Environmental Rusting of Steel Reinforcement
Steel reinforcement is widely used in concrete structures, such as reinforced concrete (RC) building structures and bridge structures to improve the strength and stiffness of the structures. Despite their importance, they are delicate and vulnerable to environmental conditions such as humidity, temperature and chemical attack, including corrosion. Corrosion of steel, whether galvanized or plain, embedded in concrete can become a significant issue but it can be prevented or properly control by proper design and construction, proper detailing and embedded protective coating.
Corrosion is an electrochemical process which occurs when steel and reinforcing bars embedded in a concrete are exposed to moisture and other corrosive elements such as oxygen, carbon dioxide and chlorides. The increase in the amount of moisture and chlorides present in the environment accelerates the process of corrosion. The corrosion of the steel reinforcement leads to cracking and the concrete cover is destroyed, resulting in gradual weakening of the structure.
The corrosion of steel in concrete structures is a complex phenomenon. It involves several factors such as the chemical composition of the concrete, combined with the presence of other impurities in concrete, the properties/composition of steel, the porosity and moisture content of concrete and the design of the structure. In addition, climate conditions also affect the corrosion process, with temperature playing a major role in the incidence of corrosion. Hot and humid climates are more prone to environmental rusting of steel reinforcement than cold, dry climates.
The effects of environmental rusting of steel reinforcement vary according to the properties of the reinforcing bar, the presence of moisture and other corrosive elements, and the design of the structure. The primary symptom of environmental rusting of steel reinforcement is the localised corrosion of the rebar, indicated by a reddish-brown colour on the surface of the bar. In most cases, the corroded steel develops a powdery layer which flakes off, exposing the base metal. This continues until the reinforcement bar is completely exposed and consequently the strength and stiffness of the structure is reduced.
Various methods have been used to protect reinforcing bars from corrosion, such as galvanizing, bituminous coatings, epoxy-coated bars, cathodic protection and corrosion inhibitors. However, depending on the environment, all of these solutions have their own limitations and therefore other solutions such as corrosion monitoring systems, maintenance programs and lifetime assessment are also suggested to counteract the effects of environmental rusting of steel reinforcement.
Although the effects of environmental rusting of steel reinforcement cannot totally be eliminated, it is possible to significantly reduce them if proper steps are taken at the design and construction stages of any project. It is necessary to understand the properties of steel, the properties of the environment, and the type of reinforcement used in order to ensure long-term protection of a structure by using the correct protective materials and structures. Taking proper measures at a project’s design stage is the best way to protect a structure from corrosion, in addition to proper maintenance, corrosion monitoring systems and lifetime assessment.
