PSI - Issue 64

Pascual Saura Gómez et al. / Procedia Structural Integrity 64 (2024) 2125–2132 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The corrosion of steel reinforcement can be recognized as one of the main causes of deterioration in reinforced (RC) and prestressed concrete (PC) structures, and a chloride environment is often the main trigger mechanism. The reinforcement in concrete is protected by both a physical barrier provided by the cover and a chemical barrier provided by the alkaline nature of the concrete pores. However, if the alkaline layer is neutralised or depassivating agents arrive, corrosion of the reinforcement can occur, as stated by Garcia et al. (2021), Sánchez Montero et al. (2024), and Torres Martín et al. (2022). The Eurocodes define various exposure classes for structures with chloride presence. The content of chloride ions in reinforced concrete is limited to a range of 0.6% to 0.8% for class XS (marine environments) and 0.4% to 0.6% for class XD (environments with melting salts). However, for prestressed concrete, due to higher susceptibility of the bearing capacity of these structures in case of corrosion, the Cl - ion content should not exceed 0.3%, except for environments with exposure class XD3, where the limit is 0.2%. After casting, since iron oxides and oxyhydroxides are thermodynamically stable, steel in alkaline concrete forms a protective layer, known as a passivating layer, which can be a few nanometres thick. This layer is composed of hydrated iron oxides, see Maurice & Marcus (2018) and Toney et al. (1997). However, the passivating layer can be locally destroyed by the presence of chloride ions which pass through the pores of the concrete cover, leading to pitting attack. More in detail, chlorides can cause a local breakdown of the protective film, which acts as an anode (active zone) in relation to the surrounding, still passive zones where the cathodic reaction of oxygen reduction occurs, as stated by Angst et al. (2009) and Angst et al. (2019). After corrosion has begun, a highly aggressive environment develops within the pit, resulting in rapid loss of reinforcement cross-section due to very high corrosion rates. Corrosion then becomes widespread where reinforcement’s surface is subjected to high chlorides’ concentration. If chloride levels are very high, the attack can occur in larger areas and the pitting may be less evident. Apart from the Cl - concentration, other factors such as the concentration of OH- ions in the pore solution, the steel potential, and the presence of capillaries and pores at the steel concrete interface should also be considered. RC and PC structures and infrastructures can be severely damaged by chlorides contained in water, such as seawater of coasts, or de-icing salts, making them susceptible to a significant pathology that directly impacts their durability. Several studies investigated pitting corrosion in RC elements, with few research works which analyze PC elements without transverse shear reinforcement, by Belletti et al. (2020), Franceschini et al. (2022), Vecchi et al. (2021 a ) and Vecchi et al. (2021 b ). Also, relationships between visible defects (i.e., surface cracks), in-situ measurements and corrosion level are still required, see Ahmed et al. (2024), and Franceschini et al. (2022). Indeed, the identification of weakness of existing PC structures thorough inspection activities can represents a key tool for the evaluation of potential risks for the structural safety, as well as for planning repairing and strengthening interventions. In this work, eleven prestressed concrete beams subjected to 10 years of natural chloride attack were analysed along their length, through visual inspection and non-destructive measurements, such as potential, resistivity, chlorides content and corrosion rate, to try to find a correspondence between surface defects and corrosion of strands. 2. Methodology The experimental campaign includes eleven prestressed concrete (PC) beams from a thermal power plant placed along the Spanish coast. The beams were exposed for 10 years to wetting and drying cycles with seawater used as a coolant. Then, due to the shear failure of some companion beams, the eleven beams were dismantled and moved to the “Instituto Eduardo Torroja de Ciencias de la Construcción” for testing. The manufacturer's identification label has remained on all beams, providing information on their position in relation to their exposure during the service period. All the beams are characterized by a transversal cross-section of 150 x 300 mm and are reinforced at the bottom with two strands (with outer wires’ diameter equal to 4.26 mm and inner one equal to 4.38 mm) having nominal diameter equal to 12.9 mm, and at the top with two bars having diameter equal to 5 mm. The concrete cover is 50 mm, refer to Fig. 1. The mechanical properties of concrete and reinforcement are assumed the same as a previous study by Belletti et al. (2020). The concrete compressive strength is equal to 45.4 MPa. The tensile strength of prestressing steel is equal to 1901.75 MPa while the strain at maximum load is equal to 5%. The tensile strength of reinforcing steel is equal to 435 MPa while the strain at maximum load is assumed equal to 18%. The elastic modulus for both bars and strands is assumed equal to195 GPa.