What is Reinforcement corrosion?
Corrosion is a natural process which converts a refined metal into a more chemically stable oxide such as rust. While steel reinforcements embedded in concrete are provided with a passive layer of protection at the steel/concrete interface due to the strongly alkaline nature of concrete, this layer can be destroyed by carbonation or chloride contamination leaving the steel vulnerable to corrosion.
Corrosion is an expansive reaction, as the steel is embedded in the concrete, this force breaks it apart from within creating corrosion cracks and spalling which in turn allow greater ingress of carbonation, chlorides, moisture and oxygen; accelerating the process
What causes Reinforcement corrosion of concrete structures?
Reinforcement corrosion requires three things: moisture, oxygen and an electrochemical difference either between different points along the rebar or those nearby that can form anodic and cathodic sites. Iron ions are oxidized at the anode sites, release electrons, and are then transported to the cathode where reduction reactions occur, resulting in the formation of expansive corrosion products like rust.
While carbonation and chloride contamination can initiate corrosion by compromising the protective layer, they do not drive the corrosion reaction itself. Instead, once the protective layer is breached, the steel becomes susceptible to corrosion from oxygen and moisture present in the surrounding environment. As corrosion progresses, it leads to the formation of rust (iron oxide) on the steel surface, causing expansion and subsequent cracking, delamination, and spalling of the concrete cover.
What are the signs of Reinforcement corrosion of concrete structures?
Corrosion cracks.
Delamination.
How can I identify Reinforcement corrosion in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
How can I prevent Reinforcement corrosion in concrete structures?
To prevent reinforcement corrosion in concrete structures carbonation and chloride ingress must be tightly controlled.
First, 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 both carbonation and 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 ingress for carbon or chlorides, increasing the cover depth to reinforcements according to the environmental exposure conditions can provide added protection against corrosion damage.
Surface protection systems such as coatings or sealants can be applied to the concrete surface to provide an additional barrier against chloride intrusion. These protective coatings act as a shield, preventing ingress of damaging materials such as carbon or chloride ions 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.