The investigation of behavior of reinforced concrete columns strengthened with GFRP rebar

A large part of our country's building stock creates of reinforced concrete buildings. The majority of existing reinforced concrete buildings pose a great risk in any earthquake that may occur because they were not constructed in accordance with the design regulations without receiving any engineering and inspection services. In order to prevent and minimize the loss of life and property that may occur during an earthquake, buildings that are not earthquake safe should be demolished within the scope of urban transformation or strengthened and made earthquake resistant. In order for reinforced concrete buildings to survive without collapsing during an earthquake, it is of great importance that especially their vertical load-bearing elements are not damaged. The load bearing systems of existing reinforced concrete buildings generally consist of frames. Since columns are the elements that keep existing buildings standing, they should be strengthened if necessary. Within the scope of the doctoral thesis, it was aimed to increase the capacity of column elements with insufficient bending strength. For this purpose, 9 reinforced concrete column specimens with ½ scale 250x250 mm cross-sectional dimensions and 1500 mm height were produced, reflecting the defects in existing buildings in our country, with low concrete strength (15 MPa), longitudinal reinforcement ratio below the minimum limit (0.5%), stirrup hooks 90˚ and without stirrup tightening (200 mm) in the support regions. One of these specimens was left as a reference, and all other specimens were strengthened with different application details using the near surface mounted method (NSM). In the strengthened specimens, the effect of the groove location (corner and edge), the wrapping material (carbon fiber reinforced polymer (CFRP) and textile reinforced polymer (TRM)) and the wrapping condition in the hinge area (unwrapped, partial and full) were examined. Tests of all specimens were carried out under constant axial load (180 kN) with displacement control by applying horizontal load. As a result of the experiments, horizontal load-displacement cyclic graphs of the samples were obtained and horizontal load carrying capacity, bending stiffness, translational ductility and energy consumption capacity were determined. By comparing the results obtained for all samples, the effectiveness of the applied strengthening details on the behavior of the column element was interpreted. It has been observed that the groove location and wrapping condition are quite effective on the element capacity. Analytical calculations of all samples were made according to ACI 440.2R (2017) and ACI 549.4R (2020) and axial load-moment diagrams were obtained. It has been observed that the horizontal load carrying capacities obtained as a result of analytical calculations and experiments are compatible.