The Occurrence of Corrosion within Reinforced Concrete
Concrete is a widely used construction material and has been for many centuries. While concrete has relatively outstanding compressive strength, it lacks in tensile strength. For this reason, reinforcement steel bars are normally used in concrete structures to compensate the tensile strength. However, the steel bars are susceptible to corrosion that can undermine the structural integrity of concrete. Once corrosion occurs, rust forms on the surface of the steel. The rust takes up more volume than the original steel, adding tensile stress to the concrete, causing concrete to eventually crack or spall as it has relatively low tensile strength. It is important to understand the mechanism of corrosion to effectively prevent corrosion from comprising reinforced concrete structures.
Steel naturally undergoes corrosion as the metal is not in its natural form. Iron ore, composed of iron oxides, is smelted and refined to produce steel. Under corrosion, steel returns back to iron oxide, known as rust, as it is the more stable form of iron. Corrosion is an electrochemical process and three requirements must be met for it to occur:
1) Metals with different potential energy (anode and cathode)
2) Electrochemically conductive environment
The reinforced steel bar in concrete frequently meets these requirements. A rebar itself generally contains areas with different potential energy. A rebar is also a single piece of connecting metal so it serves as a conductive environment. Finally, due to the inherent porosity of concrete, water vapor and liquid water can reach rebar and act as electrolytes. The combination of these three conditions causes corrosion.
Under ideal conditions, reinforced steel is actually protected by concrete. Steel naturally forms a thin oxide layer, also known as a passive layer, under high pH conditions. The pH in concrete increases during its initial curing process allowing steel to form its passive layer. Essentially, this layer physically protects the metal from environmental contact and slows the rate of corrosion significantly. This protective layer normally will prevent corrosion from causing notable deterioration to the steel. However, this protective layer is easily broken due to chloride ions and carbon dioxide, common occurrences for many reinforced concrete structures. According to The Masterbuilder, reinforced steel only experiences metal losses of about 0.1–1.0 um/year under ideal conditions. On the other hand, reinforced steel can experience metal losses of as much as several mm/year with influence from chloride ions and carbon dioxide.
Chloride ions are generally the main cause for accelerated corrosion on steel. Deicing salts, sea water, and chemical carrying relatively high chloride ion contents are an issue for steel. The exact mechanism in which chloride ion breaks down the passive layer is not fully understood, but it is generally speculated that the chloride ions can penetrate the passive layer and disrupt it. Once the passive layer is disrupted, it leaves the bare steel underneath susceptible to corrosion.
Carbon dioxide is another cause for accelerated corrosion. Carbon dioxide in the air can penetrate concrete structures and react with the hydroxides contained in concrete to form carbonates. This process is known as carbonation. The formation of carbonates reduces the pH of the surrounding solution and destabilizes the passive layer once the pH is below 9. Once again, this leaves the steel open to corrosion. The rate of corrosion influenced by carbon dioxide is generally slower than that of chloride ions, but carbon dioxide will still deteriorate reinforcement bars over time. In addition, both chloride ions and carbon dioxide can affect the steel at the same time, further accelerating corrosion.
Corrosion of steel rebar in concrete structures contributes significantly to structural deterioration. Proper preventative measures and maintenance should be followed to ensure longevity of concrete structures.