Magnesium chloride de-icers used on roadways and bridges may do more damage than previously thought, researchers have found. The damage also is unlikely to be detected using standard visual inspections, the typical method of assessing bridge health.

About 20 states use magnesium chloride for snow and ice control. Source: U.S. Air Force Photo by Staff Sgt. Kenny HolstonResearchers from Washington State University and Montana State University found that samples of concrete exposed to magnesium chloride in the laboratory with repeated freeze-and-thaw cycles lost more strength than samples exposed to rock salt, even though they showed no visual signs of damage.

The researchers report on their work in the journal, Cement and Concrete Research.

The work was funded by the U.S. and Oregon Departments of Transportation, as well as by the National Natural Science Foundation of China.

Transportation officials in the U.S. began using magnesium chloride about 20 years ago. Like rock salt, magnesium chloride prevents ice formation on roadways. It works well in extremely cold temperatures when rock salt stops working. Magnesium chloride also is thought to be a more environmentally friendly alternative to rock salt.

In the U.S., about 20 states use magnesium chloride for snow and ice control, sometimes in combination with other treatments, or in a few cases as the only treatment. In recent decades, the new treatments, including the use of magnesium chloride, have led to improved driving conditions on winter roads.

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At the same time, researchers have known that many types of road salts, including magnesium chloride, degrade concrete, causing both physical and chemical deterioration. They lacked a clear understanding of the microscopic changes in concrete that may trigger impacts at larger scales.

In the study, the researchers learned how nano-sized crystals form within concrete samples. Formation of these crystals resulted in stress buildup and calcium leaching in the concrete. Both actions reduced the concrete's strength. The researchers also found that none of the magnesium chloride samples showed any typical visible distress after 10 accelerated cycles of freezing and thawing.

In addition to the laboratory observations, the researchers tested sample cores from 10 Oregon bridge decks that had been treated annually with magnesium chloride. They found what they said was a "significant compromise in splitting tensile strength." That property affects cracking resistance and load-bearing capacity. They said the strength reduction was as much as 50%. They also saw an up to 60% reduction in the concrete's micro-hardness. The worst effects often occurred 0.5 in to 1 in inside the sample, instead of on the concrete surface.

Some of the samples that had the significant degradation within the concrete had been rated as good or satisfactory in bridge inspections.

A webinar series for transportation officials on holistic and sustainable winter road treatments is being organized by the research team.