It might seem that the extremes of weather – the “polar vortex” freeze in the U.S. and Canada during the winter of 2014, or the triple-digit temp heat waves in the American southwest regions in recent summers – might cause the most damage to asphalt and pavement. But in fact it is the oscillations above and below 32 degrees F (0 degrees C) that are the primary culprit in creating potholes.
The science of freeze-thaw cycles and its effects are fairly simple. When water freezes it expands, technically referred to as volumetric expansion. This means that water wants about ten percent more space – 9.87 percent to be exact –when it goes from a liquid to a solid, frozen state. If that liquid water was in a crack or crevasse in pavement, it will push outward and, often, make the crack bigger.
That alone is bad enough, but in “shoulder season” conditions – any time the weather hovers above or below 32F/0C, usually in October or March, but sometimes in a January thaw or May cold spell – it gets much worse. A freeze on Tuesday means the water pries itself into pavement defects; a thaw on Wednesday translates into more water getting into the pried-open area, followed by another freeze on Thursday that pushes the pavement apart further.
In most roadways, that freeze-thaw effect gets seriously destructive when water and freezing temperatures reach the sub-pavement, where gravel and soil support the pavement. The expansion-contraction cycles undermine this supporting structure, culminating in a frozen-water bubble that pushes pavement up from below. Once a heavy vehicle passes over such a bubbled-up section of asphalt, the pavement collapses into a bona fide pothole.
It’s a worldwide phenomenon and vexing problem, as engineers in Mongolia document in a paper titled, “Freeze-Thaw Effects on Roadways; Approach to Pavement Design with Special Reference to Roads in Mongolia” (Kachroo, Raju and Gombo). The authors include in their analysis the fact that Mongolia experiences sub-zero temperatures in winter that causes the subsoil to hold its cold temperature until late spring. Thaws, and freeze-thaw cycles, happen to coincide with increased road traffic, a destructive combination for the central Asian country’s roads. Traffic in North America increases as well when the weather warms up.
Drivers who routinely follow a daily commute often report “overnight potholes,” a pavement defect that truly does pop up overnight. In fact, it might have been in development for days or weeks, but invisible to humans until the pothole bubble burst from a heavy bus or truck.
Cities where freeze-thaw cycles are more frequent
Some parts of the country have more of these cycles than others. The National Institute of Standards and Technology, a bureau of the U.S. Department of Commerce, measured freeze-thaw events for a dozen geographically representative cities in the U.S., identifying the number of cycles each city experiences annually. They are:
Kansas City, MO
Freeze-thaw cycles per year
In its own study, the National Ready Mixed Concrete Association reported in 2004 the number of freeze-thaw cycles for other cities:
State College, PA
Chapel Hill, NC
Salt Lake City, UT
Freeze-thaw cycles per year
What both sources point out is that the degree of moisture is another factor, where some regions have wetter climates than others. But that said, places such as Gallup, New Mexico clearly have more freeze-thaw challenges than Tampa or Tucson.
And to be clear, factors other than freeze-thaw cycles contribute to pothole development. Those include extreme heat (pavement sometimes explodes when trapped, sub-pavement water is converted to steam), heavy traffic, poor maintenance and time. This is why places that include Tucson, Tampa, Honolulu, Miami and Los Angeles all experience crumbling roads – no polar vortexes required.