PSI - Issue 84

Alessandro Nettis et al. / Procedia Structural Integrity 84 (2026) 653–660

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moisture), oxygen and carbon dioxide, plus other external agents like chloride ions (Cl - ) coming from de-icing salts or spread in the air of marine environments. Corrosion generally affects steel embedded in concrete structures, due to the environmental conditions the structure is subjected to during its entire service life. Normally, the steel is preserved from corrosion by a protective passive film that forms on its surface due to the alkalinity of the concrete environment. Over time, the penetration of external corrosive agents into the concrete structure reduces the alkalinity of its environment, threatening the local or global destruction of the protective film, until the concentration of these aggressive substances reaches a critical threshold. At this point, the film locally breaks down and the corrosion process can get started. During corrosion, the strand section, the passive film, and the pore water form a closed-circuit electrolytic cell. Three chemical reactions take place, as two anodic-cathodic regions take shape in the closed-circuit cell: 1) oxidation reaction occurs in the anode region, where the strand loses electrons becoming ferric ions (Equation (1)); 2) reduction reaction occurs at the cathode region, where the strand gains electrons to OH - combining with oxygen and pore water (Equation (2)); 3) corrosion products are created as a result of the process, following the reactions listed in Equations from (3) to (5). − 2 − = 2+ (1) 2 +2 2 + 4 − = 4 − (2) 2+ + 2 − = ( ) 2 (3) 4 ( ) 2 + 2 = 2 2 3 +4 2 (4) 6 ( ) 2 + 2 = 2 3 4 +6 2 (5) These reactions locally erode the steel, reducing its cross-section and producing bulky molecules (Fe 2 O 3 or Fe 3 O 4 ), increasing the internal pressure on the surrounding concrete, leading to concrete cracking and steel-concrete bond weakening (Wang, 2023). Eventually, spalling of the concrete cover impairs the protection provided by the concrete to the steel and may speed up the corrosion process. At the structural level, this results into reduction of strength and ductility (Yu et al., 2022) and both ultimate and serviceability behaviours of the structure degrade. How to model this localised steel area loss has been the objective of many research works over the last years. 2.2. Literature review The research on steel corrosion process and how it can affect the concrete structure performances starts from RC structures. Over the last half century, numerous models of different levels of complexity aimed at predicting or describing the deterioration of RC structures have been developed. Even if this is not the scope of the present work, it is worth mentioning some of these studies to understand the evolution of the available steel corrosion models. One of the first and most appreciated studies on the topic was proposed by Tuutti in 1982 (Tuutti, 1982). Here, the process of corrosion is illustrated with a schematic model where the service life of a structure is divided into a period of initiation and a period of propagation. The time up to the initiation of the corrosion process is determined by the flow of penetrating substances into the concrete cover until the threshold concentration for the corrosion start is reached. In the propagation period, the corrosion process is modelled by a single parameter, i.e., the rate of corrosion, depending on the relative humidity in the concrete and the mean temperature of the structure. This study established the foundation of the steel corrosion process in concrete structures. An important study on geometric models for steel corrosion of reinforcement rebars was proposed by Val and Melchers in 1997 (Val and Melchers, 1997). In their study, they focused on the propagation period and considered general and localised corrosion, i.e., a uniform loss of metal over the whole exposed surface of a rebar and a concentrated configuration of corrosion forming corrosion pits on the steel surface, as noted earlier by Gonzalez et al. (González et al., 1995) . They provide analytical formulas to compute the steel area reduction of a rebar affected by pitting corrosion, based on a semicircular shape intersecting the cross section of a steel rebar. First studies on PC structures appear in early 2000s (Gardoni et al., 2009; Vu et al., 2009). Modelling the initiation and propagation period of corrosion in PC structures is different from RC structures. Prestressing steel works under high level of stresses during the structure service life, and this can accelerate the corrosion process (Vereecken et al., 2021). Moreover, in post-tensioned structures, the diffusion models of chlorides proposed for RC structures can barely be applied, as the steel tendons are not directly embedded in concrete, but they are surrounded by a plastic or metallic duct filled with grout. The issue is further complicated by possible construction errors during grout injections, which