Canadian winters are harsh. As temperatures dip below freezing, ice can form on roads and walkways and make them hazardous. One method of keeping ice away is the use of de-icing agents. The oldest and most commonly used compound is salt. However, the use of salt needs to be controlled as it can sometimes be more hurtful than helpful.
DE-ICING SALTS
Salt has been traditionally used to melt ice or prevent water from freezing on roads and paths of travel. Essentially, this works because salt lowers the freezing point of water. When salt is spread on a sheet of ice, it pulls some of the water away from its crystal form and mixes with a thin film of water on the ice surface. More saltwater is created, which in turn melts more ice. The salt water also keeps any new ice from being created. This process can be effective for temperatures as low as -20°C, depending on the concentration of the solution and type of salt used (sodium chloride, calcium chloride or magnesium chloride).
COMMON USES AND IMPACTS
SAFETY OF PEDESTRIANS
Slip and fall accidents are common in Canada, especially in winter when ice and snow can make walkways dangerous for pedestrians. In a slip and fall case, both the injured party and the property owner bear some of the responsibility. Property owners have to show they met their duty to exercise reasonable care by maintaining the area (prompt removal of snow and ice), while the injured party has to show they exercised reasonable care when walking on the dangerous surface. By using de-icing agents to melt the ice, and removing the snow, property owners greatly reduce their risk of liability in the case of an accident.
ROAD SAFETY
Before a snowfall or low temperature event, roads are sprayed with brine (a saline solution with a high concentration of salt). There are environmental concerns linked to the use of road salts, as they can end up being washed into freshwater bodies and harm the aquatic life. Most cities now have regulations in place that limit the amount of snow from roads that can be dumped into water bodies, mainly because of the de-icing compounds used. That being said, the Government of Canada does not ban the use of road salts, as doing so would dramatically reduce road safety.
SALT VS. CONCRETE
Most of the underlying structures on roads and walkways are made of concrete. Examples are: parking garages, bridges, tunnels, sidewalks, etc. Although they help control ice, the de-icing agents used often harm concrete and can cause major structural problems. There are three main ways in which salt can attack concrete:
- Freeze-thaw scaling
- Rebar corrosion
- Chemical attack
Concrete is a porous material that has capillaries capable of absorbing water, similar to a sponge. When water enters the capillaries and freezes, it expands by around 10% and creates pressure and stress on the surrounding concrete matrix. The concrete cannot resist to the pressure and failure is observed. De-icing salts increase the amount of pressure and intensify the freeze-thaw cycle. In fact, concrete can go through several freeze-thaw cycles in one day where de-icing agents are used.
Salt can also attack concrete indirectly by facilitating corrosion of the steel reinforcing. Like the freeze-thaw scenario, when concrete reinforcement corrodes, it also expands. As the rust layer grows, it will create pressure on the concrete that surrounds it and will eventually damage it. Salt accelerates the corrosion process by bathing the metal surface in electrolytes (solutions capable of conducting electricity) and encouraging the creation of iron-oxides, which we know as rust.
In some cases, salt can also accelerate alkali-aggregate reaction (AAR) in concrete. AAR is a reaction in concrete between the cement (an alkaline), and one or more of the concrete aggregates. This reaction causes the formation of an expansive gel around the aggregates. As the gel increases in volume, it exerts pressure inside the concrete causing damage to the concrete matrix.
EFFLORESCENCE
Another phenomenon sometimes observed on concrete is efflorescence, commonly known as leaching. Efflorescence in concrete is the migration of salt to the surface. A solution made up of salt and a solvent (usually water) makes its way to the concrete surface and evaporates, leaving behind a coating of salt. There are two types of efflorescence; primary efflorescence, in which the saline solution was present inside the concrete already, and secondary efflorescence, in which the saline solution is brought into the concrete from an external source.
PRIMARY EFFLORESCENCE
Primary efflorescence in concrete ordinarily occurs during curing. As concrete cures, it generates heat and hydration. Calcium hydroxide Ca(OH), one of the hydration products, dissolves in the water within the concrete and makes its way to the surface, where it leaches out. The water evaporates and leaves behind the solid, Ca(OH), which reacts with the atmospheric carbon dioxide CO to form calcium carbonate CaCO, a deposit on the concrete surface that appears fluffy and white.
It is also possible for primary efflorescence to occur long after the concrete has cured. In some cases, changes to the relative air humidity can draw out water from the concrete. For example, the installation of a dehumidifier in a basement can lower the relative humidity and create efflorescence, even though the concrete basement walls had never shown this phenomenon before.
Primary efflorescence is typically not structurally damaging to the concrete. The salt found internally in the concrete is not bonded to its other elements. Primary efflorescence is therefore usually an aesthetic concern only.
SECONDARY EFFLORESCENCE
Secondary efflorescence in concrete is due to the external infiltration of a saline solution. In this case, the saline solution can cause harm to the concrete by attacking the cement and weakening its bond to the aggregates. Secondary efflorescence is like osteoporosis of the concrete and usually needs to be addressed.
In Canada, we most often see secondary efflorescence on highway bridges and in parking garages. The deicing salts cause the ice to melt, and then the saline solution is absorbed in the concrete. As the solution makes its way through the concrete, it weakens the bond between the cement and aggregates and can facilitate corrosion. Secondary efflorescence will usually cause white streaking or sometimes even stalactites at cracks and joints in the concrete structure.
Once secondary efflorescence is observable by the naked eye, it is usually a sign that infiltration has been happening for some time since the solution has made its way completely through the concrete. Although not observable, the concrete and its reinforcement are most likely being attacked internally by the saline solution.
PROTECTION
There are several ways to protect concrete from the nefarious effects of salts described above. For example, the Canadian National Building Code requires exterior-use concrete to have 5% to 8% air entrainment. Entrained air provides tiny air bubbles or voids in the concrete that provide additional space for water to expand in when it freezes, thereby reducing the pressure applied to the concrete matrix.
Another method of protection is the use of sealers that keep water and dissolved salts away from the concrete. They are surface applied and work by clogging the concrete pores or by forming an impermeable layer. The main sealer types are:
- Penetrating sealers
- Acrylics
- Polyurethanes
- Epoxies
- Bituminous
Although sealing helps keep salts from penetrating the concrete surface, it does nothing for concrete that already has salt or water infiltration and can make the problem worse if used improperly (water and salt get trapped in the concrete and can’t leach out).
For protection against rebar corrosion, some measures include the use of an epoxy coating or sacrificial anodes. Galvanized, or stainless steel, or composite reinforcement can also be used, although its prevalence in Canada is limited due to its high cost.
Finally, where salt is not required, the best course of action is to avoid its use. There are alternatives that can be considered. Heat, for example, is the best method for melting ice. Sand can also be used to create traction and is generally more friendly to the environment.
CONCLUSION
It’s a catch-22. On the one hand, it’s important to keep ice away from paths of travel for public safety reasons. On the other hand, the de-icing agents used can have damaging, often irreversible effects on the integrity of the concrete infrastructure.
We depend on roads, bridges and walkways to get from one place to another. It’s important to have safe paths of travel, but also to choose the right concrete mix and protective measures to combat the attack of salt on concrete. Our experience handling a multitude of different cases, such as slips and falls or structural integrity of concrete, puts us in a unique position to handle both sides of the salt vs concrete issue.
If you have any questions or would like to learn more about this topic, please contact our Structural & Civil Engineering team at 877 686-0240 or info@cep-experts.ca