Axial behavior of low strength concrete prisms confined with hybrid fiber reinforced polymers

Abstract

Tens of thousands of people lose their lives due to natural disasters every year. Earthquakes are one of the most devastating of these natural disasters, and it is known that most of the deaths occur due to the collapse of buildings. Despite the existence of engineering solutions that can almost completely eliminate the risk of loss of life, loss of life continues to occur, and this situation presents a critical problem that needs to be examined from a sociological and psychological perspective. As engineers, it is our responsibility to solve these problems faced by society and, if necessary, produce innovative solutions. In order to prevent loss of life, it is of great importance to develop emergency strategies that will increase the earthquake resistance of buildings. But for these strategies to be effective, they must also be cost-effective and sustainable. Demolishing risky structures and replacing them with code-compliant, earthquake-resistant structures may not always be a feasible, sustainable, or fast solution. It is known that structures with poor seismic performance pose significant risks in terms of loss of life and property. Post-earthquake assessments/observations and research have shown that structures constructed without complying to seismic codes, specifications, and standards (substandard structures) sustain severe damage, often leading to collapse. Common deficiencies in such buildings include inadequate dimensions of load-bearing structural elements, the use of low-strength concrete, insufficient longitudinal and transverse reinforcement ratios, and improper reinforcement detailing. In order to minimize the damage or destruction efforts should be focused on improving the seismic performance of these structures. For this purpose, implementation of retrofitting applications has become a highly preferred practice worldwide in the last decades. A technique used to improve the seismic performance of inadequately designed reinforced concrete (RC) columns, beams and slabs is the application of fiber-reinforced polymers (FRPs) retrofitting. The practice of CFRP confinement began in the early 1990s and has become a standard practice today. The FRP confiniement method, which provides benefits in terms of time and cost by preventing the demolition and reconstruction of structures, also has important features that distinguish it from other strengthening methods. These include the ability to adapt to any geometry, high strength/weight ratio, easy applicability that allows the business to continue operating, corrosion resistance, low maintenance requirements, and rapid applicability. In RC column strengthening applications, FRP is typically wrapped around the column with fibers aligned parallel to the transverse reinforcement. Epoxy or similar resins are used to ensure the adhesion of FRP to the concrete surface and to bond multiple layers of FRP together. In recent years, research on polyethylene terephthalate (PET) fiber-reinforced polymers (FRP) with large rupture strain (LRS) capacity has gained widespread attention as an alternative to traditional FRP materials (carbon, glass, basalt, and aramid). PET is a more economical material compared to conventional FRPs. The PET fabrics used in this study were obtained through recycling, making them environmentally friendly and sustainable. In this thesis, it is aimed to obtain a hybrid confining material with both improved structural performance and reduced cost by combining carbon fiber fabrics with superior strength and rigidity, and PET (Polyethylene Terephthalate) fiber fabrics with superior deformation ability. The primary objective of hybrid confinement is to utilize the advantages of both FRP materials to enhance column ductility and maintain element integrity under severe damage conditions. Additionally, since PET is a recycled material (produced from unused fiber waste generated during the manufacturing of tire cord fabric) and is more affordable than traditional FRPs, the study also aims to reduce FRP confinement costs while achieving positive environmental benefits. An experimental study was designed to investigate the effectiveness of hybrid confinement under axial compression loading. At the initial stage of the experimental study, coupon specimens were prepared following ASTM D3039 standards to evaluate the tensile behavior of CFRP and PET FRP materials. Tensile tests were conducted on these specimens to determine the tensile strength, ultimate strain, stress-strain relationship, modulus of elasticity, and failure modes of both FRP materials. The stress-strain relationship of CFRP exhibited a linear elastic behavior, whereas PET FRP demonstrated a bilinear response. The tests show that CFRP has a tensile strength of approximately 2535 MPa, ultimate strain of 1.08%, and modulus of elasticity of 240 GPa. PET FRP has a tensile strength of 490 MPa, ultimate strain of 7.2%, first modulus of elasticity of 15.2 GPa, and second modulus of elasticity of 5.5 GPa. In the following phase of the experimental study, nineteen prismatic concrete specimens were produced using low-strength ready-mix concrete. The specimens had cross-sectional dimensions of 150 mm × 275 mm, a height of 550 mm, and a corner radius of 30 mm. The test variables included the FRP type (carbon, PET, and hybrid) and the number of PET FRP layers (1, 2, and 3). Two specimens were left unconfined as reference samples, while the remaining specimens were confined transversely with CFRP, PET FRP, and hybrid FRP. Then, specimens were subjected to monotonically increasing axial compressive loads. To measure transverse strains, strain gauges were used, while axial deformations in the vertical direction were measured using 50 mm capacity linear variable displacement transducers (LVDTs). The readings were collected every second and transferred to a computer via a data logger until specimen lost its integrity. The experimental results revealed that, compared to unconfined specimens, CFRP confinement increased the compressive strength of the specimens by approximately 20%, while PET FRP confinement significantly enhanced the deformation capacity by approximately 150% for 1P, 310% for 2P, and 430% for 3P specimens. Hybrid FRP confinement demonstrated beneficial effects in terms of strength, and axial deformation ductility, preserving the advantages of both FRP materials. An increase in the number of FRP layers further improved the strength and ductility. The compressive strength and axial deformation capacity of the tested specimens were predicted using existing analytical models. A significant discrepancy was observed between the experimental deformation values and the predicted values. The models proposed by Ilki et al. (2004) accurately predicted the compressive strength of CFRP confined specimens, while the models proposed by Pimanmas & Saleem (2019), Yuan (2022), and Zeng (2023) were able to predict the compressive strength of PET FRP confined specimens with sufficient accuracy. For hybrid FRP confinement, the models proposed by Pimanmas & Saleem (2019) and Zeng (2020) for PET FRP confinement were applied, following the hybrid confinement algorithm suggested by Ispir et al. (2018). The strength predictions of these models for hybrid FRP-confined specimens were found to be successful.