Corrosion of reinforcing steel in concrete structures is a pressing issue. While high-quality concrete typically provides robust protection against corrosion, various structures like bridges, parking garages, and marine installations are frequently afflicted by the damaging effects of corroded reinforcement.
Corrosion is exacerbated by variations in chloride content, moisture, or oxygen along a reinforcing bar or between different bars. These disparities can generate voltage differentials that significantly accelerate the corrosion rate.
The expansion from corroded reinforcements imposes additional volume on the surrounding concrete, often leading to spalling. This effect can notably diminish the cross-sectional area of the rebar, compromising the structure’s ability to safely support its intended loads.
Mechanisms of Corrosion
Corrosion occurs as metals revert to their natural states, often as oxides or hydroxides. Metals in their refined, metallic state are at a higher energy level, and corrosion is a natural process that lowers this energy. Despite this inherent tendency, an inert film of tightly adhering corrosion products usually forms in alkaline environments like concrete, protecting the steel beneath from further corrosive attack. However, chloride ions can penetrate this passive film through a process called pitting, initiating corrosion in concrete-laden reinforcements.
Corrosion is fundamentally an electrochemical process, involving the transfer or displacement of electrons. It becomes more prevalent when there are distinct anodes and cathodes at measurable distances, allowing electrical currents to flow across the metal.
Even in concrete exposed merely to air and rain, corrosion can occur. This atmospheric corrosion does not necessarily require a substantial electrolyte. A thin film of rainwater can suffice, carrying ions and completing the electrical circuit. Anodes and cathodes in this scenario may reside within the same steel bar, separated only by minimal distances.
Preventing Corrosion in Reinforcing Steel
High-quality concrete with a minimum cover of 50 mm can effectively protect embedded steel from corrosion. However, the risk of steel corrosion increases if the quality or depth of the cover is inadequate. Corrosion is likely under the following conditions:
- The concrete cover undergoes carbonation, reducing the pH from roughly 13 to about 9.
- Chloride ions penetrate the concrete, with the concrete adjacent to the steel containing at least 0.77 kg of chloride ions per cubic meter.
If corrosion occurs due to insufficient cover and carbonated concrete, it tends to progress relatively slowly. In contrast, corrosion initiated by chloride ions usually advances more rapidly than atmospheric corrosion, leading to extensive damage. This process involves voltage differentials, electron transfer, and electrical current flow, coupled with chemical reactions.
Damages Induced by Corrosion
Corrosion causes significant damage to concrete, manifesting as cracking, delamination, and spalling. Atmospheric corrosion, linked to the carbonation of concrete cover, is relatively slow to develop and may not always lead to serious deterioration, depending on the environment.
Conversely, chloride-induced corrosion is notably more destructive. It can occur even in high-quality concrete, provided there is a sufficient supply of water (approximately 75% humidity), the threshold level of chloride ions at the anode, and adequate oxygen at the cathode. The resulting voltages push the corrosion to progress rapidly, causing noticeable cracking, delamination, and spalling of the concrete cover.
The underlying cause of this deterioration is the increased solid volume of the corrosion products compared to the original metal. In the case of iron, the reaction of the metal with gases, liquids, or dissolved solids creates this volume increase. Corroded steel generates various products—commonly known as rust—dependent on the available oxygen, chloride, and water. These products often include amorphous ferrous (Fe++), ferric (Fe+++), oxides, hydroxides, chlorides, hydrates, and complexes.
In conclusion, understanding the mechanisms and effects of corrosion on reinforcing steel is essential for maintaining the integrity and longevity of concrete structures. Effective preventive measures and regular monitoring are crucial to mitigating the damaging impacts of corrosion.