PSI - Issue 64

Henrik Becks et al. / Procedia Structural Integrity 64 (2024) 1279–1286 Henrik Becks / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Transportation and energy infrastructure are cornerstones of modern society, yet their integrity faces constant threats from structural overloading, deterioration, and fatigue. Bridges, runways, and wind turbine towers are particularly vulnerable. The vast majority of our existing structures are built from prestressed and reinforced concrete (RC), many of which are in a precarious state of structural health. Fatigue of RC is one of the most complex and, thus, most dangerous factors that prevents civil engineering structures – particularly bridges – from reaching their expected lifespan. Therefore, a comprehensive grasp of the degradation processes responsible for RC failure under subcritical cyclic loading is essential. In recent years, experimental research has been conducted in various areas of concrete fatigue – under a huge variety of loading types – , showing that the overall strain development provides a very promising indication of the underlying damage condition (e.g., Becks & Classen, 2024; Blasón et al., 2019; Oneschkow, 2016; Ortega et al., 2022). To accurately estimate the remaining service life of a concrete structure, it is thus imperative to have a monitoring system that continuously measures the strain development as well as an evaluation routine capable of processing strain information in real-time, enabling strategic long-term planning of structural maintenance measures. Currently, the remaining service life is typically assessed solely through visual inspection of the concrete surface. However, this approach is inadequate for capturing the intricate relationships between fatigue-induced strain progression, deterioration, and overloading. To improve the labor-intensive process of visual inspection and establish an autonomous, intelligent monitoring system, it is crucial to integrate continuously obtained information on the structural damage state through comprehensive strain measurements. By combining this data with mechanics based service life prediction models, a more robust assessment of structural safety can be achieved. This approach enables proactive maintenance measures to be implemented before irreparable damage occurs, thereby preventing premature disruption of bridge operations. Realizing this overarching objective necessitates the development of a large-scale strain monitoring concept that is applicable to existing structures and durable over decades. A pivotal hurdle in formulating this concept lies in the heterogeneous nature of RC; both linear elastic strains and localized microcracks induced by repetitive subcritical loading are unevenly distributed across the entirety of the structure. As punctual measuring devices (e.g., strain gauges) solely capture localized phenomena, yielding a potentially deceptive or incomplete depiction of the global strain state, continuous two-dimensional measuring systems become imperative for a dependable evaluation of the complex strain profile (Becks, Baktheer et al., 2023). Digital image correlation (DIC) and fiber optic sensors (FOS) emerge as particularly well-suited methodologies for this task. However, in contrast to DIC, FOS entails lesser computational overhead, attains a higher measurement resolution, generates less voluminous data, and proves more suitable for sustained deployment in bridge structures. Within this paper, preliminary experimental studies are presented, aimed at scrutinizing the applicability of FOS on tension-loaded concrete elements. Moreover, a holistic measuring concept is proposed, wherein raw one dimensional measurement data is processed into two-dimensional strain profiles, incorporated into an array of diverse service life prediction models, thus facilitating an assessment of the structural damage condition and remaining service life. 2. Service life prediction via 2D-FOS To address the challenges outlined in Section 1, the authors engage in a sub-project (Project number: 501771082) of the Priority Program SPP 2388 "100 plus". This sub-project focuses on the high-resolution measurement and interpretation of the damage state in reinforced and prestressed concrete structures under fatigue loading, utilizing externally applied, two-dimensional fiber optic sensor technology (2D-FOS). The overarching objectives of this endeavor are: a) continuous monitoring of the evolving strain state due to fatigue-induced micro and macro-crack formation, b) interpretation of time-varying strain/damage information and its integration into a digital twin of the structure, and c) prediction of the remaining service life based on selected physical models (Figure 1).