Can bacteria help us prevent salt damage to concrete roads and bridges?

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 10 million tons 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. Yaghoob Farnam, CC BY-SA
According to the U.S. Federal Highway Association, winter road maintenance accounts for roughly 20 percent of state department of transportation maintenance budgets, through spending more than US$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, fixing potholes, patching and reinforcing roads and dealing 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 $276 billion, approximately 3.1 percent of the nation’s gross domestic product.

While research is underway to develop new types of concrete, such as self-heating concrete, 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.