Retrofitting of Beam Column Joints Using Confined High Performance Hybrid Fiber Reinforced Concrete

Abstract

Presently, various multi storey buildings are serving to the societies which were constructed before the implementation of seismic codal guidelines. The change in occupancies of building, amendment in codal provisions and to meet the present seismic demand, there is an urgent need for up-gradation of these structures. In the past, various methods, such as steel plate adhesion, jacketing, and fiber reinforced polymer (FRPs), have been successfully deployed to retrofit the pre-seismically detailed structures. Nonetheless, the demerits associated with these techniques depict the limitation in the performance of the retrofitted BCJ. The initial damage level also significantly governs the retrofitting strategies and affects the performance of the retrofitted BCJ. The newly developed high performance hybrid fiber reinforced concrete (HP-HFRC) revealed excellent properties (post-elastic ductility, strain hardening, higher tensile strain, higher bond strength and durability) which are expected from the retrofitting materials. Hence, it can be used as a new retrofitting material. Moreover, in past studies, much attention was not paid to the fact that what is the initial damage level of the beam-column joint before the time of retrofitting. Therefore, the present study explores the possibility of using HP-HFRC in the retrofitting of beam column joint which is initially damaged to different damage levels. In the study firstly, the HP-HFRC is developed using trial and error method and fresh, mechanical and durability properties were evaluated. Thereafter, the design mix is refined using Andreasen and Andersen (A&A) packing model. The fresh, mechanical, durability and microstructural properties are evaluated after 28 days of curing and are further used for retrofitting the initially damaged beam column joint. In the second phase of the study, sixteen beam column joint specimens are cast and set of two specimens are tested as control specimens for each retrofitting strategy. The complete load displacement hysteresis behaviour of control specimens subjected to quasi-static reversed cyclic loading is recoded. Based on the obtained load displacement hysteresis of control specimen, the different damage levels viz., slight, moderate, severe and complete have been calculated using Park and Ang model. Further beam column joints have been initially damaged to achieve the above mentioned damage levels. Thereafter, the initially damaged specimens have been retrofitted using HP-HFRC and wire mesh confined HP-HFRC and tested under cyclic loading. The performance of retrofitted beam column joints has been measured in terms of load displacement behaviour, ultimate lateral drift capacity vi displacement ductility, energy dissipation, stiffness, strength degradation, and principal tensile stress. The proposed retrofitting strategies significantly affect the performance of the initially damaged beam column joint. An improvement of 14.28% and 28.57%, in ultimate drift ratio (UDR), as compared to control specimens has been observed after retrofitting with HP-HFRC and wire mesh confined HP-HFRC beam column joint, respectively. The ductility of HP-HFRC retrofitted beam column joint and wire mesh confined HP-HFRC depicts the maximum improvement of 45.29% and 63.23% respectively than control specimen. The energy dissipation capacity of HP-HFRC confined and wire mesh confined HP-HFRC retrofitted specimen is 8894 kN-mm and 11827.67 kN-mm, respectively. The maximum improvement in energy dissipation in wire mesh confined retrofitted specimen exhibited the 1.32 times improvement over the HP-HFRC confined retrofitted specimen. The maximum improvement in stiffness and strength retention has been observed in wire mesh confined retrofitted specimen than HP-HFRC retrofitted specimen. The excellent improvement in principal tensile resistance capacity along with lower rate of post elastic principal tensile resistance degradation is observed in wire mesh confined HP-HFRC. The effect of initial damage level on the performance of HP-HFRC and wire mesh confined HP-HFRC retrofitted specimen is also evaluated. It has been found that the ultimate drift capacity (UDC) of confined HP-HFRC retrofitted initially complete damaged specimen decreased up to 28.57% and at the same initial damage level, wire mesh confined HP-HFRC retrofitted specimen regains the original drift capacity. Also, the increment in initial damage level decreases the ductility or vice versa. The slope of stiffness and strength degradation is decreased as the initial damage level shifting from complete to slight damage level which reflects the improvement in post elastic behaviour. The maximum improvement in energy dissipation has been found in initially slight damage specimen in both types of retrofitting strategy. Therefore, it is concluded that retrofitting of beam column joint using HP-HFRC and wire mesh confined HP-HFRC improves the overall performance of the beam column joint. However, the behaviour of BCJ retrofitted with wire mesh confined HP-HFRC is superior to HP-HRFC retrofitted joints, due to confinement provided by wire mesh. Further, the initial damage level significantly affects the performance of the retrofitted beam column joint. The performance of the retrofitted specimen improves as the damage level shifts from complete to slight.