PSI - Issue 78

Francesco Bianco et al. / Procedia Structural Integrity 78 (2026) 41–48

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microorganisms exhibit a strong tendency to colonize and grow on carrier surfaces, leading to a net mass gain even in the absence of significant polymer breakdown (Biswas et al. 2013).

Tab. 1. Concrete weight loss after bioleaching experiments.

Samples

Initial weight (g) 0.1164 0.2709 6.0585 6.0662 7.0326

Final weight (g) 0.1214 0.2826 5.1991 4.7482 6.7055

Weight loss (%)

F

+4.30 +4.15 –16.05

FR CP

C

–22.05

ALL

–4.65

Conclusions Fiber-reinforced systems are widely used to strengthen concrete and masonry elements, improving both their load carrying capacity and ductility. While their effectiveness under static loads is well established, their response under repeated cyclic actions still needs further investigation, particularly in relation to bond degradation at the interface with the substrate. This degradation reduces the system’s effectiveness and may require partial or full removal of the composite reinforcement. Therefore, the disposal of these materials highlights the importance of introducing environmentally friendly processes to manage their end-of-life phase within a sustainable life-cycle approach. This study, conducted within the framework of the SAFER-REBUILT project, addresses a dual objective. On one hand, it presents a numerical model for simulating the cyclic bond behavior of FRP–concrete interfaces through a spring-based formulation. On the other hand, it contributes to sustainability goals by supporting the development of bioleaching-based processes for the selective recovery of detached FRP materials. The proposed model is based on a tri-linear bond-slip relationship and includes simplified rules for unloading and reloading, allowing for realistic simulation of stiffness degradation during cyclic loading. Comparisons with experimental results has confirmed the model’s ability to reproduce key behaviors such as load reduction, interface damage, and the influence of previous slip history. These outcomes are particularly valuable for predicting the damaged portions of FRP systems after seismic events and for estimating the amount of material to be processed through bioleaching. The results of the bioleaching tests confirmed that dark fermentation can support both the cement matrix degradation of and the partial recovery of reinforcement fibers. In particular, the highest bio-H₂ yield was obtained in the CP samples (i.e., 1512 mL), which suggests that the presence of concrete, especially when it is old or damaged, can stimulate microbial activity due to the fact that the calcium hydroxide present in the material can work as a natural pH buffer. The sample with the complete CFRP system (ALL) also showed increased hydrogen production (i.e., 1055 mL) and a slight loss of mass, showing that the process can work even when resin and primer are present. The weight loss was enhanced in the C and CP samples, reaching 22 and 16%, respectively. This would indicate that VFAs can be effective for dissolving calcium compounds, such as Ca(OH)₂ and CaCO₃, leading to a pH increase and progressively weakening the cement matrix. This aspect is important for releasing embedded fibers, such as CFRP. In contrast, the F and FR samples showed no significant changes in terms of weight loss as well as DF gas production compared to control samples, thus suggesting that fibers alone are resistant to biological attack and that degradation mainly happens when fibers are still attached to concrete. These results showed that DF was not only a proper approach for treating these types of waste but also a practical solution for managing the end-of-life phase of FRP-strengthened elements in a sustainable way. The production of