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Several different types of deicing chemicals exist. Those covered this section include chloride-based deicers, acetate-based deicers, and carbohydrates. A list of the chemicals approved for use by the MNDOT can be found here. This article presents information on non-environmental impacts from use of deicers. A discussion of environmental impacts can be found here.
Contents
Chloride based deicer effects on infrastructure
Chloride based deicers are corrosive (Levelton Consultants Ltd, 2008; Shi et al., 2009a) and can impact vehicles, bridges, and roadways, (National Research Council, 1991). Magnesium chloride and calcium chloride may be more corrosive than sodium chloride due to a longer time of wetness, however the corrosivity of all three is still high (Nixon and Xiong, 2009; Shi et al., 2009b). Often times corrosion inhibitors are added to lessen the deicers corrosive effects. Every corrosion inhibitor has limitations as to which metals it can protect (Levelton Consultants, Ltd, 2008).
Effects on Concrete and Asphalt Roadways/Bridges
There are several ways in which chloride based deicers can negatively impact concrete, roads and bridges (Shi et al., 2009a; Levelton Consultants Ltd. 2008; Public Sector Consultants, 1993.):
- Physical deterioration such as “salt scaling”.
- Deterioration in the cement matrix due to reactions with the cement paste.
- Enabling and/or accelerating the corrosion of the rebar used to reinforce the concrete. This is primarily a concern for bridges, but there is a secondary risk in concrete pavement with doweled joints or continuous reinforcement.
Scaling of Roadway
Scaling of concrete is caused by the breakdown of the top surface of the road, which results in exposure of the aggregate below (Public Sector Consultants, 1993). While scaling can be the effect of many things, there are two main causes of scaling. The first is related to the temperature difference between the road surface and underlying portion of the road. The deicing agent reduces the freezing point at the road’s surface, but the layers below can still freeze. The resulting temperature differential can cause stress on the road and cause the top layer to chip away. Another cause of scaling could be related to the cracks. Crystals could begin to form in the cracks, causing the road surface to flake off (Public Sector Consultants, 1993). The occurrence of scaling is less dependent on the type of deicing agent, but rather on the quality of the concrete and the prevailing weather conditions (Levelton Consultants Ltd., 2008).
Reactions with Cement Paste
Chloride based deicers can affect the cement paste in both a physical and chemical manner. The physical effects were discussed above in the scaling section. The chemical effects are the result of chemical reactions between the deicers and cement paste, and through aggravation of the expansive aggregate-cement chemical reactions (Sumsion and Guthrie, 2013).
Of the various chloride-based deicers, magnesium chloride causes the greatest degradation of concrete (Cody et al., 1994; Lee et al., 2000; Sutter et al., 2008; Shi et al., 2009a). This is due to the way in which the magnesium chloride reacts with calcium-silicate-hydrate (C-S-H) and calcium hydroxide [Ca(OH)2], the two components of cement paste. The magnesium chloride will replace the calcium in the C-S-H compound with a non-cementitious compound called magnesium-silicate-hydrate (M-S-H) and calcium chloride (Lee et al., 2000). The M-S-H compound is not as strong as the C-S-H compound, and as such, the cement pasted is weakened and the concrete loses some of its strength. In addition, the magnesium will form magnesium hydroxide and calcium chloride when it reacts with the calcium hydroxide (Shi et al., 2009a). The magnesium chloride will cause expansive forces which can further accelerate the deterioration of the concrete (Levelton Consultants Ltd., 2008). Calcium chloride causes a similar degradation pattern in concrete but it occurs at a slower pace and to a lesser degree (Cody et al., 1994; Shi et al., 2009a).
Sodium chloride was found to have the least impact on cement paste (Cody et al., 1994; Sutter et al., 2008). Long-term use of this deicer does slowly accelerate the alkali-silica reaction but has not been shown to result in any significant loss in concrete strength (Shi et al., 2009a). It is important to note, while sodium chloride does not appear to have a significant effect on the cement paste, it does significantly corrode the reinforcing steel. Sutter et al. (2008) found sodium chloride had the highest rate of ingress into hardened concrete.
Corrosion of Reinforcing Steel
The reinforcing steel (rebar) embedded within concrete structures is usually protected from corrosion due to the high alkalinity of the surrounding concrete. This highly alkaline environment creates a chemical coating, often called a “passive” layer on the steel surface. This reduces the rate of corrosion to an almost negligible level (Levelton Consultants Ltd., 2008). The ingress of chloride into the concrete is one of the main driving forces behind the deterioration of reinforced concrete structures. Localized corrosion of rebar occurs when oxygen and water are able to access the rebar surface. This could be brought about by a decrease in the pH of a concrete pour solution, or the presence of enough water-soluble chloride ions (Shi et al., 2009b).
The chloride-induced corrosion causes a pitting corrosion on the rebar. Figure 1.4 provides an illustration of this process. Corrosion of the reinforcing steel leads to delamination and spalling, which ultimately decreases the strength of the structure.
Impact on Motor Vehicles
The impact of road salt on motor vehicles can be broken into three categories: functional, structural, and cosmetic. Functional and structural damage result in a loss in operating performance while cosmetic damage affects only the appearance of the vehicle.
Costs to Infrastructure
The cost of using road salt does not only include the price of materials and application. The cost associated with the damage to infrastructure and automobiles must also be considered. One study, Vitaliano (1992), estimated road salt adds an additional $332 per ton of salt per season for associated bridge maintenance. There is also an additional $1,460 per ton of damage to bridges in terms of corrosion. Corrosion to vehicles results in about a $133 per ton of salt. These associated costs could total toan extra $1,925/ton of applied salt (Vitaliano, 1992). Actual costs will vary by season and location but generally the use of deicers will require additional infrastructure maintenance costs..
In 2014, Fortin Consulting compiled the results of seven studies conducted between 1976 and 2010 that looked at the damages associated with road salt, which is summarized in Table 1.1.