Many types of chemicals are used to melt ice and snow on bridges and roads.
In this article, we’ll provide an overview of three of them, offering information about how they affect bridge and road infrastructure, along with their impact on vehicles and the environment. The three types we’ll look at are:
- Chloride-based deicers.
- Acetate-based deicers.
- Carbohydrates.
We hope this information will make it easier for you to decide which solutions are best to use on your bridges and road surfaces.
Chloride-based deicers
The three primary types of chlorine deicers are sodium chloride, calcium chloride, and magnesium chloride. Each has a different lowest melting point (the temperature at which they are no longer able to melt ice or snow):
- Sodium chloride: 15 degrees Fahrenheit
- Calcium chloride: -20 degrees Fahrenheit
- Magnesium chloride: -10 degrees Fahrenheit.
Chloride-based deicers are known to be corrosive and can have a significant impact on bridges and roadways, along with the vehicles that cross them and the environment.
Metal components
All three types of chloride deicers have been shown to cause corrosion on the metal parts of bridges. This includes the superstructure and supporting elements, which can be weakened over time if exposed to chloride solutions regularly.
Concrete
All chloride deicers are known to damage concrete on bridges and roads. Magnesium chloride has a bigger impact than the others. The two most common types of damage are scaling of concrete and reactions with the cement paste.
Scaling of concrete is caused by the breakdown of the upper road surface. This results in exposing the aggregate below it. While scaling can occur for many reasons, there are two primary ones.
- The first is related to the difference in temperature between the upper road surface and the layer below it. The deicing agent lowers the freezing point at the road’s surface, but not the layers below, which are still able to freeze. The temperature difference stresses the road, which makes the top layer break and chip away.
- The second happens because of cracks in the road surface. Chemicals make crystals form in them, which causes the top surface of the road to flake away.
Reactions with cement paste take place when chloride-based deicers affect it both physically and chemically. The physical effects are similar to the scaling described above. The chemical-related ones are caused by reactions between the deicers and cement paste, which lead to expansion.
Rebar
All chloride deicers have a high likelihood of corroding the steel rebar that reinforces concrete on bridges. This is one of the biggest issues with using them.
Magnesium chloride has the highest negative impact of all of them. That’s because of how it reacts with calcium-silicate-hydrate (C-S-H) and calcium hydroxide [Ca(OH)2], the two substances that make up cement paste.
The magnesium chloride is known to replace the calcium in the C-S-H compound with a non-cement compound called magnesium-silicate-hydrate (M-S-H) and calcium chloride. The resulting substance is not as durable, and the concrete loses its strength.
In addition, the magnesium is known to form magnesium hydroxide and calcium chloride when it reacts with the calcium hydroxide. The magnesium chloride causes expansion, which accelerates the deterioration of concrete.
Calcium chloride leads to similar degradation, but it happens at a slower rate and to a lesser degree.
The steel in rebar is usually protected from corrosion because of the relatively high alkalinity of the surrounding concrete. When chloride seeps into concrete, it leads to deterioration. This allows oxygen and water to break through the protective concrete surface and causes pitting and corrosion to the rebar. Over time, this weakens reinforced concrete, which reduces the strength of bridges over time.
Note: Damage to rebar is a big issue for bridges, but it can also affect pavement with doweled joints or continuous reinforcement.
Impact to vehicles
Road salt causes functional, structural, and cosmetic damage to vehicles. Functional and structural damage affect performance, and cosmetic issues affect how the vehicle looks. Road salt costs drivers a significant amount every year.
Effect on the environment
- Chlorine-based substances are known to have a:
- Moderate impact on water and aquatic life.
- Low impact on air quality.
- Relatively high impact on soil because it breaks down its structure, decreases permeability, and reduces stability.
- High impact on plants, causing foliage and root damage.
Cost of using salt to melt snow and ice
Researchers have studied the dollar cost of using sodium deicers beyond the price of materials and hiring people to apply them. They’ve calculated costs related to infrastructure, vehicle, and environmental damage.
In 2014, Fortin Consulting pulled together the results of seven studies conducted between 1976 and 2010 to quantify the annual cost of damage associated with the use of one ton of road salt per year.
- Vehicle corrosion: Approximately $30 to $110.
- Extra road maintenance: More than $600.
- Infrastructure damage: $1,460.
- Ecosystem damage: Approximate $170 to $230.
- Overall damage: $800 to almost $3,400.
Acetate-based deicers
The three primary types of acetate deicers are calcium magnesium acetate, potassium acetate, and sodium acetate. Each has a different lowest melting point:
- Calcium magnesium acetate: 20 degrees Fahrenheit.
- Potassium acetate: -15 degrees Fahrenheit.
- Sodium acetate: -10 degrees Fahrenheit.
Overall, acetate deicers are known to impact bridge and road infrastructure, vehicles, and the environment, but less so than chlorine.
Metal components
Calcium magnesium acetate and potassium acetate have a relatively low possibility of causing metal corrosion. Sodium acetate has negligible impact on metal as well.
Overall, studies have found that acetate-based deicers are less destructive to steel than chloride-based ones. Specifically, the corrosion rate for steel that comes into contact with acetate-based solutions is about two to four times lower than that for sodium-based deicers. When it comes to aluminum, acetate is approximately 90 percent less corrosive than salt.
Concrete
Calcium magnesium acetate and potassium acetate have a relatively high likelihood of causing damage to concrete on bridges and roads because of scaling. There is little evidence that sodium acetate affects it at all.
One study looked at what happens when good quality concrete is exposed to acetate-based solutions non-stop for eight months. It found that there was a measurable decrease in load capacity. Researchers were also able to see degradation to the concrete. However, the same study found that when slag-blended cement was tested, the loss of strength was not as great.
Acetate deicers may also damage asphalt because of chemical reactions, emulsifications, distillations, and by causing added stress within it.
Rebar
There is little or no evidence that acetate deicers have any negative impact on the steel rebar used on bridges. This clearly gives it an edge over sodium-based solutions.
Impact to vehicles
When it come to automobiles, studies show that acetate-based solutions have less of an effect on brake lines, tubes, and other automobile parts than salts do.
Effect on the environment
Acetate-based substances are known to have a:
- Moderate impact on water and aquatic life because it affects oxygen demand.
- Low impact on air quality.
- Relatively low but positive impact on soil (it may actually improve its structure and increase permeability).
- Little or no impact on plants.
Carbohydrate deicers
The three primary types of carbohydrate deicers are beet juice, molasses, and corn syrup. The lowest melting point for these substances has not been documented.
Metal components
There is a very small possibility that these substance may initiate and accelerate metal corrosion because of elevated conductivity. There is minor evidence that they may prevent it, but these claims need more study and research.
Concrete matrix
There is little or no proof that carbohydrate deicers have a negative impact on concrete.
Concrete reinforcing
There is little or no evidence that carbohydrate-based deicers cause rebar to corrode. Claims that it could be protective require more research.
Impact to vehicles
Carbohydrate deicers have no demonstrated impact to vehicles.
Impact on the environment
- Carbohydrate-based substances are known to have a:
- Relatively high impact on water and aquatic life because it affects oxygen demand and may cause other issues.
- Low impact on air quality.
- Low impact on soil, although there is limited research available.
- Little or no impact on plants.
Summary
Every chemical used to deice bridges and roads comes with its own pluses and minuses. You owe it to the people you serve to ensure that you select the ones that are most effective, cause the least damage to bridges, roads, and vehicles, and have the lowest possible impact on the environment.
