PSI - Issue 64

Yunus Harmanci et al. / Procedia Structural Integrity 64 (2024) 2067–2074 Author name / Structural Integrity Procedia 00 (2019) 000–000

2068

2

1. Introduction Building owners are increasingly interested in strengthening existing reinforced concrete (RC) structures to meet current load demands, address aging-related problems, and extend the design life, as a cost-effective and sustainable alternative to rebuilding. RC elements primarily subjected to bending can be strengthened either by adding material to the compression zone (e.g. UHPFRC (Martin-Sanz, Chatzi, and Brühwiler 2016)) or the tension zone (Teng et al. 2003). There has been substantial interest in strengthening the tension zone, with carbon fiber reinforced polymers (CFRP) being a common choice. CFRPs can be applied either as an externally bonded reinforcement (EBR) or as a near-surface mounted (NSM) reinforcement. NSM strengthening offers improved bonding due to a higher perimeter to-cross-sectional area ratio (Zhang, Yu, and Chen 2017), which can be further enhanced by prestressing the CFRP (Badawi and Soudki 2009), enhancing its static and fatigue performance (Hosseini, Dias, and Barros 2014). However, NSM-CFRP requires complex prestressing systems, limiting its widespread use. Emergence of low-cost iron-based shape memory alloys (Fe-SMA) introduced a simplified prestressing procedure, leveraging the “shape memory” effect (SME) to generate prestressing through heating (Janke et al. 2005), also referred to as “activation”. This effect relies on the reversible phase transformation between martensite and austenite phases (Yang, Breveglieri, and Shahverdi 2021), which allows the deformed alloy to recover to its original shape after heat stimuli, but builds a “recovery stress” if the deformations are blocked from both ends (Yang et al. 2021; Schranz et al. 2019). While several studies have investigated the short-term behavior of RC elements strengthened with Fe-SMA, including bond behavior (Schranz et al. 2020; Saim Raza et al. 2022), lab-scale tests (Shahverdi, Czaderski, and Motavalli 2016; Rojob and El-Hacha 2017; Schranz et al. 2021; Czaderski, Shahverdi, and Michels 2021), numerical studies (Hawileh 2012; Omran and El-Hacha 2012; Rojob and El-Hacha 2015; Yao and Wu 2016), and real-world applications (Michels, Shahverdi, and Czaderski 2018; Schranz et al. 2019), the long-term behavior has not been extensively explored. Within the few studies available, relaxation behavior of ribbed Fe-SMA bars were investigated and a 10% relaxation loss was observed after 1000 hours (Michels et al. 2018). In another study, unribbed NSM Fe SMA bars in RC beams showed spalling of the grout after exposure to freeze-thaw cycles, resulting in minimal deterioration compared to unexposed beams after subsequent bending loading (Rojob and El-Hacha 2018). Interim results of this work after four years of natural environmental exposure and sustained loading, showed stable mid-span deflection trends, with negligible reduction in prestressing force (Shahverdi and Czaderski 2019). This study extends previous findings by comparing short-term experiments (Shahverdi, Czaderski, and Motavalli 2016) against an 8-year period of natural environmental exposure and sustained loading, followed by four-point loading. A CSA and sensitivity analysis was introduced in (Harmanci, Czaderski, and Shahverdi 2024) to assess the applicability of existing tools for predicting future behavior as well as failure prognosis while incorporating long-term models. A more in-depth discussion on the contribution of long-term effects on total deformation is provided herein. 2. Experimental Investigations The natural environmental exposure and subsequent experimental investigations were conducted at the Swiss Federal Laboratories for Materials Science and Technology (Empa), on two additional two-meter span beams, which were cast simultaneously with the six beams previously discussed in (Shahverdi, Czaderski, and Motavalli 2016). Over the course of roughly eight years, both beams were subjected to sustained four-point loading outdoors. The experimental program, including samples of the previous study, is outlined in Table 1.

Table 1: Overview of specimens

Beam No.

Properties of Beams No strengthening

Exposure Conditions

B1 B2

Short-term ambient exposure

Two non-activated FeSMA strips Two activated FeSMA strips Two activated FeSMA strips Two non-activated FeSMA strips

B3 – B4 - B6

B7 B8

Eight year long-term environmental exposure and sustained loading