The effect of waste plastic replacement and CFRP wrapping on the behavior of reinforced concrete columns under cyclic loading

This thesis examines the effects of substituting PET wastes for fine aggregate in concrete on various mechanical properties of concrete, as well as the impact of the presence of PET replacement in reinforced concrete columns under cyclic loading. Additionally, the effects of wrapping using carbon and glass fiber polymers have been tested. Solid waste has emerged as one of the most pressing environmental challenges of our time, primarily due to its increasing volume and improper disposal methods. Among various types of waste, plastic stands out as a particularly problematic material. Its widespread use across industries and households, combined with its non-biodegradable nature, has resulted in a rapid accumulation of plastic waste in landfills, oceans, and urban environments. This proliferation of plastic waste not only degrades ecosystems but also poses serious threats to wildlife and human health. In recent years, the urgency of effective waste management strategies has intensified. One of the most viable and sustainable approaches to addressing the plastic waste crisis is through recycling and reusing plastic materials. By diverting plastic waste from landfills and repurposing it for new applications, we can significantly reduce environmental pollution and conserve natural resources. A promising innovation in this regard is the incorporation of plastic waste into concrete. Utilizing plastic as a partial replacement for traditional aggregates or as reinforcement in concrete not only offers a cost-effective solution for the construction industry but also serves as an environmentally responsible method of waste disposal. This practice reduces the demand for virgin materials, lowers carbon emissions associated with concrete production, and helps to manage plastic waste in a sustainable manner. Developing and adopting such sustainable solutions is essential in mitigating the adverse environmental impacts of plastic waste. Through research, innovation, and policy support, plastic waste can be transformed from an environmental liability into a valuable resource, contributing to a more sustainable and circular economy. In recent years, researchers have focused on the use of recycled plastics in concrete. Incorporating plastic waste into concrete mixtures offers a range of environmental and economic benefits, primarily by diverting plastic materials from landfills and reducing the reliance on natural aggregates, which are finite and often energy-intensive to extract. One of the key advantages of using recycled plastics in concrete is the potential improvement in certain material properties. Depending on the type and proportion of plastic used, concrete mixtures can exhibit enhanced characteristics such as reduced density, increased ductility, improved thermal insulation, and better resistance to chemical attacks. These modifications can be particularly beneficial in specific applications, such as lightweight concrete for non-load-bearing structures, insulation layers, or construction in seismic zones where ductility and energy absorption are crucial. The use of recycled plastic aggregates also contributes to a significant reduction in the environmental footprint of the construction industry. By minimizing the extraction and processing of natural aggregates, this practice reduces greenhouse gas emissions and conserves valuable natural resources. Additionally, it promotes circular economy principles by reintroducing waste materials into the production cycle, thus improving overall resource efficiency. Among the various types of plastics suitable for incorporation into concrete, polyethylene terephthalate (PET) stands out due to its widespread availability, high recycling potential, and favorable physical properties. PET, commonly found in beverage bottles and packaging materials, can be processed into granules, flakes or fibers and used as partial replacements for traditional aggregates or reinforcement in concrete. Its use not only enhances sustainability but also offers a cost-effective alternative to conventional materials, potentially lowering construction costs without compromising performance. Overall, the integration of recycled plastics into concrete technology represents a promising advancement in sustainable construction. This innovative approach not only contributes to reducing environmental pollution and alleviating the burden on landfills but also aligns with the global movement toward greener, more cost-efficient building practices. Studies on replacing PET waste in concrete have generally focused on cube or cylinder samples. While these initial investigations provide valuable insight, they do not fully capture the performance of PET-based concrete in structural elements subjected to real-world loading conditions. In modern construction, reinforced concrete (RC) structures are widely used due to their strength, versatility, and durability. Therefore, it is essential to investigate the compatibility of PET-modified concrete with steel reinforcement, particularly in structural members that play a key role in ensuring the integrity of buildings, such as columns. The application of PET waste in reinforced concrete elements introduces new variables that could influence the interaction between concrete and steel reinforcement. Specifically, the behavior of reinforced concrete columns incorporating PET waste under axial, lateral, and seismic loads may differ from that of conventional concrete columns. As columns are critical load-bearing elements in any structural system—especially during seismic events—their performance under earthquake loading is of paramount importance. Inadequate reinforcement detailing, combined with low ductility in the concrete matrix, can lead to severe damage or collapse during seismic activity. To