PSI - Issue 62

Silvia Caprili et al. / Procedia Structural Integrity 62 (2024) 355–360 Caprili et al./ Structural Integrity Procedia 00 (2019) 000 – 000

356

2

1. Introduction A large part of the existing infrastructural heritage in Italy was realized in the period 1960s/1970s; most of existing bridges are therefore progressively approaching the end of their nominal life, with following deterioration of the maintenance conditions and of the structural safety. Local and global failures happened in recent years, think for instance to Morandi (2019), Fossano (2017) and Santo Stefano (2020) bridges, highlighted the need for urgent vulnerability and risk assessment of existing infrastructures. The vulnerability of prestressed concrete (PC) bridges with post – tensioned (PT) cables is essentially related to the presence of ‘ hidden’ defects, such as the presence of voids into the mortar of cables due to incorrect grouting injection operations, that cannot be detected with traditional investigation techniques. The grouting has the role of guaranteeing the efficient performance of the cables into the PT component and of protecting prestressing strands from the corrosion; if injection defects are present, wires and strands may suffer aggressive environmental actions, such as rain, air, etc., potentially leading to the activation of corrosion phenomena. Different corrosion mechanisms have been recognized in prestressing steels, ranging from localized/pitting corrosion – where the degradation is mainly related to the cross-section reduction (Li et al. 2011) of the wire and therefore a decrease of the bearing performance can be appreciated even at strand level – to hydrogen embrittlement, responsible for a sudden modification of the deformation capacity leading to unexpected brittle failures even without relevant mass loss or pit depth. In this last case (Nürnberger, 2002) the depassivation of the steel strands, followed by a decrease of the pH in several occluded areas lead to possible Hydrogen emission and to the following absorption by the prestressing steel; the wire is not cracked, but in relation to the H-content absorbed and to the specific H-sensitivity, the prestressing steel may turn into a brittle performance. Furthermore, the presence of the high stress level characterizing prestressing products is responsible for the so-called stress-corrosion phenomenon, strongly accelerating the corrosion process. Deepening the knowledge about corrosion effects on the residual capacity of strands becomes then fundamental; the possibility of having reliable tools for predicting deterioration of wires and strands, possibly calibrated basing on experimental results, can help in the assessment of the residual performance of prestressing products and, in a wider framework, prestressed concrete members and whole infrastructure. In the present work, the preliminary results of an experimental campaign executed on naturally corroded strands are reported, with the aim to understand how the corrosion and external aggressive actions impact on their mechanical characteristics. Preliminary results of FEM analyses on strands’ three -dimensional models calibrated basing on the results of experimental tests are even reported, being the first step to elaborate reliable tools describing the relationship between a specific corrosion damage indicator (dependant on the corrosion mechanism) and the modification of the mechanical performance. The strength of the present work lies in the availability of a wide set of naturally corroded strands, extracted from an existing prestressed concrete bridge with PT system and exposed to environmental corrosion conditions (water, rain, etc.) due to incorrect operations during constructions. Corrosion of strands has been caused by the simultaneous action of relevant high stress in the cables (already pre-tensioned) and exposure to water and oxygen coming from the injection nozzles before grouting, for several months, causing Hydrogen embrittlement on prestressing strands. After the extraction, n. 2554 strands were catalogued according to visual judgment on damage conditions (Sason et al. 1992), leading to the final selection of more than 500 samples for the following mechanical characterization, generally including tensile and bending/re-bending tests. Physical tests were also performed on a reduced set of strands (about 80) through X-Ray tomography, to investigate the entity of corrosion. FEM analyses are still ongoing to calibrate the results of experimental tests. In following paragraphs, preliminary results are presented. 2. Mechanical and physical characterization The experimental campaign consisted in the execution of tensile and bending/re-bending tests with the aim to evaluate the effects of corrosion due to hydrogen embrittlement (HE) on the mechanical performance of prestressing strands, both in terms of strength and deformation capacity and modification of the failure modality. X-Ray tomography tests to investigate the corrosion entity were also executed. Mechanical tests were performed at ‘Laboratorio Ufficiale per le Esperienze sui Materiali da Costruzione ’ of University of Pisa ; X-ray tomographies were executed with the support of Pontlab laboratory, close to Pisa. Tensile tests were executed on n.556 strands according to EN ISO 15630-3:2019, through the adoption of a video-