If past years are any indication, trucks will spread 900,000 tons of salt on Pennsylvania’s state roadways this winter, lowering the melting point of snow and ice to where some of it turns into harmless liquid. Their counterparts in New Jersey will disgorge an additional 300,000 tons.
Great for motorist safety (assuming the snow actually falls — whoops on last weekend), but harsh on roads and sidewalks.
In a new study, Drexel University engineers have identified an odd weapon to neutralize some of the damaging effects of salt: bacteria.
When the researchers made concrete using a certain type of bacteria and nutrients, they found that the surface was better able to withstand the damage from a type of road salt called calcium chloride.
The way it works requires a bit of explanation, and the engineers say more research will be needed to determine how long their microbially enhanced concrete retains its protective powers. Briefly, it seems to prevent the formation of a road-busting substance called calcium oxychloride, a byproduct of road salt that highway engineers have started to learn about only in the last five years.
“The bacteria are capable of changing the micro-environment around them,” said Christoper M. Sales, one of the authors of the study in Construction and Building Materials.
The Drexel project, led by assistant professor Yaghoob Farnam, offers a reminder that damage from road salt is a man-made problem of recent vintage.
Before the 1940s, highway departments relied primarily on plows to clear snow and ice. Road crews sometimes put down abrasive materials, such as sand and cinders, to improve traction, according to a history by the Transportation Research Board. Motorists often equipped their tires with chains. (Remember those? Didn’t think so.)
In the winter of 1941-42, New Hampshire became the first state to use salt to any significant degree, treating its highways with a modest 5,000 tons. After the boom in highway construction, public works departments now apply tens of millions of tons each year, according to the U.S. Geological Survey.
It is something of a surprise that it took us so long. History does not record the identity of the first person to notice that saltwater is harder to freeze than fresh water, but the knowledge presumably goes back as long as people have lived near the ocean in cold-weather regions.
In the 1600s, British chemist Robert Boyle conducted extensive experiments on the topic, noting that brine was among the liquids that “do not freeze.” Philadelphia’s Science History Institute, the museum and research organization on Chestnut Street, has a 1683 edition of his New Experiments and Observations Touching Cold.
Boyle’s work notwithstanding, brine will freeze if the temperature is low enough, though the exact point depends on the type of salt and its concentration. Saltwater can exist as a liquid at temperatures well below zero degrees Fahrenheit, though salt’s effectiveness as a road de-icer tails off below 15 degrees because so much is needed, the USGS says.
The reason salt lowers the freezing point of water is hard to put into everyday language, but the effect has to do with the relative amounts of energy and entropy — the degree of chemical “disorder” — in the liquid and solid phases of saltwater and freshwater, said Tom Stephenson, a chemistry professor at Swarthmore College.
The short version is that when salt is dissolved in water, the liquid phase becomes more stable, enabling it to exist as a liquid at lower temperatures, he said.
Yet it causes a wide range of damage. Salt fosters corrosion of the steel used in bridges, roadways, and vehicles, and it harms freshwater ecosystems and wildlife. Ecologist Gene E. Likens, who is among scientists being honored by the Franklin Institute in April, has been sounding the alarm on that topic for years.
Then there’s the damage to the pavement itself. Engineers have known for decades that salt can harm concrete, but only in the last five years have they started to understand one of the key culprits, said Cameron D. Murray, an assistant professor of civil engineering at the University of Arkansas.
When roads are treated with calcium chloride salt, one of the byproducts is calcium oxychloride, which expands inside concrete surfaces and causes cracking, Murray said. That sets the stage for intrusion of water, which expands as it freezes.
“If there are some initial cracks, then water gets in and it freezes, it takes off from there,” Murray said.
That is where the Drexel research on bacteria comes in. Farnam and his colleagues knew that microbially treated “bio-cement” had been studied as a way to repair cracks in concrete. They decided to try making concrete with bacteria to prevent the cracks from occurring in the first place.
The bacteria they used, called S. pasteurii, alters the conditions inside concrete in such a way that when calcium chloride is applied as road salt, the byproduct is not harmful calcium oxychloride, but calcium carbonate — the scientific term for limestone. The bacteria accomplish this feat in part by producing an enzyme that raises the pH of the surrounding material, said Drexel’s Sales, an environmental engineer.
Compared with the regular variety, concrete made with microbially enhanced cement did not crack when treated with calcium chloride, said Farnam, an assistant professor of civil, architectural, and environmental engineering at Drexel. The team envisions using the bacteria and nutrients in new roadways, not for treating existing ones.
“We want to see how long the bacteria are active,” Farnam said. “Maybe you would want to reapply it after four or five years."
It is not the first time Farnam has used unconventional methods to combat the woes of winter.
In 2017, he published research about an experimental concrete that he and his colleagues had impregnated with waxy paraffin oil. The researchers found that snow and ice melted faster on that surface than on regular concrete.
More research is needed on that innovation, too.