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This research study includes an experimental and analytical study of steel fiber-reinforced concrete columns using natural and recycled aggregate (RA). To improve the structural application of recycled aggregate and to protect the environment and preserve natural resources, it is crucial to use recycled aggregate in construction. The recycled coarse aggregate reinforced concrete columns with the addition of steel fiber subjected to concentric and eccentric loadings for short and slender columns are examined experimentally and analytically in this research. Forty two column specimens were cast to examine the impact of steel fiber, recycled aggregate, slenderness, and eccentricity on the behavior of reinforced concrete columns. In addition to concentrically loaded columns, columns were loaded at 50% and 100% eccentricity, corresponding to e/h ratios of 0.5 and 1.0. Three different slenderness ratios were selected to examine the effects of height: 17.24 for short columns, 26 for moderately slender columns, and 34.5 for highly slender columns. The research examined the failure mode, maximum load-carrying capacity, strain in the concrete, and strain in the reinforcement, mid-height lateral displacement, vertical displacement and ductility. Based on the results of the current study, it can be concluded that employing recycled concrete aggregate is a potential approach that can meet design codes. Columns produced with recycled concrete aggregate behaved similarly to columns made with natural aggregate (NA). The addition of 1% steel fiber effectively prevented concrete from crushing and spalling. Steel fiber, however, improved the columns' ductility and strength. According to experimental results, the steel fiber addition narrowed the crack width which visually observed and had a comparable effect on columns constructed with recycled aggregate and columns constructed with natural aggregate. The experimental test maximum load carrying capacity agreed well with the results using ACI-318-19 equations. Furthermore, a model has been proposed for columns with both natural and recycled aggregate and accounts for eccentricity and slenderness to forecast the load-carrying capability. Additionally, the second-order effect due to the intentional such as given eccentricity and unintentional eccentricity such as alignment errors was investigated. The second-order effect is considered an excellent theoretical method to examine the behavior of columns. Using this method theoretical load path was drawn for each column tested as well as an experimental load path for comparison, later, using the same method, an axial load bending moment interaction diagram was plotted for all the tested columns. The outcomes demonstrated that the design principles were met well. Plots of load-moment interaction diagrams for short, slender columns prepared using the ACI-318-19 equations, 2nd order effect method, and proposed method. The experimental findings were added to the interaction diagrams for comparison.

Finally, it needs to be mentioned that recycling concrete waste cubes and other recycled materials decreases the amount of waste sent to landfills. This practice helps avoid the consumption of natural resources, thereby preventing their quick depletion and cutting down on the expenses and distractions linked to their extraction. Utilizing sustainable materials and creating a new pathway for their reuse, such as incorporating recycled aggregates, can reduce waste and conserve natural resources.