For many existing reinforced concrete (RC) building columns and bridge piers designed and constructed in the 1960s and 1970s in many countries, including South Korea, there is a need to increase flexural and shear capacities, as well as ductility. Fiber reinforced polymer (FRP) wrapping using carbon fiber (CF) and glass fiber (GF) has been widely employed and successfully completed for retrofitting RC columns, which have insufficient flexural strength and ductility for seismic actions. Although CF is an excellent material with high strength, high elastic modulus, and excellent durability, it has its own deficiencies, such as high cost, very small rupture strain of about 1%, and electric conductivity. GF is more economical than CF and has good strength and elastic modulus, but it also has deficiencies, including a small rupture strain smaller than 3%, and its performance may not be reliable when exposed to certain environmental conditions, such as alkalinity, moisture, and ultra violet. Aramid fiber (AF) is also an excellent material with mechanical properties between CF and GF and has good durability, but its rupture strain is about 3% (ACI,
2003; ACI,
2007).
Recently, some researchers have focused on the use of new fibers with high to very high ultimate strain in tension, such as polyethylene naphthalate (PEN) and polyethylene terephthalate (PET), for seismic retrofitting of RC columns (Anggawidjaja et al.,
2006; Dai et al.,
2012; Fahmy & Wu,
2010; Liu & Li,
2018; Liu & Sheikh,
2013; Mirmiran et al.,
1998; Saleem et al.,
2018; Ueda et al.,
2006; Zhang et al.,
2017; Obaidat et al.,
2021) PET and PEN have a large rupture strain and good strength, but the elastic modulus is low for both fibers. The stress
–strain behaviors of both fibers are non-linear. On the weight-based comparison, PET is more economical than PEN.
Anggawidjaja et al. (
2006) used FRP such as PEN and PET with large fracture strain for seismic retrofit of RC columns. Through an experimental study of 15 shear-deficient square RC piers, they concluded that RC piers wrapped by multiple PET or PEN layers with large fracture strain could efficiently enhance the ductility of RC piers, and PET and PEN could be used for shear strengthening of RC piers lacking transverse reinforcement if an adequate amount of fiber is provided, regardless of their low stiffness. They also proposed an analytical model to predict the pier’s deformation capacity based on the experimental results. Liu and Sheikh (
2013) conducted tests on nine reinforced concrete circular columns subjected to simulated seismic loads. The specimens were designed with a wide-space steel spiral, and eight specimens were retrofitted with FRP. The test results showed that FRP confinement effectively improved the seismic resistance of columns but displayed different characteristics from steel-confined columns. Curvature ductility and the energy dissipation capacity of a section increased with an increase in FRP confinement. Youssf et al. (
2015) conducted a parameter study using LS-DYNA to investigate the plastic hinge length for FRP wrapped RC columns. Parghi and Alam (
2017) performed nonlinear static pushover analysis to study influencing parameters including concrete strength, yielding strength of rebar, amount of longitudinal steel rebar, level of axial load, shear span
–depth ratio, and carbon FRP confinement layer for RC bridge piers retrofitted with FRP. Zhang et al. (
2017) performed experiments to examine the influence of stirrup corrosion on the shear contribution of a PET
–FRP sheet and substrate columns wrapped by PET. The shear resistance of the PET-wrapped RC square columns was enhanced by the volume ratio of the PET sheet at the peak load and decreased as the corrosion level of the stirrups increased. Based on the test results, a prediction model was proposed to capture the shear capacity of corroded RC columns strengthened by PET
–FRP sheet. In 2018, Liu and Li (
2018) investigated the seismic behavior of corroded RC square and circular columns wrapped with carbon FRP sheets and PET-600 composites. To corrode the steel bar, the electrochemical corrosion method was used with 15% of corrosion rate. It was concluded that PET-600 and CFRP had anti-seismic capacity in terms of hysteretic hoops, failure modes, residual displacement, stiffness degradation, damping ratio, and energy dissipation. Saleem et al. (
2018) studied the lateral response of PET-confined concrete with circular, square, or rectangular cross sections using small-scale specimens. A total of 54 specimens were tested under monotonic axial compression, while test variables were cross-sectional shape, corner radius (in the case of square or rectangular sections), and number of PET layers. In circular specimens, the PET’s large strain capacity was utilized to enhance the strength and lateral ductility of confined concrete. In square and rectangular specimens with low effective confinement, PET mainly contributed to recovering strength loss, while in sufficiently confined specimens, it also resulted in significant strength gain, with a significant increase in lateral ductility. In 2019, Cao and Pham provided the guideline for determining CFRP/GFRP for confinement retrofitting of RC structures poorly confined based on experimental study. Naser et al. (
2019) reviewed the FRP composites and summarized the state-of-the-art experimental, analytical, and numerical works involving FRPs applied to infrastructures including building. Mhanna et al. (
2020) investigated the mechanical properties of PET FRP in terms of thermal effect and developed temperature-dependent models. To investigate the effectiveness of CFRP, seismic-retrofit tests of circular RC bridge piers were carried out by Zhou et al. (
2021). The authors clearly showed that CFRP can reduce vulnerability under lateral loading, and a semi-empirical model for maximum displacement was successfully proposed based on the test data. Although there are many studies for FRP to strengthen RC columns, at present there is no strong consensus in the literature on seismic retrofitting of existing RC columns using various FRPs, such as CF, GF, and PET. Based on the authors’ knowledge, few large-scale laboratory tests have been performed on tied RC circular columns wrapped with PET FRP.
The purpose of this study was to experimentally and analytically compare the structural performance of columns retrofitted by different fibers or fiber combinations in terms of strength and ductility improvement using three different types of fibers (CF, GF, and PET) and one fiber combination (AF/PET, called hybrid fiber reinforced polymer (HF)) for seismic retrofitting of RC columns by fiber wrapping. The study’s main emphasis was to identify the behavior of the PET-strengthened columns and draw comparisons on the behavior of the RC columns strengthened by relatively new ductile fibers with the behaviors of the columns confined by more conventional CF- and AF- strengthened columns. A total of 11 columns were tested in 3 test groups (TGs): 3 control columns and 8 retrofitted columns. The test scheme was pseudo-dynamic lateral reverse cyclic loading with constant axial force simulating seismic action. The main purpose of retrofitting was to improve flexural capacity and ductility in the plastic hinge region. The stress–strain behavior of concrete confined by different fibers was then analytically investigated using an existing model. Section analyses were performed to construct moment–curvature diagrams, including the material properties of confined concrete, reinforcing steel, FRP, and adhesive used in the experiment. The analytical investigation concentrated on the confinement effect of different fibers or fiber combinations, which should be directly related to strength and ductility improvement.