Choudhury, Arshad Hussain2026-10-092026-10-092022http://nits.ndl.gov.in/handle/123456789/205The most critical component of reinforced concrete (RC) moment resisting frame is the beam-column joint. Beam-column joint plays an important role in load transfer mechanism between horizontal and vertical components of a RC framed structure. Integrity of beam column joint influences structural stability of RC frame during an earthquake. During lateral loading, beam-column joint goes through large inelastic displacement. Substandard beam-column joint which were designed, detailed and constructed prior to the development of any seismic code of practice lacks in adequate confinement reinforcement and sufficient anchorage length of the beam reinforcement. Such substandard beam-column joint when subjected to large lateral loads during an earthquake, may lead to formation of diagonal cracks and concrete crushing in joint core resulting in sudden brittle shear failure. With advent of seismic code of practice, beam-column joints are being constructed with special confining hoop reinforcement and adequate anchorage. Such seismically detailed beam column joints exhibit better stiffness, ductility and higher energy dissipation capacity. Recently some studies revealed that reinforced concrete (RC) structures designed as per seismic code of practice exhibited damage in joint region when maximum force acting on the joint exceeded the code expectation level. If the damaged joint is not repaired and strengthened, it may cause instability in structure. In order to mitigate structural instability, rehabilitation of damaged beam-column joint is carried out. Rehabilitation is a simple and effective process of repairing and strengthening the damaged structures to the service level so as to provide safety to the occupant. Rehabilitation by removal and replacement technique is generally used for severely damaged beam-column joint. In removal and replacement technique, the fragmented concrete is removed from the damaged joint region and replaced with suitable repair material. From reported literature it was observed that the repair materials that were used to rehabilitated beam-column joint adopting removal and replacement technique were mostly cement based (concrete, cement paste, cement mortar and commercial micro concrete). Due to utilisation of natural resources and huge environment cost associated with production of cement, scientist and researchers were looking for a sustainable binding material. Geopolymer has immerged out as an environmentally friendly cementitious material alternative to ordinary Portland cement (OPC). Use of industrial by-products like ground granulated blast furnace slag (GGBS), fly ash as source material contributed to lower CO2 emission and energy consumption during production stage makes geopolymer an eco friendly green material. In last decade, geopolymer due to better physical and mechanical properties, has shown immense potential as repair material alternative to cement-based materials. But the repair works reported in literatures was associated with structures subjected to monotonic loading only. Till date, no study was carried out to investigate suitability of geopolymer based material on rehabilitation of structures subjected to cyclic loading. In the present investigation, an attempt has been made to rehabilitate substandard as well as seismically detailed beam-column joints following removal and replacement technique using geopolymer mortar (GM). Two one-third scaled substandard beam-column joints having different percentages of beam and column longitudinal reinforcements designed as per pre 1993s design code (substandard) of Indian Standard were used as control specimens. Both control and rehabilitated specimens were tested under quasi-static cyclic loading. Test results indicated that geopolymer mortar were successful in restoring the cyclic performance of damaged substandard beam-column joint in terms of peak load, stiffness degradation, ductility, energy dissipation and damage index up to the level of control specimens but exhibited undesirable brittle joint failure, similar to that of substandard control specimens. For seismically detailed specimens, two one-third scaled beam-column joints of two different groups categorized on flexural strength ratio (1.2 and 1.4), designed as per seismic code of practice were considered in the study. To compare performance of geopolymer mortar with that of cement mortar, another set of seismically detailed beam-column joint (one from each group) were rehabilitated with cement mortar following same rehabilitation technique. Seismically detailed specimens rehabilitated with geopolymer mortar exhibited 20 – 21% higher initial stiffness, 19 – 22% higher displacement ductility and 23 – 24% higher cumulative energy dissipation compared to specimens rehabilitated with cement mortar. But irrespective of repair material, removal and replacement technique was only able to partially restore the cyclic performance of rehabilitated specimens. This was attributed to presence of yielded hoop reinforcement and cold joints in seismically detailed specimens. When removal and replacement technique was used to rehabilitate seismically detailed beam-column joint, only fragmented concrete was removed and yielded hoops from the damaged joint core are not removed and it remained inside the joint core after rehabilitation. Therefore, investigation was carried out to study the effect of yielding of hoops on cyclic performance of seismically detailed beam-column joint. For this purpose, four numbers of one-third scaled beam-column subassemblies with flexural strength ratio 1.4 were prepared. All four specimens were designed and detailed following relevant provisions of Indian Standard Code of Practice. Out of the four specimens, one was used as control specimen and the rest three were cast with different magnitude of pre-yielded hoop reinforcement in the joint core. It was observed that specimens with yielded hoop reinforcement showed lower peak load, lower ductility and lower energy dissipation along with higher stiffness degradation and damage index. On the other hand, due to removal and replacement, there exists cold joint between old concrete and new repair material. Moreover, yielded hoops remain inside the repaired joint core after rehabilitation adopting removal and replacement technique. Till date, no attempt has been made to investigate combined effect of cold joint and yielded hoop reinforcement on cyclic behaviour of seismically detailed beam-column joint. When tested under cyclic loading, specimens with both cold joint and yielded hoop reinforcement shown the highest reduction in cyclic performance in terms of initial stiffness, stiffness degradation, ductility and cumulative energy dissipation compared to individual effect of either cold joints or yielded hoops. Between individual parameters – cold joint and yielded hoop reinforcement, cold joint played a more dominant role. This was established by reduced cyclic performance of specimens with cold joint in terms of initial stiffness (15 -17% lower), displacement ductility (7 – 9% lower) and energy dissipation (18 – 19% lower) in comparison to specimens with yielded hoops. It was observed that though GM was able to enhance performance of rehabilitated specimens but was not able to alter mode of failure brittle joint failure to ductile beam failure in substandard specimens. Furthermore, when seismically detailed specimens were rehabilitated with geopolymer mortar, rehabilitated specimens displayed beam failure accompanied by joint failure as well as specimens exhibited lower performance compared to control under cyclic loading. Such failure is undesirable and unacceptable as per current seismic design philosophy. To mitigate underperformances in beam-column joints, steel fiber reinforced geopolymer mortar (SFRGM) was used for rehabilitation of both substandard and seismically detailed specimens adopting same repair technique. For substandard specimens, SFRGM not only increased performance of rehabilitated specimens substantially above the level of controls but also alter mode of failure to ductile beam failure. In case of seismically detailed beam-column joints, specimens rehabilitated with SFRGM exhibited beam failure along with 28 – 34% increased peak load, 20 – 26% increased initial stiffness, 109 – 113% increased ductility, 177 – 212% higher cumulative energy dissipation, substantial lower stiffness degradation and lower damage index.en