Cyclic behavior of HPFRC-repaired reinforced concrete interior beam-column joints

A large number of old buildings have been identified as having potentially critical detailing to resist earthquakes. The main reinforcement of lap-spliced columns just above the joint region, discontinuous bottom beam reinforcement, and little or no joint transverse reinforcement are the most critical details of interior beam column joints in such buildings. Five 1/3-scale interior beam column joints representing these critical reinforcement details under seismic loads were constructed and tested under cyclic lateral loading simulating seismic excitation. These specimens behaved weakly such that they attained low load carrying capacity, small energy dissipation, and failed in diagonal shear in the joint region. After testing, the specimens were repaired using high performance fiber reinforced concrete (HPFRC) jacket, all around the joint column regions, and retested up to failure. Higher load levels were attained, more ductile behavior was achieved, substantial energy dissipation was observed, slower stiffness degradation was also noted. Failure modes were transformed from brittle shear in the joint diagonals to a ductile one in the beam region through plastic hinge formation in the beam maximum moment sections. Based on the findings of this study, the strengthening technique used promises to provide a cost effective, long-term repair and retrofit solution that can be implemented in the field.