Salt and snowplows can make potholes worse. But blame time, traffic, temperature and precipitation for most asphalt pavement deterioration.
A common misperception is that road salt and snowplows are what cause potholes. Both can be at least partially responsible, but that doesn’t explain why there are potholes in Los Angeles, Miami and Honolulu.
Potholes are a product of precipitation, temperature, traffic and time. To be more specific, temperatures fluctuating above and below freezing lead to freeze-thaw cycles of moisture that undermine pavement sub-surfaces; extreme heat that cooks moisture below the road surface can explode sections of asphalt, a less-common phenomenon. Heavy traffic is another factor, especially with large trucks and buses. Over time, when repairs of small cracks and potholes are inadequate those fissures grow naturally. All of these factors conspire to damage our streets and highways in a variety of climatic conditions.
But there are more potholes in regions affected by snow and freezing temperatures, and that includes a majority of pavement that crisscrosses the U.S. According to The Salt Institute, about 70% of American roads are in areas that get five or more inches per year of snowfall, in addition to freezing rain.
Consequently, in the U.S. an annual 13.6 to 18 million metric tons of deicing road salt is used.
That’s lot of salt. It goes without saying that sodium chloride (NaCl) and its deicing cousins (calcium chloride, magnesium chloride, potassium acetate and calcium magnesium acetate) are necessary for public safety. Getting rid of slippery ice and compacted snow goes a long way toward reducing vehicular accidents in winter. (There is an environmental argument against overuse of road salt, a discussion we will save for another day.)
These agents, commonly referred to as rock salt, are used all over the world. The salt doesn’t just stay on the roads, however, as salty water is sprayed on roadsides and even aerosolizes upward. A study conducted by TMR Consulting (Pittsburgh, Pennsylvania) in cooperation with the International Molybdenum Association – molybdenum is an element used to add strength and corrosive resistance to various metal building materials – found that road salt has several deleterious effects on building materials that include aluminum, zinc, carbon and weathering steel and some types of stainless steel. Road salt is implicated in the deterioration of reinforcing steel in concrete bridges. The effects range from discoloration to damaging oxidation of the materials. Building designers and managers are encouraged to study proximity to roadways that are deiced with salt in order to specify which materials should be used in new construction and what type of cleaning methods and maintenance schedules should be maintained to reduce its effects.
Of note, road salt is implicated in the deterioration of reinforcing steel in concrete bridges.
So if road salt is damaging to metal, can it mean that salt also damages road asphalt? Yes, but not in the same way. Salt causes potholes by altering the temperature at which water freezes, basically at 15 degrees F instead of 32 degrees F. The plus side of that is the freeze-thaw cycles happen at a lower temperature, sparing roads that rarely experience temperatures that low. The negative is that those cycles still happen where air and ground get that cold or colder. In places where temperatures might vacillate above and below 15 degrees, usually in the dead of winter, that means those potholes are being created just when it’s hardest to repair them.
Most of the country experiences some degree of freezing temperatures. And every state but Hawaii has seen below zero temperatures (F), according to the National Oceanic and Atmospheric Administration (NOAA). The polar vortex of January 14-21, 2014 brought frigid, below zero temperatures as far south as Texas, very low readings in parts of Florida and most states north in the Midwest, Mid-Atlantic and Northeast regions. Extreme temperatures recorded in that period included the following:
- Washington, DC +6 degrees F
- Chicago, Illinois -16
- Indianapolis, Indiana -15
- New York City, New York +4
- Pittsburgh, Pennsylvania -9
- Green Bay, Wisconsin -18
Of course, each of those cities plan for cold, snow and potholes just about every winter. But cities of the Southeast occasionally experience very cold temperatures, too. Some examples of record low temperatures in various winters from the Southeast Regional Climate Center (University of North Carolina, Chapel Hill, NC):
- Huntsville, Alabama -11 degrees F
- Pensacola, Florida +5
- Atlanta, Georgia -8
- Asheville, North Carolina -16
- Greenville-Spartanburg, South Carolina -6
- Richmond, Virginia -12
Even Miami, Florida once saw temperatures dip down to 30 degrees. So as should be clear, almost everywhere in the U.S. is subject to freeze-thaw conditions that lead to potholes. But what should be noted is the cycling above and below the freezing point – freeze-thaw-freeze-thaw, multiple times over a number of days, for example – is what leads to the rapid creation of potholes. Whether that happens around 32 degrees (without salt) or 15 degrees (with salt), the outcome is the same. The unpredictable nature of weather and temperatures is what makes the pothole damage of one year vary against another in the same locale.
As for snowplows, the upward heave of pavement can cause already weakened areas to get caught on snowplow blades. Most municipalities adjust the blade height to account for this, but inexperienced snow removal personnel might make errors at setting the blade too low.
One need not wait until springtime, or the first big thaw, for the net effect of each of these factors to become apparent. Potholes in winter are a common scourge to motorists. Some municipalities repair with temporary patch material and make permanent repairs when conditions are warmer and drier. Higher-performing cold mix asphalt is also used during winter where the two-step process is identified as too inefficient and expensive.