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A Secret Weapon to Combat Road Salt Damage

It takes 10 million tons of road salt to keep roads safe in winter. Bacteria could help ease the toll of salt on roads and bridges

Road salt helps make highways safe in winter, but it takes its toll on roads and bridges made with concrete. Do bacteria offer a solution? [Photo credit: Portland Department of Transportation / CC BY-NC-ND / Flickr]

Bacteria, which have been working for millennia as nature’s stonemasons, could soon be enlisted to help neutralize the destructive effects of road salt.

According to the Transportation Research Board, it takes about of road salt to keep roads safely navigable in the winter. And while it’s certainly an effective method for staving off snow and ice, around this time of year, we start to see the toll it takes on our infrastructure in the form of cracks, potholes, and bumps.

It turns out, those bumps aren’t just the inevitable annoyances that come with wear and tear — they’re actually caused by a chemical that forms when road salt reacts with the surface of roads, bridges and sidewalks that are made from white-gray concrete.

As a civil materials engineer at Drexel University, I spend my time teaching and developing advanced materials that we can use to build more robust roads, bridges, buildings and infrastructure.

The concrete killer

The chemical causing the havoc is called calcium oxycholoride — CAOXY, in chemistry shorthand — and it forms when a common type of road salt, calcium chloride, reacts with the calcium hydroxide that is an ingredient in concrete.

CAOXY is a destructive component. When it forms inside concrete, it expands, creating internal distress and cracks that are then amplified by the chiseling effect of the freeze-thaw cycle.

CAOXY forms when calcium hydroxide, an ingredient in concrete, reacts with a common road salt called calcium chloride. [Image Credit: Yaghoob Farnam]

According to the U.S. , winter road maintenance accounts for roughly 20% of state department of transportation maintenance budgets, through spending more than $2.3 billion on snow and ice control. This does not include the billions of dollars needed to repair infrastructure damage caused by snow, ice, and deicing salts; to fix potholes; to patch and reinforce roads; and to deal with the corrosion that salt causes on the metal parts of vehicles. The annual direct losses caused by corrosion on U.S. highway bridges are estimated at , approximately 3.1% of the nation’s gross domestic product.

While is underway to develop new types of concrete, such as , that can melt snow and ice without the need for road salt, it might be more feasible to treat roads with something that would still allow the salt to do its job while counteracting its negative side effects by preventing the formation of CAOXY.

Bacterial blockers derailing chemical reaction

My multidisciplinary group at Drexel University, which includes civil, environmental, and materials engineers, decided that any antidote for CAOXY-related damage would need to prevent the chemical reaction that forms it. But curtailing the reaction is tricky because it can occur at temperatures above freezing. This means that CAOXY can start forming almost as soon as the salt hits the road.

One of the best ways to block the reaction from happening is to make sure there aren’t enough ingredients for it. So, we wanted to create another chemical reaction that could use up the calcium in road salt before it reacted to form CAOXY.

Nature provided the perfect solution in the form of some talented bacteria.

On the other side of our lab, students were examining bacteria called Sporosarcina pasteurii to understand how they performed their magic. The bacteria, which are commonly found in the soil, have the unique ability to convert nutrients and calcium into calcium carbonate or calcite — also known as limestone, a common stone in Earth’s crust. This bacterium, S. pasteurii, is credited with depositing limestone as a binder (or glue), aiding the formation of coral reefs and helping to bind and stabilize soil.

But the S. pasteurii findings that interested my colleagues and me were in . showed how bacteria like S. pasteurii could make their own sort of concrete, a biomortar, that could be used to repair damaged marble surfaces, such as sculptures or historical buildings.

Through their metabolism of the nutrients, the bacteria produce an enzyme that acts as a catalyst for calcite formation. The process also increases the alkaline nature of the surrounding environment, which also enables the reaction.

was hoping to put the bacteria to work repairing cracks in concrete, most of which has limestone as its main ingredient. The breakthrough came when we noticed one of the primary ingredients S. pasteurii needed to make its limestone is calcium.

Could the little bacterial masons help us thwart CAOXY?