address these vulnerabilities, numerous studies have explored various strengthening techniques aimed at enhancing the seismic performance of RC columns. Among these, the use of Fiber Reinforced Polymer (FRP) systems has gained significant attention in both research and practical applications. FRP materials, known for their high strength-to-weight ratio, corrosion resistance, and ease of installation, offer a promising alternative to traditional retrofitting methods. When applied as external confinement through wrapping, FRP restricts the lateral expansion of the concrete core under load, thereby increasing confinement pressure. This confinement not only improves the compressive strength and ductility of the column but also enhances its energy dissipation capacity during seismic events. The integration of FRP systems in retrofitting strategies has been shown to significantly improve both the strength and deformation capacity of RC columns, making them more resilient to earthquake-induced forces. As the construction industry continues to seek sustainable and resilient solutions, the combined use of recycled PET waste in concrete and advanced strengthening systems such as FRP presents a unique opportunity. This approach not only promotes environmental sustainability by reducing plastic waste but also contributes to the development of earthquake-resistant structures with improved long-term performance. In the first phase of the study, 10%, 20%, and 30% of the sand by volume in the concrete was replaced with PET granule material. Experiments were conducted on both the reference concrete without any replacement and the concrete samples with PET replacement, including slump, density, compressive strength, splitting tensile strength, and flexural strength tests. The results of the experiments showed that the PET replacement reduced the values of slump, density, elasticity modulus, compressive strength, splitting tensile strength, and flexural strength of the concrete by certain percentages. It is considered that the decrease in slump values due to the increase in the PET replacement ratio in concrete may pose a disadvantage for the workability of fresh concrete. However, it has been concluded that this reduction in density could contribute to reducing the weight of the structure or achieving lightweight concrete when the concrete is used structurally. The PET replacement in concrete has decreased the modulus of elasticity, compressive strength, split tensile strength, and flexural strength, while increasing the ultimate strain values. Therefore, it has been concluded that PET replacement can be used in situations where displacement demand is required, but it is not suitable for increasing strength. In the second phase of the study, the concrete with 10% PET replacement, which yielded the best results, was strengthened by wrapping with carbon and glass fiber polymers, and the same tests were repeated. As a result of the wrapping process, the damage caused by the PET replacement was significantly compensated. In the concrete material tests, the reduction in compressive, split tensile, and flexural strengths due to PET replacement was compensated for by using FRP, even surpassing the reference strength. Thus, it has been observed that using PET as a fine aggregate replacement within the concrete and FRP as a wrapping material outside the concrete is a beneficial method for obtaining concrete that is both lighter and stronger. Furthermore, this study has compiled experimental data from studies where PET replacement was made by volume or weight in place of fine aggregates in concrete, providing a comprehensive database. Based on the results of the experiments conducted in this study and the data in the literature, model suggestions for PET-replaced concrete have been proposed. The models suggested in this study were subjected to applicability tests alongside previously proposed models. Similarly, the model suggestions for wrapped concrete in the literature were tested with the experimental data from this study, and their suitability for PET-replaced concrete was discussed. In the third phase of the study, the PET replacement was scaled up from the material size to the structural element size, and reinforced concrete columns were produced from PET-replaced concrete. The columns were categorized based on the presence of PET replacement, the use of carbon fiber polymer, and axial load levels. The created columns were tested under horizontal cyclic loading. The horizontal load-displacement curves obtained from the experiments were analyzed. The maximum horizontal loads carried by the reinforced concrete columns, their ductility coefficients, stiffness values, and the energy they dissipated were determined. When 10% PET replacement was used in the concrete of reinforced concrete columns tested under earthquake-like horizontal cyclic loads, a slight negative trend was observed in the maximum horizontal loads, stiffness values, and energy dissipation. However, their overall behavior remained similar. An increase in the axial load level of the reinforced concrete columns tested under horizontal cyclic loads led to an increase in the maximum horizontal loads and stiffness values, but it caused significant decreases in ductility and dissipated energy values. This limited the displacement capacity of the columns and led to a more brittle behavior. The use of carbon fiber polymer wrapping around the reinforced concrete columns resulted in increases in the maximum horizontal loads, ductility coefficients, stiffness values, and energy dissipation, providing positive results in all aspects. While the carbon fiber polymer wrapping caused slight increases in the maximum horizontal loads and stiffness values, it significantly enhanced the columns' behavior under earthquake-like cyclic loading by providing substantial increases in ductility and energy dissipation values.