corrosion protection of reinforcement by galvanization
Corrosion of steel reinforcement is problematic and posing a threat to reinforced concrete structures. Severe corrosion of reinforcement can result in concrete spalling and damaging structure integrity. Various factors can affect the rate of reinforcement corrosions. The environment is one of these factors. For example, the rate of reinforcement corrosion will accelerate for structures submerged in seawater compared to structures built above the ground—concrete properties playing an important role in reducing the rate of reinforcement corrosion. Concrete with low permeability will reduce the ingress of chemicals that cause the corrosion of reinforcement, which can effectively slow the corrosion of reinforcement. The concrete cover is another factor that can effectively slow reinforcement corrosion. Usually, a higher concrete cover is used for members of the structure in contact with groundwater or soil.
In a severe environment, galvanized reinforcement can be used to reduce the risk of reinforcement corrosion and to prevent the deterioration of structures as a result of reinforcement corrosion. Galvanized rebar is ordinary reinforcement rebar coated with a layer of the protective coating of zinc metal. The zinc layer will act as a protective barrier that protecting the reinforcement bars from the corrosive chemicals.
The corrosion of reinforcement will pass through different stages. To understand how the corrosion occurs, we need to know the components of concrete and how it hardens. The concrete is composed of cement, aggregates, and water. The cement and water react together to form cement paste. The chemical reaction between cement and water know as the hydration process. The cement paste will bind the aggregate together and form a solid matrix that can stand loads. The consumption of water during the hydration process will leave capillary and gel pores on the hardened concrete. These pores will act as a passage for the chemicals, gases, and water. At the initial stage, the PH of concrete will be almost 12.5. the high value of PH will create a passive environment that protecting the reinforcement from the corrosion. However, the ingress of chemicals and gases such as carbon dioxide and sulfur oxide will reduce the PH of concrete. The process of lowering the PH of concrete is known as the carbonation of concrete. The lowering of concrete PH below 11.5 will cause reinforcement to rust. The ingress of chloride will also accelerate the rate of corrosion of reinforcement.
The galvanizing of reinforcement will offer high protection against corrosion. The zinc coating has a higher chloride threshold (2 to 4 times of bare rebar). That means the galvanized reinforcement will require a higher concentration of chloride before it begins to rust. Also, the zinc has a greater PH passivation range than bare steel. This property will make the galvanized reinforcement more resistant to the carbonation process as the concrete ages.