Concrete vs. Salt: Mitigating the Chemical Deterioration of Roadways from Deicers
In areas prone to winter precipitation, the removal of snow and ice from roadways is crucial to maintaining the safety and efficiency of highway facilities and motorized vehicles. Highway maintenance organizations use large quantities of ice melting materials – called deicing chemicals – to maintain clear and safe pavements. Sodium chloride, more commonly known as table salt, is one of the most commonly used chemical deicers in the United States; however, magnesium chloride and calcium chloride are two widely used alternatives to table salt.
Deicing chemicals are capable of melting ice because they lower the freezing point of water. Pure water will freeze at a temperature of 32 degrees Fahrenheit, or 0 degrees Celsius. When the deicing chemicals are added to a frozen roadway, they disrupt the organized crystal structures of ice, melting the ice and making it harder for the water to freeze. This property, called freezing point depression, varies with different types of deicers. For example, table salt only works for temperatures higher than about 16 degrees Fahrenheit.
“While deicers greatly improve roadway safety and efficiency, they also have negative effects on the environment and surrounding infrastructure,” explains Prannoy Suraneni, an assistant professor in the College of Engineering’s Department of Civil, Architectural, and Environmental Engineering. “The increasing use of these deicing chemicals has raised concerns regarding the extent of detrimental effects attributed to their use on pavement surfaces, motor vehicles, roadside vegetation, water quality, and wildlife.”
Specifically, the use of chloride-based deicing salts creates several durability issues in concrete such as salt scaling damage and reinforcement corrosion. In addition, chloride-based deicing salts such as calcium and magnesium chloride also interact chemically with concrete to produce a salt known as calcium oxychloride, which damages the concrete, although the exact mechanism is not well understood. After about ten years of exposure to calcium and magnesium chloride deicers, the joints and surrounding concrete in improperly designed pavement slowly deteriorate, forming cracks that expand into potholes, which are difficult and expensive to repair.
“Calcium oxychloride formation and its ensuing damage in concrete is not well studied and understood,” says Suraneni. “Although there are certain specifications and guidelines to mitigate the damage from calcium oxychloride, there are some potential drawbacks to these specifications that need addressing.”
Suraneni recently received funding from Ready Mixed Concrete (RMC) Research and Education Foundation to critically evaluate certain criteria in the American Association of State Highway and Transportation Officials (AASHTO) document PP 84 that deal with concrete pavement joint damage in the presence of calcium and magnesium chlorides, and to potentially improve these criteria.
With this research, Suraneni expects to see multiple benefits to the concrete industry. “We need to develop more realistic specifications that everyone can follow,” he says. “This research project moves us one step closer to providing a more scientific approach to mitigating concrete infrastructure damage caused by deicing chemicals, potentially saving local, state, and federal governments millions of dollars in roadway repair.”
Ultimately, the guidelines developed from this project grant more freedom to the concrete industry in their pavement mixture designs, while also providing more confidence that these concretes will be durable in cold regions when deicing salts are used.
The research project is officially titled, “The role of air content and supplementary cementitious materials replacement in deicing salt joint damage in concrete.”