Salt corrodes concrete primarily by penetrating its porous structure and initiating a deterioration process from within.
The Mechanism of Salt Corrosion in Concrete
When salt, typically sodium chloride (NaCl), is used for de-icing and melts with snow or ice, the resulting saltwater solution is readily absorbed into the concrete's pores. This is the first step in a cascade of events that leads to corrosion.
- Water Absorption: Concrete is a porous material. Water, especially saltwater, is absorbed into these pores.
- Crystallization and Pressure: As the saltwater evaporates, salt crystals form inside the pores of the concrete. The formation of these crystals exerts significant pressure on the pore walls.
- Internal Stress and Cracking: This pressure, caused by crystal growth, creates internal stress within the concrete, leading to micro-cracks and, eventually, larger cracks and surface scaling. This process is often referred to as salt weathering.
- Freeze-Thaw Cycles: The problem is exacerbated by freeze-thaw cycles. Water expands when it freezes. If saltwater has penetrated the concrete and then freezes, the expansion further increases the pressure on the pore walls, accelerating cracking and deterioration.
- Reinforcement Corrosion: In reinforced concrete, the presence of chloride ions from salt can break down the passive protective layer on the steel reinforcement bars (rebar). This allows the steel to corrode. As the steel corrodes, it expands, putting even more stress on the surrounding concrete, leading to cracking and spalling (concrete breaking off).
Visual Symptoms of Salt Corrosion
- Scaling: Flaking or peeling of the concrete surface.
- Cracking: Visible cracks, often starting as hairline fractures and expanding over time.
- Spalling: Chunks of concrete breaking away, exposing the aggregate and/or reinforcing steel.
- Efflorescence: White, powdery deposits on the concrete surface, indicating the presence of salt deposits.
Factors Influencing the Rate of Corrosion
- Salt Concentration: Higher concentrations of salt accelerate the corrosion process.
- Exposure Frequency: Frequent exposure to saltwater leads to more rapid deterioration.
- Concrete Quality: Concrete with higher density and lower porosity is more resistant to salt corrosion.
- Climate: Freeze-thaw cycles significantly worsen the effects of salt corrosion.
Mitigation Strategies
- Sealers and Coatings: Applying sealers or coatings to the concrete surface can reduce water and salt penetration.
- Air Entrainment: Incorporating air-entraining admixtures into the concrete mix creates microscopic air bubbles that relieve pressure during freeze-thaw cycles.
- Using Alternative De-icers: Consider using alternative de-icers such as calcium chloride, magnesium chloride, or calcium magnesium acetate, which may be less corrosive than sodium chloride. However, each has its own environmental and performance considerations.
- Proper Drainage: Ensure proper drainage to minimize water pooling on the concrete surface.
- Cathodic Protection: For reinforced concrete structures, cathodic protection can prevent or reduce corrosion of the steel reinforcement.
- Use of Corrosion Inhibitors: Add corrosion inhibitors to the concrete mix to reduce or prevent corrosion.
In summary, salt corrosion of concrete is a complex process initiated by saltwater absorption, followed by salt crystallization, pressure build-up, and cracking, often exacerbated by freeze-thaw cycles, which ultimately leads to the deterioration of the concrete structure.
