PSI - Issue 67

Gabriele Milone et al. / Procedia Structural Integrity 67 (2025) 90–106 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000 – 000

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1. Introduction Reinforced concrete (RC) is the most widely used construction material in the world (Gagg, 2014). By combining concrete's ability to withstand compression with steel's ductility to tension, this material is an ideal composite for all types of structures (Fehling, Leutbecher, and Roeder, 2011; Wight, 2016). However, as concrete is prone to cracking (Safiuddin et al. , 2018), one of the main challenges for RC structures revolves around reinforcement corrosion (Popov, 2023). The corrosion of steel rebars within concrete is initiated through the penetration of chlorides or carbonation, reducing the alkalinity of concrete and, consequently, breaking the natural passivation of steel (Zhu et al. , 2016). This leads to the progressive weakening of the rebar’s cross -sectional area and adversely affecting the bond between steel and concrete (Syll and Kanakubo, 2022), which is crucial for the structural performance of reinforced concrete elements (Coccia, Imperatore, and Rinaldi, 2016). The repair and maintenance of corroded structures lead to environmental issues and high costs, equivalent to half of the yearly construction budget spent on renovation of existing structures to extend their service life (Cailleux and Pollet, 2009). Hence, the development of effective prevention and protection techniques aims at limiting corrosion for maintaining safety and serviceability levels of existing structures (Angst, 2018; Abbas and Shafiee, 2020). Different strategies include corrosion inhibitors, alternative reinforcement materials, coatings, and electrochemical methods (Goyal et al. , 2018). Despite the variety of techniques available, each corrosion monitoring method is characterized by different efficiencies and limitations, such as invasiveness, cost, and the inability to provide early warning signs (Komary et al. , 2023). For instance, while electrochemical methods provide precise corrosion rates, they require direct access to the steel (Popova and Prošek, 2022). Similarly, fiber Bragg grating (FBG) based acoustic emission, though capable of real-time monitoring, struggles with background noise and interpreting data accurately (Jinachandran and Rajan, 2021). Infrared thermography offers a non-destructive method for corrosion assessment. Its accuracy, however, can be affected by external thermal variations and the placement of the reinforcement within concrete (Kobayashi and Banthia, 2011). The emergence of self-sensing construction materials offers a novel solution to overcome the limitations of traditional corrosion detection in reinforced concrete, by integrating sensing capabilities directly into construction materials (Han, Ding, and Yu, 2015). These smart materials are cement systems, such as pastes, mortar or concrete, that have been doped with a single or a combination of electrically conductive fillers to enhance their sensing properties (Ding et al. , 2019; Tian et al. , 2019). Monitoring is made possible through the analysis of electrical changes in electrical resistance of these conductive based material under external stimuli such as strain, damage, temperature and moisture (Chung, 2023). Self-sensing materials also detect and monitor corrosion, addressing some of the issues associated with more traditional methods, as a consequence of the deformation and cracking induced in the concrete cover due to the generation of iron oxides. Jin et al. (2017) explored how the incorporation of various chloride ion contents affects the electrical response of carbon-based cementitious composites. They found that the presence of ions supported graphene fillers in creating additional conductive networks within the matrix. Alternatively, carbon-based cementitious binders can be used as a pseudo reference electrode which, embedded in concrete, allows to monitor the different corrosion states of steel (Jin et al. , 2019). Nonetheless, the methodology for these applications – generally referred to as “bulk” – present significant limitations, i.e., invasiveness, due to the substitution of concrete with the conductive material, and cost, associated with the large filler amount needed to enhance the conductivity of an entire structural element. Moreover, this methodology does not represent general corrosion observations since carbon-based particles may act as protective layers on steel which suppress both metal oxidation and oxygen reduction, limiting the overall rebar corrosion (Coating, 2012). Alternatively, cement-based systems can be applied as external sensors – “coatings” – which have the potential to monitor corrosion development on a reinforced substrate. Hence, this research aims at exploring the potential of carbon black (CB)-based cementitious coatings as smart, multifunctional systems for early corrosion detection and monitoring in reinforced concrete structures. Coatings provide greater flexibility in terms of deployment for new and existing infrastructure, and also in fabrication due to their smaller size and thus lower costs (Chung, 2023; Ding et al. , 2019). The corrosion monitoring capability of these sensing coatings, similar to conventional gauges, was achieved by measuring the substrate’s inner state of strain and relating it with the movements of corrosion products between the rebar and the cementitious matrix (Grattan et al. , 2009; Routoulas and Batis, 1999). Differently from the traditional