An Experimental Study on Retrofitting of Strong Beam- Weak Column Joint by using UHP-HFRC

Abstract

Currently, various multi-story buildings serve the societies that were built before the seismic codal rules were implemented. Because of changes in the building's occupation, changes in codal provisions, and the need to satisfy current seismic codal norms, their early built structures had to be improved, as the structure is the most impacted by seismic vibrations (or earthquakes). The connection of various members or joints with the various elements of the structure is the most affected aspect of RC structural building. However, the Beam-Column Joint (BCJ) assembly is the most affected. They are an essential part of constructing reinforced concrete (RC), which is vital for the structure's stability during an earthquake. However, many of the pre-existing RC structures built before the new code came into effect may prematurely fail due to high stresses because of poorly detailed beam-column joints with a larger flexural capacity in the beam than that in columns and insufficient transverse reinforcement in the joint core, which significantly reduces the ductility of the structure. The beam-column connections, particularly the exterior ones, and the weak column-strong beam effect contributed to the collapse of the entire building. It has been observed in many of the places like Turkey in 1999 and 2011, Kashmir (North Pakistan) in 2005, and Indonesia in 2004 are very active earthquake regions; these earthquakes result in thousands of deaths and enormous economic losses. The structures built in these locations are often made of reinforced concrete and feature several flaws, including poor concrete quality, non-seismic steel detailing, improper structural elements, and various architectural defects. The primary shortcomings include over or under reinforced beams, and weak column - strong beam formation; problems in the codal recommendations also include the absence of lateral or transverse reinforcement in beam-column junctions and column confinement zones. Also, If the column is weaker than the beam, it is found that the plastic hinges are likely to be formed at the column. It is observed that the development of plastic hinges in beams aids in the construction of the structure's most ideal and acceptable energy-dissipating mechanism under seismic circumstances. If the column's ends are created with plastic hinges, the column cannot distribute its flexibility and collapses, which may result in systemic failure. So, diverse retrofitting approaches using various materials and procedures are used to overcome structural damage caused by earthquakes. Pre-seismically designed buildings have

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been repaired using various methods, including steel plate adhesion, jacketing, and fibre- reinforced polymers (FRPs). The degree of initial damage affects the retrofitting methods and

the functioning of the BCJ that has been modified, but these retrofitting processes have some drawbacks. The superior properties such as post-elastic ductility, strain hardening, higher tensile strain, and stronger bond strength and durability often associated with retrofitting materials have been

shown up by novel Ultra High Performance – Hybrid Fibre Reinforced Concrete (UHP- HFRC). It can therefore be used for retrofitting.

The behaviour of the initially damaged, inadequately defined beam-column connection (strong beam-weak column) is thus being retrofitted in this study utilizing UHP-HFRC in combination with confinement supplied by wire mesh. The design mix of M20 grade is designed according to IS: 10262:2009 for the casting of the control specimens. The control samples are then put through a quasi-static reverse cyclic loading test, and a load-displacement hysteresis plot is generated. Using the Park and Ang model, the Damage Indices/levels (complete, severe, moderate, slight) are determined from the hysteresis plot. Wire mesh has been used in earlier studies (published literature) shows excellent results. In this study, two types of retrofitting schemes have been adopted. Firstly, UHP-HFRC is used for retrofitting initially damaged BCJ. The second set of four samples is retrofitted with wire mesh confined UHP-HFRC. The behaviour of load displacement, displacement ductility, energy dissipation, stiffness degradation, and principal tensile stress have all been used to evaluate the performance of retrofitted beam-column joints. The recommended retrofitting techniques have a substantial impact on how well the initially damaged beam-column junction performs. Retrofitting samples adopting UHP-HFRC and Wire Mesh Confined UHP-HFRC exhibit improved peak load capacity, ductility, energy dissipation, stiffness degradation, and primary tensile stress when compared to control specimens. Wire mesh confined UHP-HFRC retrofitted specimen performance and the impact of initial damage level are both assessed. While the wire mesh confined UHP-HFRC retrofitted specimen exhibits a considerable improvement in Peak load carrying capacity, it has been noticed that the confined UHP-HFRC retrofitted initial complete damaged specimen's Peak load carrying capacity was able to restore its original strength. Also, in both the retrofitting strategies, there is shifting of hinge from strong beam weak column to weak beam strong

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column. Thus, it can be said that the overall performance of the beam column joint is improved by retrofitting BCJ with UHP-HFRC and wire mesh confined UHP-HFRC. However, because of the confinement that wire mesh provides, the behaviour of BCJ retrofitted with confined UHP-HFRC is superior to that of simply UHP-HFRC retrofitted Joints. Furthermore, the performance of the retrofitted BCJ is considerably impacted by the degree of initial damage. As the damage degree decreases from complete to slight, the performance of the modified sample gets improves.