PSI - Issue 42

Haya H. Mhanna et al. / Procedia Structural Integrity 42 (2022) 1190–1197

1191

© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of the scientific committee of the 23 European Conference on Fracture – ECF23 2 Mhanna et al./ Structural Integrity Procedia 00 (2019) 000 – 000 Keywords: Strengthening; hybrid; CFRP; PET-FRP; flexure

1. Introduction Fiber-reinforced polymer (FRP) materials are being extensively used in the construction industry to retrofit existing RC structures. In fact, due to its noncorrosive nature, FRPs have been replacing traditional strengthening methods that are corrosion-vulnerable such as steel plating. Other advantages of FRP materials include high strength to weight ratio, durability, and versatility (Mhanna et al. (2021); Salama et al. (2019)). The FRP materials, in the form of sheets and plates, are usually bonded externally to concrete surfaces using epoxy adhesives to enhance the flexural, shear and confinement capacity of RC members. The enhancement in flexural capacity of RC beams is achieved by bonding the FR P laminates along the longitudinal direction of the fibers on the tension side of the beam’s cross -section (Gao et al. (2017)). The shear capacity of RC beams is enhanced by bonding the FRP laminates perpendicular to the longitudinal axis of the beam at the beam’s side face, either by using the side bonded, U -wrapped or completely wrapped strengthening schemes (Chen et al. (2017); Mhanna et al. (2021)). The RC column capacity and ductility can be enhanced by confining the column with FRP la minates with the fibers oriented along the perimeter of the column’s cross-section (Abokwiek et al. (2021); Zeng et al. (2018)). The conventional FRP composite sheets (laminates) used in the construction industry are carbon (CFRP), glass (GFRP), aramid (AFRP), and basalt (BFRP). Those FRPs behave as linear elastic materials when loaded in tension and rupture in a brittle manner at strains ranging between 1.5 and 4% (Hawileh et al. (2022); Kang et al. (2014)). CFRP is the most used type of FRP in strengthening RC beams, slabs, columns, and walls due to its high elastic modulus (stiffness) and tensile strength. Comparatively, GFRP, AFRP, and BFRP have lower stiffness and tensile capacity than CFRP, but higher rupture strain. Hence, these types of FRPs are mainly used in column confining and for seismic retrofitting applications, in addition to flexural and shear strengthening of RC beams. Although conventional FRPs are being effectively used by engineers and researchers in external strengthening of RC beams, the non-yielding characteristic of FRP materials is a major concern, and often results in sudden and brittle failure mode of the strengthened member. Typical failure modes of RC members externally strengthened with FRP include the premature debonding of the FRP laminates, concrete cover delamination, and concrete failure followed by FRP rupture (Naser et al. (2019)). Recently, a new type of FRP materials composed from polyethylene naphthalate (PEN), polyethylene terephthalate (PET) and polyacetal (PAF) fibers have been developed to encounter the drawbacks of conventional FRPs (Anggawidjaja et al. (2006); Dai et al. (2011)). Compared to conventional FRPs, PET-FRP have large deformability (rupture strain exceeds 7%) and possess a nonlinear stress-strain relationship. Another key advantage of PET-FRP is that it is corrosion resistant, environmentally friendly, and cost-effective material since it is manufactured from recycled scrap polymer products (for e.g., PET bottles). Using PET-FRP in the retrofitting industry reduces construction waste, enhances the capacity of structures and provides a solution that encourages the concept of sustainability. However, these types of FRPs have lower stiffness and tensile strengths than conventional FRPs, which could be compensated by using more FRP layers (thicker sheets) (Dai et al. (2011)). Due to its large deformation capacity, the use of PET-FRP in external strengthening applications has been limited to special structures such as columns, underground structures, and structures that require high ductility (Zeng et al. (2020)). These structures should be designed to sustain large deformations due to several factors such as earthquake, impact, and blast. As a result, the literature mainly focuses on the use of PET-FRP in enhancing the confinement capacity of concrete and RC columns (Han et al. (2020); Pimanmas and Saleem (2018)). The preceding studies showed that strengthening of RC columns with PET-FRP laminates resulted in significant enhancement in the ductility while providing approximately the same strength enhancement as conventional FRPs. This is because PET-FRP is less likely to fracture before the columns reach their ultimate deformation capacity (Anggawidjaja et al. (2006)). The elastic and brittle nature of conventional FRPs causes a significant reduction in the ductility of the strengthened beam, which results in unfavourable and sudden brittle mode without any warning signs. In order to enhance the beam’s strength , several research studies proposed hybrid strengthening systems by stacking high stiffness and strength CFRP sheets with that of lower stiffness sheets (such as GFRP or BFRP) to the soffit of RC beams via epoxy adhesives (Hawileh et al. (2018)). Such hybrid systems improved the mechanical properties of the strengthening composite laminates that enhanced both the flexural strength and ductility of RC beam specimens. However, all these