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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substrate interface, joints bonded with the rubber-toughened epoxy exhibited cohesive failure within the concrete substrate. This indicates an enhancement in the adhesive interface capacity due to the rubber toughening process. Similarly, Irshidat et al. (2015) explored the behavior of glass and carbon FRP-concrete joints using a double-lap shear test setup. Their study demonstrated that the incorporation of carbon nanotubes into the epoxy resin significantly improved bond strength and slip at failure. Compared to their untoughened counterparts, specimens bonded with the toughened epoxy exhibited a 35% and 52% increase in bond strength and slip at failure for carbon fiber and a 26% and 83% increase for glass fiber, respectively. Moreover, while specimens bonded with the investigated un-toughened epoxy primarily experienced debonding at the interface between the fiber sheet and concrete surface, the use of toughened epoxy resulted in cohesive failure within the concrete substrate. 5. Bond-slip behavior of toughened epoxy cohesive interface Debonding of EB FRP reinforcements could occur according to several modes, namely: (i) within the substrate, (ii) at the composite-adhesive interface or (iii) at the substrate-adhesive interface, (iv) within the composite (composite delamination) and (v) within the adhesive layer (cohesive debonding). While substrate failure is often inevitable in existing concrete structures, it is excluded in steel applications. Proper substrate preparation can mitigate substrate adhesive interface debonding, and careful selection of composite and adhesive types can prevent debonding at the composite-adhesive interface. Additionally, composite delamination can be prevented through proper design of the CFRP plate's geometrical and mechanical properties. When debonding occurs within the adhesive layer, the fracture toughness of the epoxy adhesive plays a crucial role in joint bond behavior. Accordingly, the use of toughened adhesives could be a promising solution to improve the performances of the strengthening/repair applications. Under a pure Mode-II loading condition, the cohesive behavior of the adhesive layer can be described by the so called bond-slip law, which relates the shear stress exchanged at the cohesive interface with the relative slip between the two adherends, Papa et al. (2023). Traditional un-toughened epoxies are generally characterized by an idealized bi-linear bond-slip behavior (see Figure 2c), consisting of an initial linear response, followed by a decreasing softening branch. At the end of the softening branch, no residual stress is exchanged at the interface, which could then be considered fully debonded. Consistently, the area below the bond-slip curve represents the interface fracture energy. Recent studies on toughened epoxy bonded joints agree on the fact that toughening could affect the bond-slip behavior of the adhesive layer, which shifts from ideally bi-linear to tri-linear or trapezoidal, Calabrese et al. (2024). Figure 2c shows an example in this regard, comparing the different bond-slip behaviors experimentally measured on traditional and toughened adhesives. As inferred by Figure 2c, the toughened epoxy is characterized by wider fracture energy and slip capacity, resulting in higher strain absorption capacity. This entails the necessity of a longer joint length needed to fully establish the stress transfer for the toughened adhesive compared to the traditional one. However, when fully established, the higher fracture energy leads to a higher bond capacity for the joint. 6. Conclusions The study investigated the use of toughened epoxy adhesives for the repair and strengthening of existing structures with externally-bonded FRP composite. First, an overview of the main toughening mechanisms occurring in modified epoxy resins was presented, with reference to rubber- and nanomaterial-toughened adhesives. The effect of the use of toughened adhesives in externally-bonded FRP strengthening/repair applications was then discussed, also in light of the bond-slip behavior of such adhesives. This allowed for drawing the following conclusions: • The incorporation of a second microphase of dispersed toughening agents such as liquids, fibers or particles enhances the fracture toughness of epoxy polymers, due to the development of toughening mechanisms such as crack pinning/deflection, crack bridging and cavitation/plastic void growth. • Traditional toughening methods, such as rubber-toughening, are effective in increasing crack and impact resistance, albeit at the expense of reduced strength and modulus. Furthermore, this can induce high viscosity of the epoxy pre-polymer mixture, and the potential decrease of the modified-polymer glass transition temperature. Epoxy toughening with nanoparticles has overcome such limitations, in light to their high surface area-to-volume ratio and to the presence of active groups on their surface.