PSI - Issue 78

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

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1. Introduction The use of composite materials for strengthening and retrofitting reinforced concrete and masonry structures has become increasingly widespread in recent decades. Their appeal lies in a combination of high strength-to-weight ratio, excellent durability and straightforward installation procedures. In particular, externally bonded (EB) fiber-reinforced composites, such as Carbon Fiber Reinforced Polymer (CFRP) sheets, have proved highly effective in boosting the load-bearing capacity and ductility of beams, columns, walls, arches and other elements. A key factor in the success of these strengthening systems is the quality of the bond between the reinforcement layer and the supporting substrate. Consequently, much research has been devoted to monotonic shear-lap tests, which explore how different variables, reinforcement type, substrate properties, surface preparation and application techniques, influence the debonding mechanism and ultimate load capacity Xiao and Zha (2004); Yao et al. (2025); Mazzotti et al. (2009); De Santis et al. (2017); D’Ambrosi et al. (2013); Oliveira et al. (2011); Ghiassi et al. (2013); Seo et al. (2013). These experimental insights have, in turn, supported the development of ever more accurate theoretical and numerical models for FRP-to-concrete interfaces Sun and Yu (2017); Ascione et al. (2020). Despite the seismic motivation for many FRP applications, since earthquakes impose cyclic rather than purely monotonic demands, most laboratory work and computational studies still concentrate on monotonic behaviour. Only in the past few years have researchers begun to look specifically at bond performance under repeated load–unload cycles. This gap is important, because cyclic degradation may significantly reduce the effectiveness of a retrofit during service. This degradation inevitably requires the removal of strengthening systems, leading to large quantities of special waste. Consequently, it highlights the need to develop eco-friendly treatment and recycling processes within a life cycle assessment framework. This represents a significant gap for these new materials. In the context of sustainable end-of-life management of construction materials, bioleaching represents an emerging biotechnological method for coupling the recovery of structural materials with circular economy strategies. This process uses anaerobic microorganisms, usually during a phase called dark fermentation (DF), to produce volatile fatty acids (VFAs). These organic acids biochemically attack and dissolve cement-based materials. For FRP systems that detach from concrete substrates by removing a thin concrete layer, VFAs can help selectively release valuable reinforcing fibers (such as carbon fibers) while also producing biohydrogen (bio‑H₂), a clean energy source that increases the sustainability of the process (Trancone et al., 2022). DF includes the first steps of anaerobic digestion (i.e., hydrolysis, acidogenesis, and acetogenesis) by excluding the methanogenesis pathway that produces methane-rich biogas. Therefore, to perform DF properly, methanogens should be inhibited via an inoculum pretreatment, such as thermal shock or particular operating conditions (e.g., low pH, short hydraulic retention time) (Elreedy et al., 2024). The inoculum can be constituted by a mixed microbial culture to improve the conversion of organic substances to VFAs and bio‑H₂ under mesophilic or thermophilic conditions (i.e., 38 – 52 °C). VFAs, such as acetate and butyrate, can play a key role in breaking down the cement matrix. The aim of this paper is twofold: first, to numerically investigate the cyclic bond degradation of FRP-strengthening systems applied to concrete specimens; and second, to experimentally evaluate the effectiveness of a bioleaching process for the disposal of FRP systems after detachment from structural substrates. These objectives are closely linked, as accurate prediction of bond degradation informs the design of the subsequent removal and recycling phase. In the first part of the paper, the authors present their numerical approach for simulating bond degradation under load–unload cycles. The second part reports the experimental results of the bioleaching process. 2. Cyclic Bond behavior: numerical modeling The proposed modeling approach simplifies the simulation of a concrete block strengthened with an FRP strip. Following a common assumption in literature, the nonlinear bond behavior is concentrated in the interface between the FRP and the concrete (Grande and Imbimbo 2016). The model includes three main components (Figure 1): (i) the concrete block, idealized using linear-elastic axial springs (“support springs”), (ii) the FRP strip, also modeled with linear-elastic axial springs (“FRP springs”), and (iii) the FRP-to-concrete interface, represented by nonlinear shear springs (“interface springs”).