Potholes from freezing weather aren’t the only thing that can go wrong with asphalt and concrete roads. The lessons from heat dome regions should be part of engineering, policy, and infrastructure planning.
There are two misperceptions about road pavement that could be increasingly problematic in the years to come. They are:
- All asphalt is made the same (it’s not)
- The only threats to roads are the freeze-thaw cycles that cause potholes (high temperatures can be a problem, too)
As the “heat dome” conditions of June and July 2021 demonstrated – roads buckled and were deformed in the US and Canadian parts of the Pacific Northwest – extreme temperatures at either end can lead to pavement failure. That of course begs the question: if high temperatures such as 108ºF in Seattle and 116ºF in Portland, Oregon can have these effects on asphalt and concrete, how are there functioning roads in Death Valley, California and Phoenix, Arizona, two places with perennial and protracted triple-digit temperatures?
In fact asphalt and concrete have varied compositions that are designed for the local climates. The bitumen that holds asphalt together, derived from petroleum, has different chemical structures and degrees of viscosity in roads laid in places like Minneapolis, Buffalo, and Cleveland versus those put down in warmer regions. Concrete slabs have cushions in the seams that are intended to absorb concrete expansion in warm weather, which again vary by region, as well as different kinds of crushed rock in the slurry mix.
Relative precipitation factors are a consideration as well in these compositions.
Which is why reports from Everson, Washington this summer were that State Route 544 failed when temperatures there reached 105ºF. On some sections of the route, the asphalt was laid over concrete. When the slabs of concrete underneath expanded from the high temperatures, buckling because of the extreme heat squeezed the seam buffers, the bitumen mix in the top layer of asphalt simultaneously softened, resulting in a rutted and cracked road. It was closed for a period of time. (The heat also disabled a Portland streetcar line when copper overhead wires stretched, sagged, which consequently rendered the system unusable.)
Streets of either type of pavement also suffer in extended heat events because they tend to retain solar heat into the night, when air temperatures usually cool down a bit. During the 2021 heat wave the interior communities of British Columbia (Lytton, Kamlooops, Prince George, and Williams Lake), where ocean-fed cooler air is blocked by mountains, the ground temperatures reached 133ºF. Slightly lower temperatures were recorded in the eastern portions of Washington and Oregon. Hospital emergency rooms in these areas treated skin burn injuries of people who had the misfortune of falling onto the pavement.
Now, back to Phoenix road building, where pavement withstands hot summer temperatures. Why not build them that way in Seattle and other northern cities? The problem with that is Seattle, Portland, Vancouver, Minneapolis, Chicago, Buffalo, Boston and New York will still experience winter and freeze-thaw cycles, requiring the types of pavement appropriate for those seasons. So the pressure is on engineers to invent pavement compositions that work in hot summers as well as cold, cold winters. The other problem is these extreme weather events aren’t at all predictable, given the capricious nature of a changing climate.
One additional point on unpredictable weather: Precipitation levels are breaking norms as well, particularly as we see 100-to-1000 year weather events of both rain and drought. Even with minimally above-average rainfall, snow, and ice, pavement is threatened. All forms of water find ways into small cracks, undermining the subsurfaces of the roa, which eventually leads to potholes. In drought, wildfires are more likely and with flash flooding mudslides in the aftermath when rains at last return.
With infrastructure a large part of what the public expects from government, and literally trillions of dollars in play, let’s hope that extreme temperature swings and ways to manage them are, pardon the pun, baked into the plans.