PSI - Issue 42

Haya H. Mhanna et al. / Procedia Structural Integrity 42 (2022) 1190–1197 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1192

3

conventional FRPs rupture at low strain levels (1.5- 3%), which limits the beam’s ductility enhancement . Several studies were conducted to investigate the effect of hybridizing CFRP and GFRP laminates (Dong and Davies (2018); Hawileh et al. (2015)) on the flexural performance of RC beams. Similarly, other studies investigated the effect of combining Basalt (BFRP) with CFRP sheets (Choobbor et al. (2019); Sun et al. (2018)). In general, most of these studies agreed that the use of hybrid FRPs utilizes the superior properties of its components. The high stiffness of CFRP combines with the low stiffness of either GFRP or BFRP sheets and lead to a new and enhanced FRP laminate, which is stiffer than GFRP/BFRP and more ductile than CFRP laminate. In addition, hybrid FRP laminates have better fire resistance and durability (Hawileh et al. (2016)). Moreover, the use of hybrid FRPs to strengthen and retrofit RC beams in flexure had proven to enhance the beam’s ultimate capacity, stiffness, and ductility. The order of attaching the FRP layers is also an important factor in strengthening with hybrid FRPs. The optimum strengthening scheme that gave the best results in terms of improving strength and ductility of RC beams is via attaching GFRP or BFRP sheets to the beam’s soffit with epoxy adhesives followed by a layer of CFRP. All studies investigating hybrid combinations of FRPs have involved conventional FRPs with linear stress-strain behavior. Thus far, there are no studies conducted on RC beams to investigate the hybrid effect of combining high strength conventional CFRP with PET-FRP sheets that has a bilinear stress-strain relationship and significantly higher rupture strain, exceeding 7%. Therefore, the main aim of this study is to propose a new hybrid and ductile strengthening composite system to externally strength RC beams in flexure by bonding CFRP with PET-FRP sheets to the beams’ soffit in diffe rent combinations via epoxy adhesives. To satisfy the aim of this project, four RC beams that were externally strengthened with CFRP and PET-FRP sheets and their hybrid combinations are tested, in addition to a control unstrengthened beam specimen. In addition, standard coupon specimens were prepared and tested to obtain the mechanical properties of the proposed hybrid strengthening laminates applied to the beam ’s soffit . 2. Experimental Program 2.1. Beam specimens The experimental program consisted of a total of five RC beams, four of which were strengthened along the longitudinal direction with CFRP/PET-FRP laminates and one beam was left unstrengthened to serve as a control benchmark specimen. The beams had dimensions of 1840 mm (length) x 240 mm (height) x 125 mm (width) and were reinforced with 2 ϕ 10 and 2 ϕ 8 steel bars in the tension and compression zones, respectively. In addition, the depth from the concrete top compression fiber to the center of the tensile steel reinforcement is 205 mm. To guarantee flexural failure, the beams were reinforced in shear with ϕ 8 stirrups spaced at 100 mm center-to-center. Figure 1(a) summarizes the specimens’ geometry and reinforcement detailing. The external FRP reinforcement consisted of FRP sheets that had a width of 125 mm and span length of 1740 mm between the supports, as shown in Fig 1(b). The FRP sheets were installed onto the soffit of the beams using the wet layup process after preparing the concrete surfaces. Four RC specimens were strengthened with one layer of CFRP laminate (C), one layer of PET-FRP laminate (P), one layer of CFRP followed by a layer of PET-FRP (CP), and one layer of PET-FRP followed by a layer of CFRP (PC), respectively. In addition, one beam was cast and tested as a control beam (CB). 2.2. Material properties To obtain the compressive strength of concrete, three cubes of dimensions 150 x 150 x 150 mm were prepared and cast with the beams to be tested in the testing period in accordance with BS EN 12390-2:2019 (CEN (2019)). The average compressive strength of the cubes was recorded as 46.31 MPa and the calculated average cylindrical compressive strength ( ′ ) was 38.44 MPa. The mechanical properties of the steel bars were obtained by conducting tensile tests on three coupon steel bars with diameters of 8 and 10 mm in accordance with ASTM A370-18 (ASTM International (2018)). The average obtained yield strength, tensile strength, elastic modulus, and elongation at rupture for the No. 8 and No. 10 bars were 557 MPa, 640 MPa, 200 GPa, 11.8% and 549 MPa, 645 MPa, 200 GPa, 12.9%, respectively.