Behind the Forecast: The salty situation wrecking roads

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Science Behind the Forecast 2/22/19

LOUISVILLE, KY (WAVE) - Potholes. They are a drivers’ worst nightmare and are seemingly ubiquitous during the winter and spring. Our wild temperatures this season have not helped. From a three degree low on January 30th to near 70° multiple times the following week.

The crazy swings between warm and very cold temperatures cause asphalt to expand and contract respectively, eventually leading to cracks, weakening it and reducing its longevity.

In Kentucky, a hot-mix asphalt is used for paving many roadways, according to the Kentucky Transportation Cabinet (KYTC). "We specify a liquid asphalt binder in our mixes with performance grade (PG) classifications that span the average temperature ranges experienced in Kentucky based on weather collection data,” KYTC Spokesperson Andrea Clifford explained.

However, despite precautions taken, potholes are an inevitability. When water oozes into pavement, freezes, and expands. The increasing volume (water expands in volume by 9% when it turns to ice) can damage the pavement and raise the internal pressure. Water remaining in it’s liquid state can also cause issues. “Hydrostatic pressures caused by water sitting in cracks and openings in the asphalt and vehicles tires compressing that water into the pavement is what causes the majority of our problems,” Clifford said. “Most potholes develop following a rain or snow event especially as snow melts to the liquid state.”

Potholes form when vehicles travel over areas weakened by cracks caused by expanding ice, standing water or when a prolonged freeze-thaw cycle exacerbates existing damage. “The low-temperature grade of our asphalt binders should protect our pavements from thermal cracking. It is true that such cracking can eventually lead to the creation of potholes, but potholes and other forms of deterioration may be caused by a wide variety of issues,” Clifford said. “Certainly, the number of freeze/thaw cycles that we experience in Kentucky is a major problem.”

Salt vital for keeping roads safe in the winter. Sodium chloride (NaCl), calcium chloride, magnesium chloride, potassium acetate and calcium magnesium acetate are some of the deicers used across the country. Salt works by lowering the freezing point of water which makes it harder for winter precipitation to stick to roads. A 10-percent salt solution will freeze at 20°F while a 20-percent solution freezes at 2°F. The brine used on roads typically mixes with snow and ice, lowering the freezing temperature and melting the snow around it. Research from Drexel University found that the copious amounts of salt used also can react with pavement, gradually degrading it. Highly concentrated salt solutions are used by many cities while salting before a storm. Dry salt and sand are typically used after the snow has already fallen. Salts can actually increase freeze-thaw damage and scaling problems, according to research. Scaling, or the peeling of a finished surface of hardened concrete, occurs when the top surface of a roadway breaks down, exposing the aggregate below. Research has found that there are two main causes of scaling: temperature differences between the road surface and cracks in the roadway. A de-icing agent reduces the freezing point of the road’s surface but the layers below can still freeze. The temperature gradient leads to additional stress on the road which can cause the top layer to chip away, according to the Minnesota Pollution Control Agency. “Snow and ice treatments, particularly plowing operations, can exacerbate pavement distress,” according to Clifford.

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Salt not only damages the road itself but it eats away at steel used to reinforce concrete. The Federal Highway Administration said that high chloride concentrations eat away at the coatings on steel surfaces. Delamination caused by the salt, when steel separates from its coating, and rust can ruin the structural integrity of the metal.

Of course, to fix the problems that arise, the roads often need to be shut down. The best time for many crews to do this work is overnight in order to minimize the impact on travel. However, patches commonly used by road departments often do not bond and settle properly over a short time frame. “For potholes repairs, we try to patch all of our routes as we learn of pothole locations. When prioritizing work, routes with higher traffic volumes (such as interstates and parkways) are typically repaired first,” KYTC said.

During the 2017-18 winter, apparent consumption of salt in the United States rose to 50,000 metric tons, according to the United States Geological Survey. Forty-four percent of that salt was used for highway deicing equally 22,00 metric tons. That’s approximately equal to the following:

  • The weight of 16,989 Toyota Corollas
  • The weight of 4,042 Asian elephants
  • The weight of 606 full semi trucks plus their trailers
  • The weight of 713 Big Bats outside the Louisville Slugger Museum

So if salt is so bad for roads, is there something else we can use? One batch of scientists at Drexel University have an interesting solution. Bacteria! No, they don’t want to sprinkle bacteria on roads before a winter storm. Their solution involves creating a type of concrete enhanced by particular bacteria which may be more resistant to damage. Calcium chloride, a common deicer, reacts with concrete helping to create a substance called calcium oxychloride that damages roadways. Their research shows that a bacteria, S. pasteurii, creates an enzyme that raises the pH of the material preventing the creation of calcium oxychloride when it’s treated with calcium chloride and instead creating limestone.

Other options already being used to deice roads across North America. In Wisconsin, a byproduct of the cheese making process, cheese brine, is mixed with salt and used to clear roads. Cheese brine is the liquid used to soak particular cheeses. Toronto uses beet juice as a part of their deicing process. Ankeny, Iowa has taken a more appetizing approach to removing ice from their roads. The suburb used outdated garlic salt from a local factory in 2008 to help battle the winter weather.

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