An experimental study on the behavior of square short concrete columns confined with hybrid frp under monotonic axial compression stress

Abstract

Column elements of structural systems are often in need of strengthening due to aging, damage, increased load demands, or adding floors to existing multistory buildings. The potential collapse of these columns necessitates retrofitting to boost their strength and ductility. Consequently, enhancing the resilience of building columns has become a focal point of recent research. The primary aim of such structural reinforcement is to improve strength and ductility, increasing the structure's ability to bear loads and reducing the risk of damage or failure during catastrophic events like earthquakes. A common technique for reinforcing such columns involves the application of Fiber Reinforced Polymer (FRP) jackets. This method wraps the concrete member with FRP jackets, which consist of fibers positioned at right angles to the member's longitudinal axis. FRP jackets provide passive confinement pressure to the concrete column and are activated (strain) only when the column faces additional axial load, leading to its expansion. The degree of strengthening and the overall increase in strength of the reinforced concrete depend on various factors. such as fibers used (modulus, thickness, and rupture strain) and the physical characteristics of the column (size and shape, whether circular, square, or rectangular). Traditionally, composite structures mainly utilized fibers like carbon, aramid, basalt, and glass for strengthening. However, the introduction of new fibers, such as PET and PEN, has been a game-changer in the construction industry. These materials have led to the development of new building materials that are crucial for strengthening and offer environmental advantages. Compared to traditional fibers, these new fibers exhibit distinctive mechanical properties, including a higher capacity to stretch before breaking (rupture strain) and lower strength values. Our research is primarily focused on hybrid fiber-reinforced polymers (FRP), particularly the combination of Carbon and Polyethylene Terephthalate (PET) fibers. Hybrid FRP involves using two different types of fibers together, each with its unique set of mechanical characteristics. By using these fibers together, the resulting composite material can show improved mechanical performance and may also be more cost-effective. The experimental part of our study 18 specimens, with two identical samples for each FRP wrapping configuration. Each sample was 300x300x550 mm in size and had a corner radius of 30 mm. The specimens were categoriaed with respect to the type and number of FRP layers used: a control group without FRP, a specimen group wrapped conventionally, and a hybrid-FRP wrapped specimen group that combined both fiber types. The last specimen group addresses a significant gap in existing research by examining the hybrid use of FRP types and assessing their practical application in real-world settings. Prior to the sample preparation phase, careful attention was given to the concrete mix design. The aim was to ensure that the produced concrete possessed lower strength characteristics, mimicking the quality commonly found in older buildings. The second phase of the thesis study is to characterize tensile behavior of CFRP and PET FRP. For each FRP type, five coupon specimens were prepared and tested under tensile loading in accordance to ASTM D3039 [1]. Thanks to these tests, the strength, strain capacity and modulus of elasticity were determined for each FRP type. This methodical approach was intended to provide a clear and comprehensive understanding of how each FRP material behaves when subjected to forces that aim to stretch it, ensuring a robust evaluation of their potential for retrofitting older building structures. The main variable parameters considered were the type of FRP material, the application of hybridization, and the number of layers used to wrap the columns. This allowed for a detailed comparison of various layer combinations under monotonic compression axial loads. Precise measurements of the hoop strain of the FRP and the longitudinal strain were obtained by strategically positioning strain gauges and LVDTs. Compression tests were conducted utilizing an Instron testing device with a maximum capacity of 5000 KN. The main goal of this study is to examine the axial strain ductility and strength of a column by using PET-FRP sheets that are wrapped around a carbon FRP layer. The sheets are layered on top of each other. The whole system is expected to exhibit a ductile behavior compared to the brittle behavior seen in concrete wrapped with conventional FRP application. The purpose of this research is carry out the following studies: (a) Investigate the efficiency of hybrid FRP composites in terms of strength and strain enhanchements (b) Conduct comparative studies to evaluate the performance of FRP wrapped and unwrapped specimens. (c) Analyze failure patterns (d) Contribute new insights to the existing knowledge on FRP wrapping techniques. (e) Perform theoretical analyses to predict axial behavior in confined samples and correlate these predictions, with empirical observations. The test results show that the use of CFRP wrapping greatly improved the mechanical characteristics of concrete, notably enhancing both its strength and strain capacity. It was discovered that employing two layers of CFRP material significantly magnified these effects. While the application of PET-FRP wrapping significantly improved the axial ductility of the specimens, that of CFRP wrapping greatly improved the strength.Additionally, PET FRP ensured that the integrity of the specimens was almost maintained even when the specimens were subjected to severe damages. The enhancement of concrete columns wrapped with PET-FRP layers reveals a direct correlation between the number of layers and the degree of enhancement in both strength and strain. Specifically, columns wrapped with a single layer of PET-FRP, designated as 1P, exhibit an ultimate strength enhancement of 1.23 and an ultimate strain enhancement of 15.51. When a second layer is added, creating 2P specimens, the strength enhancement increases to 1.29, and strain enhancement shows a more pronounced increase to 27.47. This trend continues with 3P specimens, where three layers of PET-FRP wrapping provide the highest recorded enhancements, with strength enhancement reaching 1.44 and strain enhancement escalating to 30.54. These figures suggest that each additional layer of PET-FRP not only contributes to a higher load-bearing capacity but also significantly improves ductility, allowing the column to undergo larger deformations before failure. This is particularly beneficial in seismic zones where energy absorption and ductility are critical for the structural integrity of buildings. The key findings of the test results of hybrid-FRP wrapped specimens show that the hybrid wrapping method works well at improving both the strength and axial strain ductility of concrete. Each extra layer of PET-FRP makes a big difference, especially when it comes to strain capacity, which makes the concrete stronger against forces that cause it to deform. PET-FRP significantly contributes to the ductility of the specimens, a desirable property, especially in seismic-prone areas. The hybrid-FRP system, combining the advantages of both materials, emerges as a good solution, striking an optimal balance between strength and flexibility. The thesis study also looked at existing predictive models and compared them to test results. This showed how hard it is to accurately model the behavior of FRP-confined concrete, especially in hybrid configurations. The discrepancies observed in strain predictions underline the need for more sophisticated models to accurately capture the complexities of material behavior under confinement. Looking ahead, the research opens several promising avenues for future exploration. The adaptability and performance of the hybrid system will be investigated across different column geometries and in conjunction with traditional reinforcement methods such as reinforced concrete (RC) columns. Furthermore, the durability of the hybrid system under various environmental conditions and loads will be a key focus, ensuring its long-term effectiveness and reliability as a structural reinforcement solution. In the future, researchers could look into the best ways to combine different type of FRP layers for different structural needs and also look into how well hybrid-FRP retrofitted structures work over time.