Bridge Inspector

PULLMAN - The magnesium chloride deicers commonly used during the winter months on many roadways throughout Washington state and the U.S. may be doing more harm to most bridges than was previously thought.

Even worse, the damage they are causing is unlikely to be detected using standard visual inspections, which are the typical means of examining a bridge’s condition. 

A recently published article in the journal Cement and Concrete Research, conducted by researchers 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 work was funded by the U.S. and Oregon Departments of Transportation and the National Natural Science Foundation of China.

Magnesium chloride was introduced as a deicer two decades ago as a means of preventing ice crystals from forming on paved road surfaces. The agent became a popular choice of many state and regional transportation agencies due to its ability to continue working in extremely cold temperatures long after traditional substances, like rock salt, would stop working. The chemical has also long been considered to have fewer environmental impacts than salt.

About 20 states currently use magnesium chloride for controlling ice and snow and many have reported an overall improvement in road conditions when using the compound.

Researchers have long known that many types of road salts, including magnesium chloride, can degrade concrete and cause both physical and chemical deterioration. However, they lacked a clear understanding of the changes in concrete at a microscopic level which can create considerable impacts on a larger scale.

In the study, led by Xianming Shi, associate professor in WSU’s Department of Civil and Environmental Engineering, the researchers were able to unravel how nano-sized crystals form within concrete samples. The formation of these crystals resulted in stress buildup and calcium leaching in the concrete, both of which significantly reduced its strength.

They also found that none of the magnesium chloride samples showed any trademarks of visible distress, even after 10 accelerated cycles of freezing and thawing.

In addition to the laboratory observations, the researchers tested sample cores from ten bridge decks in Oregon that have been receiving annual treatments with magnesium chloride. These samples revealed a significant compromise in splitting tensile strength, which is a property that affects cracking resistance and load-bearing capacity, by as much as 50 percent. They also saw an up to 60 percent reduction in the concrete’s micro-hardness. The worst effects often occurred half an inch to one inch 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.

Shi compared the chemical’s invisible degradation and concrete softening to what happens with aging osteoporotic bones.

“With rock salt, you can see there’s a problem, due to visible symptoms such as scaling and spalling;” he said. “In the case of the magnesium chloride, however, when you can see something, it might be too late to fix.”

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

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