PSI - Issue 64

Angelo Savio Calabrese et al. / Procedia Structural Integrity 64 (2024) 1832–1839 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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pulmonary effects, as well as central nervous system disruption, leading to neurotoxicity or immunotoxicity. Environmental impacts include translocation in the food chain and contribution to atmospheric dust clouds, ultimately affecting marine life, Al-Zu'bi et al. (2022). 4. Use of toughened epoxy in EB-FRP structural joints The use of toughened epoxy adhesives has emerged as a compelling alternative to traditional epoxy for bonding FRP reinforcement to existing structures, offering significant advantages in performance for both steel and concrete elements. In such applications, the adhesive plays a critical role in facilitating stress transfer between the substrate and the FRP strengthening, see Bocciarelli (2021). Toughened epoxies, which typically exhibit greater fracture energy, ensure higher bond capacity within the structural joint compared to traditional counterparts, with cohesive failure within the adhesive layer being the typical mode of failure. Moreover, the improved fracture toughness provided by modified polymers enhances fatigue life and long-term effectiveness, making toughened adhesive structural joints particularly suitable for structures subjected to dynamic actions such as bridges, viaducts, and industrial buildings. Calabrese et al. (2024) investigated the effect of adhesive toughening, referred to as S370 (Sika Italia S.p.A., 2024), on the bond behavior of CFRP-steel joints under quasi-static Mode-II loading conditions, using experimental results from single-lap direct shear tests. Results are also available for fatigue loading conditions, and they will soon be submitted for publication. The study for quasi-static loading compared a traditional un-toughened adhesive with a rubber-toughened one. As shown in Figure 2a, the toughened adhesive exhibited a non-linear tensile stress-strain behavior, while the traditional adhesive showed a linear elastic response with higher strength and elastic modulus. With the same bonded area for the joint, the toughened adhesive demonstrated an 82% increase in the average bond capacity, compared to the traditional adhesive (i.e. from 17.7 kN to 31.2 kN, see Figure 2b). Furthermore, the bond slip behavior of the cohesive interface showed a 272% higher fracture energy in the case of the toughened adhesive compared to the traditional one (i.e. from 1.195 N/mm to 4.377 N/mm, see Figure 2c). This adhesive toughening resulted in a wider amplitude of the stress transfer zone, leading to a higher resultant of the shear stress along the bonded interface.

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(b)

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Fig. 2. Comparison between the (a) tensile, (b) bond and (c) interfacial strain behaviors of CFRP-steel cohesive interfaces made with traditional and toughened epoxy resin S370. Calabrese et al. (2024).

The effect of using the same rubber-toughened epoxy adhesives on the behavior of pultruded CFRP-strengthened concrete substrate was experimentally investigated by Achillopoulou et al. (2024) by means of double-lap direct shear tests. Their findings revealed a notable alteration in the interface where cohesive failure of CFRP-concrete joints occurred. While the investigated traditional adhesive experienced debonding within the adhesive or at the adhesive-