Strengthening of reinforced concrete columns with CFRP plates

Since there is an earthquake risk in a large part of our country, building systems must be resistant to earthquakes. However, when the buildings constructed in our country before 2000 are examined, it is frequently encountered with structural system elements that do not meet the conditions of the Earthquake Regulation in force at that time, with low concrete strength, no stirrup tightening, stirrup hooks bent 90°, and insufficient lap length. These weaknesses increase the destructiveness of earthquakes in our country. The places we live in usually consist of multi-storey reinforced concrete buildings. The elements that keep the buildings standing are columns. Therefore, if the design of the column elements is safe, the buildings are also safe. Reinforced concrete column elements usually have a lap joint at each floor. In order to ensure safe load transfer, it is necessary to apply this lap length in reinforcements in sufficient and required length. Insufficient lap length can cause serious structural problems in reinforced concrete columns and jeopardize structural safety. For this reason, the lap length should be carefully considered in the design and application processes and defects in existing elements should be eliminated by taking precautions. Within the scope of this thesis, nine reinforced concrete cantilever columns were fabricated and retrofitted in order to prevent stripping damage and/or improve the behavior of reinforced concrete columns with insufficient lap length. The specimens were produced with a lap length deficiency (15 db) and with a hook with stirrups bent at 90° and without stirrup tightening in the bearing areas. One of the specimens was left bare as a reference and the remaining eight specimens were strengthened with different parameters. In the strengthening process, Near Surface Mounted (NSM) method was used in which CFRP plates were placed inside the column elements by cutting a groove along the mat layer. In some specimens where NSM was applied, EBR (Externally Bonded Reinforcement) method was also used for hybrid reinforcement. Plate orientation (parallel and perpendicular), number of plates (2 and 3), wrapping material (carbon fiber reinforced polymer (CFRP) and textile reinforced polymer (TRM)) were selected as reinforcement parameters. The specimens were tested under constant axial load and cyclic horizontal load. The effects of the selected parameters on the damage mechanism, load carrying capacity, energy dissipation capacity, initial stiffness and ductility of the element were investigated. The horizontal load carrying capacities of the specimens due to bending were calculated theoretically and compared with the capacities obtained from the experiments. In the theoretical calculations, the maximum strength of the lap reinforcement was determined according to different methods in the literature and the variation of these methods according to the capacity values obtained in the experiments was analyzed. In the retrofitted specimens, increasing the number of CFRP plates and adding wrapping to the section increased the load carrying capacity, energy dissipation capacity, initial stiffness and ductility values of the member compared to the reference specimen. However, the effect of changing the plate orientation was less than 10%. The best behavior among the retrofitted specimens was observed in specimen N7-D3-F where three plates were used and lateral wrapping with CFRP sheet was performed. In general, the increase rates in the specimens wrapped with CFRP sheet were higher than those in the specimens with TRM. As a result, reinforced concrete columns with insufficient overlap length can achieve significant improvements in their structural properties if they are laterally wrapped with CFRP sheet by adding a sufficient number of CFRP plates anchored to the foundation. In some regulations, it is recommended to reduce the yield strength of the longitudinal reinforcement by a certain percentage in the calculation of the flexural capacity of elements with insufficient lap length. Within the scope of the thesis study, capacity values were obtained according to different sources and comparisons were made with the experimental results.