Where the S. pasteurii meets the road

We put our idea to the test using samples of ordinary portland cement, the kind used to build roads, bridges, and sidewalks. In addition to a control sample that was made with no bacteria, we treated one sample in the lab with S. pasteurii and nutrient solution.

Then, we exposed our samples to calcium chloride solution at varying temperatures, to simulate the winter environment in which a typical road salt-concrete interaction occurs.

When calcium hydroxide (from the concrete) mixes with the road salt calcium chloride in the presence of bacteria, the microbes produce limestone that patches the road. [Image credit: Yahhoob Farnam]

By measuring temperature changes indicative of CAOXY formation, measuring CAOXY amount, and monitoring the acoustics of the samples with small sensitive microphones for the sounds of cracking, we saw that the bacteria-treated samples were left unscathed.

And the S. pasteurii actually converted some of the road salt into calcite that helped to seal up the micropores that are precursors to cracks and potholes.

So, can using bacteria before the salt assault really save us from road damage? I think so.

S. pasteurii are a particularly hardy type of harmless bacteria that can be found in soil. They can form spores in order to survive in a wide range of temperatures and high- or low-acidity environments. This means they may lie dormant in the off-season and spring into action with the first road salting of the winter. And more importantly, the calcium carbonate they form seems to be harmless to their immediate ecosystem – unlike road salt, .

Of course, more work is needed to fully understand the interactions of S. pasteurii with deicing salt and its effect on concrete performance. My colleagues and I don’t yet know how quickly bacteria perform this chemical reaction, and we are working on promising ways for how we would add the bacteria to the roads in a real world situation. But this is a path worth pursuing, because it’s unlikely we’ll be able to kick our addiction to road salt any time soon.

The Conversation

, Assistant Professor of Civil Engineering, . This article is republished from under a Creative Commons license. Read the .

5 Comments

  1. Jason D | | #1

    Is there a similar chemical reaction when using magnesium chloride? Seems simpler to use a different deicing chemical, if that's possible.

  2. Jon R | | #2

    > The annual direct losses caused by corrosion on U.S. highway bridges are estimated at $276 billion, approximately 3.1% of the nation’s gross domestic product.

    NACE says something quite different - "The annual direct cost of corrosion for highway bridges is estimated to be $13.6 billion." $276B is for all corrosion.

  3. Peter L | | #3

    Mag Chloride was used in Colorado when I resided there. It destroyed anything aluminum on my vehicle, leaving these weird pits and surface damage. It also destroyed electrical wiring on city street poles. Somehow the mag chloride dust entered into the wiring and shorted it out. It also polluted streams and rivers with runoff.

    The reality is that dumping salt on roads causes structural and environmental damage. That's a fact. Ideally not dumping salt or mag chloride would be best practice but most people can't drive and the amount of accidents would be staggering if they didn't salt the roads.

  4. Roger Berry | | #4

    Not sure what part of Colorado you resided in Peter L, but in the part of Colorado, where I have lived now for seven years, heavy mag chloride usage for dust control and dirt road stabilization has had no effect on my aluminum wheels. Nor on my wife's aluminum wheels or on any wiring on the electric grid that I am aware of. Errant backhoes and heavy ice have caused some problems, but I suspect that contact with the mag chloride, even from road dust, is rather minimal. I will ask the linemen when I see them.

    Perhaps your wheels were actually magnesium wheels not aluminum. True magnesium wheels might very well suffer damage as the chemistry processes that make mag chloride could be at work. Wet mud soaked with mag chloride could be attempting to continue production so to speak when drying out on your rims.

    Biggest problem with it that I can see is constantly having to hose it off the truck and also having it throw your wheel balance off if you don't clean evenly. It has been in use in the area for at least 20 years that I know of and mag chloride's ability to cause corrosion is not something I hear people cuss about. I do know that this area has the highest number of 70's and 80's Ford pickups I have ever witnessed.

  5. John Clark | | #5

    It's unfortunate that the geography* of the US is such that salting roads is a necessity.

    * S/N winter time travel typically results in a 30 degree swing in temps that would chew up dedicated snow tires in the warmer regions of the country.

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