What is Chloride contamination?
Chloride contamination is the most severe threat to reinforced concrete structures accounting for approximately 40% of all structural failures. Damage is caused by the infiltration of chloride ions into concrete structures, from diverse sources, such as contaminated mixing water or aggregates, atmospheric exposure in marine environments or the application of de-icing salts in colder regions.
Chlorides can enter the concrete by diffusing through pores or through surface cracks, and may be chemically bound with cement hydration compounds, or exist as free ions in the interstitial pore solution. These free chlorides can travel to the steel reinforcement where they react with the passive layer surrounding to steel to form soluble metal chloride complexes which disrupt the protective oxide layer on the steel surface.
When the chloride content reaches a critical value at the concrete/steel interface, the passivating layer breaks down and localized corrosion is initiated. According to BS EN 206-1:2013 critical values for reinforced and prestressed concrete are 0.4% and 0.1% chloride by weight of cement respectively.
Chloride ion penetration within the concrete is usually not uniformly distributed, steel areas exposed to higher concentrations of chlorides can start to corrode, while at other areas the steel remains passive. The interaction of these two zones will generate corrosion macro-cells which accelerates the corrosion processes at anodic zones.
What causes Chloride contamination of concrete structures?
Exposure to marine environments.
Use of de-icing salts.
High water-to-cement ratio (w/c).
High porosity of concrete.
High air permeability.
Contaminated aggregates, water, or admixtures.
What are the signs of Chloride contamination of concrete structures?
Chloride contamination is hard to identify visually until damage has already been done.
Chloride contamination is the primary corrosion inducing process and in general only becomes apparent when symptoms of that corrosion can be visually observed (rust staining, corrosion cracks, spalling, etc).
Chlorides within the concrete matrix however can be brought to the surface through the capillary action of moisture. As moisture moves through the concrete pores, it dissolves the chlorides along its path. When the moisture reaches the surface and evaporates, it leaves behind the dissolved chlorides, which crystallize and form the characteristic white, powdery deposits of efflorescence.
Thus, the appearance of efflorescence on the surface of concrete can serve as an early indicator of chloride contamination within the concrete structure before severe corrosion has taken place.
How can I identify Chloride contamination in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
How can I prevent Chloride contamination in concrete structures?
To prevent chloride contamination in concrete structures, several measures can be taken.
Firstly, utilizing low water to cement ratios and supplementary cementitious materials (SCMs) such as Ground Granulated Blast Furnace Slag (GGBS) or Fly Ash to replace the Portland cement in concrete mixtures can reduce permeability and enhance resistance to chloride ingress. Additionally, incorporating mineral additions like limestone powder can further densify the concrete matrix, making it less susceptible to chloride penetration.
While it does not reduce the rate of chloride ingress, increasing the cover depth to reinforcements according to the environmental exposure conditions can provide added protection against the corrosion damage caused by chloride contamination by mitigating the risk of chlorides reaching the steel surface and reaching the critical value.
Surface protection systems such as coatings can be applied to the concrete surface to provide an additional barrier against chloride intrusion. These protective coatings act as a shield, preventing chlorides from reaching the concrete and embedded steel reinforcement.
By combining these strategies, the risk of chloride contamination in concrete structures can be effectively minimized, ensuring their long-term durability and performance even in the most aggressive of exposure conditions.