” or “Directed Undergraduate Research in Japan.” Outside of work, he can be found in the community theater, partnering with artists and art historians, or enjoying the company of his spouse.