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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Keywords: Epoxy; Toughened; Adhesive; FRP; polymer; bond.

1. Introduction The preservation of existing built heritage stands as a paramount concern in the field of civil engineering, primarily driven by the inevitable ageing and structural deterioration of such constructions over time. Issues like fatigue and corrosion of materials exacerbate the structural vulnerabilities, necessitating robust solutions for structural strengthening. Concurrently, the imperative for sustainable construction practices urges a critical examination of the environmental implications associated with new building constructions, where the refurbishment and reinforcement of existing structures emerge as a more sustainable alternative. Within this context, the utilization of externally-bonded (EB) composite materials has emerged as a pivotal and sustainable solution, owing to a multitude of advantages it offers. Notably, these composites exhibit a high strength to-weight ratio, facilitating ease of application and installation, while their inherent resistance to corrosion and lightweight nature further augment their appeal. Among the traditionally adopted composites, fabric-reinforced polymers (FRP) hold a prominent position, often externally bonded to structural components using epoxy adhesives. Conventionally, thermosetting epoxies are widely used for this purpose due to their fast curing time, low curing shrinkage and creep, and good thermal stability. However, significant drawbacks associated with the use of epoxy adhesives are their limited fire resistance, low moisture permeability and possible strength loss following the exposure to temperature higher that their glass transition temperature (55-75 °C), Tatar and Milev (2021). Moreover, the amorphous and highly cross-linked microstructure of epoxy polymers makes them relatively brittle and susceptible to dynamic actions such as cyclic loading and impacts , Dušek and Galina (1980) , increasing the concern about the longevity and effectiveness of the composite strengthening. One of the primary weaknesses of EB-FRP applications lies in the bond between the composite and substrate, susceptible to various modes of debonding under both in-plane (Mode-II fracture mechanics condition) and out-of plane (Mode-I) actions, or their combination (Mixed mode). Notably, cyclic loading can induce the formation of cohesive cracks in the adhesive layer, leading to eventual propagation and diminished composite effectiveness over time. To address these challenges, recent efforts have focused on enhancing stress transfer between substrate and bonded reinforcements through the development of modified epoxy adhesives, Al-Zu'bi et al. (2022). These formulations incorporate new phases of material to confer higher fracture toughness, thereby mitigating debonding issues. Fracture toughness represents the ability of composites to prevent crack propagation. This toughening is typically achieved through the introduction of dispersed phases such as rubber-type particles or nanomaterials into the polymer structure, Johnsen et al. (2007). The increased fracture toughness of toughened epoxy adhesives offers several advantages, including enhanced fracture energy and strain capacity. Consequently, this leads to improved structural capacity and retardation of crack propagation under high stress concentrations at fatigue crack tips. However, it is essential to acknowledge that toughening may entail trade-offs, potentially reducing mechanical properties such as tensile strength and elastic modulus, while also increasing the resin viscosity before curing and decreasing glass transition temperature, Johnsen et al. (2007). This paper provides an overview of the toughening mechanisms inherent to epoxy polymers, followed by a discussion of main methods for epoxy toughening. Additionally, pertinent literature studies were examined, focusing on the effect of the use of toughened adhesives for the strengthening of structural elements with FRP composites, including comparative analyses with traditional epoxy formulations. Finally, the cohesive bond-slip behavior of traditional brittle adhesives is compared with that of toughened ones, shedding lights on the advantages of the toughening effect on the structural efficiency and bond capacity of FRP-substrate joints. 2. Toughening mechanisms in epoxy polymers Traditional thermosetting resins are characterized by their ability to undergo a chemical reaction called cross linking during curing. This process involves the formation of covalent bonds between polymer chains, creating a network-like structure , Dušek and Galina (1980) . It implies that a significant portion of the polymer chains in the material are linked together, forming a three-dimensional network. Once the cross-linking reaction is complete, the