What is chloride attack and how can it be prevented?

Short Answer:

Chloride attack is a type of chemical damage in concrete where chloride ions penetrate the concrete and corrode the embedded steel reinforcement. This corrosion causes the steel to expand, leading to cracking and spalling of concrete, and reduces the strength and durability of the structure.

Chloride attack can be prevented by using low-permeability concrete, providing proper cover to reinforcement, using corrosion-resistant steel or coatings, and adding chemical admixtures. Regular maintenance and waterproofing are also helpful in protecting RCC structures in chloride-rich environments like coastal or industrial areas.

Detailed Explanation

Chloride Attack and Its Prevention

Chloride attack is a serious threat to reinforced concrete structures, especially those located in coastal regions, marine environments, or areas exposed to de-icing salts and industrial chemicals. It involves the penetration of chloride ions (Cl⁻) into the concrete, which eventually reach the steel reinforcement and break down the protective passive layer on the steel surface.

Under normal conditions, the high alkalinity of concrete (pH > 12) forms a protective oxide film on the steel that prevents rusting. However, when chlorides accumulate beyond a certain threshold at the steel level (usually 0.4% by weight of cement), this protective layer is destroyed, and corrosion begins. Rust formed during corrosion expands, leading to internal stress, cracks, delamination, and spalling, which ultimately weaken the structure.

Chloride Attack in Concrete

  1. Sources of Chlorides
  • Sea water or marine air (salt spray in coastal areas)
  • De-icing salts used in cold regions on roads and bridges
  • Groundwater or soil contaminated with chlorides
  • Admixtures or aggregates that may contain chloride impurities
  1. Mechanism of Attack
  • Chloride ions enter through pores and microcracks in the concrete.
  • They migrate inward over time, especially in wet–dry cycles.
  • Once they reach the steel reinforcement, they destroy the passive film, exposing it to corrosion.
  • Corrosion products (rust) occupy more volume than steel, causing internal pressure and cracks.
  1. Effects of Chloride Attack
  • Corrosion of reinforcement bars
  • Cracking and spalling of concrete cover
  • Loss of bond between steel and concrete
  • Reduction in load-carrying capacity
  • Shortened life span and increased repair costs

Prevention of Chloride Attack

  1. Use of Low-Permeability Concrete
  • Design concrete with low water-cement ratio (≤ 0.45) to reduce porosity.
  • Use blended cements with fly ash, slag, or silica fume to make concrete denser.
  • Ensure good compaction and proper curing to avoid cracks and voids.
  1. Adequate Concrete Cover
  • Provide sufficient thickness of cover over steel reinforcement as per IS 456 or design codes.
  • This acts as a physical barrier against chloride penetration.
  1. Corrosion-Resistant Reinforcement
  • Use epoxy-coated bars, galvanized steel, or stainless steel where chloride exposure is expected.
  • Alternatively, use corrosion-inhibiting admixtures in concrete.
  1. Surface Protection and Waterproofing
  • Apply protective coatings, sealants, or membranes to prevent water and chloride ingress.
  • These are especially useful in bridges, marine structures, and basements.
  1. Quality Control in Materials
  • Avoid using aggregates, water, or admixtures that contain harmful chlorides.
  • Test and verify materials before use.
  1. Design Detailing and Maintenance
  • Ensure proper drainage and slope in structures to avoid standing water.
  • Inspect regularly for signs of corrosion and damage.
  • Perform timely repairs to stop chloride progression.
Conclusion

Chloride attack is a major cause of corrosion in reinforced concrete structures exposed to salt-laden environments. It damages the steel by breaking its protective layer, leading to expansion, cracking, and loss of strength. Prevention involves using dense, low-permeability concrete, adequate reinforcement cover, protective coatings, and corrosion-resistant steel. With proper design and maintenance, chloride attack can be effectively controlled, extending the life of RCC structures.