Presented by: Asst. Lect. Kawther Hayder Abdulateef Department of Civil Engineering – Al-Mustaqbal University Abstract Corrosion of reinforcing steel is one of the most common and costly causes of deterioration in concrete structures worldwide, as it shortens their service life, raises maintenance costs, and threatens their structural safety. This article reviews the corrosion mechanism in reinforced concrete, its most important factors, and its effects on the behavior of structural elements, followed by methods of detection and assessment and approaches to prevention and treatment. It concludes that designing for durability, coupled with periodic inspection and early intervention, is more cost-effective than repair after the damage has become severe. Keywords: reinforcement corrosion, carbonation, chlorides, concrete cover, durability, reinforced concrete, cathodic protection. Introduction Reinforced concrete relies on the complementary action of concrete, which resists compression, and reinforcing steel, which resists tension. Concrete provides the steel with dual protection: a physical barrier that isolates it from the environment, and an alkaline environment (pH between 12.5 and 13.5) that forms a thin, stable layer of iron oxides on its surface, known as the passive film. As long as this film remains intact, the corrosion rate is very low. However, its loss, due to reduced alkalinity or the arrival of chloride ions at the steel surface, starts a chain of deterioration that ends in cracking and spalling of the concrete and loss of part of the effective steel cross-section. Corrosion Mechanism Corrosion in concrete is an electrochemical process that requires four elements: an anodic area where the iron dissolves, a cathodic area where oxygen is reduced, an electronic conductor (the steel itself), and an electrolytic medium (the pore water in the concrete). The basic reactions can be summarized as follows: At the anode: Fe → Fe²⁺ + 2e⁻ At the cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ The iron ions later convert into rust products whose volume is about two to six times that of the original steel, depending on the type of rust (approximate values), generating internal pressure that cracks the concrete cover. The progression of damage is usually described by a two-stage model (Tuutti’s model): Initiation stage: The causative agent (carbon dioxide or chlorides) advances through the concrete until it reaches the steel surface and breaks down the passive film. Propagation stage: The steel corrodes at a rate that depends on moisture, oxygen, and temperature, and structural damage becomes apparent. Main Causes 1. Carbonation. Atmospheric carbon dioxide reacts with calcium hydroxide in the concrete, lowering the alkalinity of the medium from about 13 to below 9, so the passive film loses its stability. The carbonation front advances approximately in proportion to the square root of time, and it is faster in highly porous concrete and at moderate relative humidity (about 50–70%). 2. Chlorides. They come from seawater, salts dissolved in soil and groundwater, de-icing salts, or contaminated concrete constituents. When their concentration at the steel surface exceeds a critical threshold (often cited as about 0.4% by mass of cement, although it varies considerably with conditions), localized pitting corrosion occurs. It is more dangerous than uniform corrosion because it reduces the steel cross-section at specific points without visible warning. Influencing Factors Concrete quality: Its permeability increases with a higher water-to-cement ratio and with poor compaction and curing, which accelerates the penetration of the causative agents. Concrete cover thickness: The greater the thickness, the longer the initiation stage. Cracks: They represent fast pathways for water and chlorides to reach the steel. Environment: Coastal areas and areas exposed to salts, alternating wetting and drying, and high temperatures all accelerate corrosion. Type of cement and admixtures: Mineral admixtures, such as fly ash, silica fume, and slag, improve the concrete’s resistance to chloride penetration. Structural Effects Reduction in the cross-sectional area of the steel, and consequently a decrease in the element’s resistance to bending and shear. Cracking and spalling of the concrete cover, which exposes the steel to further corrosion. Weakening of the bond between steel and concrete, which affects the integrity of the reinforcement and the effectiveness of splices. Reduced ductility of the steel and of the element’s capacity to deform before failure, which is extremely important in seismic regions. Deterioration of serviceability through increased cracking and deflections, followed by a decline in the structure’s overall safety factor. Detection and Assessment Assessment begins with a visual inspection that identifies cracks, rust stains, and spalling, and is then supplemented by field and laboratory tests, the most notable of which are: Half-cell potential test (ASTM C876): estimates the probability of corrosion by measuring the electrical potential of the steel surface. Carbonation depth measurement: by spraying phenolphthalein solution on a freshly broken sample, where the pink color remains in the non-carbonated zone. Chloride content analysis: on powdered samples taken from different depths. Measurement of concrete electrical resistivity and of corrosion rate using the linear polarization technique: to estimate the speed of corrosion. Rapid chloride permeability test (ASTM C1202): to evaluate the quality of new concrete. Non-destructive tests: such as ground-penetrating radar to locate the reinforcement and cover, and ultrasonic pulse velocity tests. Methods of Prevention and Treatment First: Prevention at the design and construction stage Reducing the water-to-cement ratio (usually 0.40–0.45, or lower in severe environments) and ensuring good compaction and curing. Providing sufficient concrete cover as specified by the code according to the exposure class (often ranging between 40 and 75 mm or more). Using mineral admixtures that reduce permeability, and corrosion inhibitors. Using epoxy-coated, galvanized, or stainless steel reinforcement in highly aggressive environments. Applying surface paints and protective coatings that limit the ingress of water and chlorides. Designing good drainage details that prevent water from ponding on surfaces. Second: Treatment after damage appears Local repair: removing the damaged and contaminated concrete, cleaning the rust from the steel, treating it with a protective material, and then restoring the section with a suitable repair mortar. Strengthening the section: when a significant part of the steel is lost, by adding reinforcement, wrapping the elements with fiber-reinforced polymers (FRP) or steel plates, or enlarging the section. Cathodic protection: using impressed current or sacrificial anodes, which is among the most effective methods for structures exposed to chlorides. Electrochemical chloride extraction and realkalization: specialized techniques used in selected cases. Coatings and surface treatments: to slow the ingress of causative agents after repair. This should be accompanied by a periodic monitoring program, because local repair alone may shift corrosion to adjacent areas (the incipient anode, or ring-anode, phenomenon). Conclusion Corrosion of reinforcing steel represents a major challenge to the durability of concrete structures, because it begins unnoticed and then accelerates until it appears as cracking, spalling, and loss of strength. The cost of dealing with it varies greatly depending on the timing of intervention: prevention through sound design and construction quality is less costly than repair, and early detection allows limited treatment instead of comprehensive strengthening. This requires integrating durability considerations at the design stage, establishing periodic inspection and maintenance programs, and selecting the treatment method according to the cause and degree of corrosion and the importance of the structure. The numerical values given are approximate and vary depending on the adopted code, site conditions, and type of structure. References Bertolini, L., Elsener, B., Pedeferri, P., Redaelli, E., & Polder, R. B. (2013). Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair (2nd ed.). Wiley-VCH. Broomfield, J. P. (2007). Corrosion of Steel in Concrete: Understanding, Investigation and Repair (2nd ed.). Taylor & Francis. Tuutti, K. (1982). Corrosion of Steel in Concrete. Swedish Cement and Concrete Research Institute (CBI), Stockholm. Angst, U., Elsener, B., Larsen, C. K., & Vennesland, Ø. (2009). Critical chloride content in reinforced concrete: A review. Cement and Concrete Research, 39(12), 1122–1138. Neville, A. M. (2011). Properties of Concrete (5th ed.). Pearson. ACI Committee 222. (2019). ACI 222R-19: Guide to Protection of Reinforcing Steel in Concrete against Corrosion. American Concrete Institute. ASTM C876. Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete. ASTM C1202. Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration.